Simon Ince's Blog : Patterns

Do you need web client development help?

simonince — Tue, 21 Jul 2009 15:01:00 GMT

Then tell us what you need! Blaine has blogged a link to a survey that patterns & practices will use to help them understand what challenges that are affecting you the most... so that they can choose the right areas to focus effort in providing guidance.

If you’ve got a couple of minutes spare head over to the survey...

What tasks or areas are difficult? What consumes the most time? What is it that causes you post-deployment headaches? What parts of the process are tricky? What don't you do because it is hard?

Enforcing Unobtrusive JavaScript

simonince — Fri, 10 Jul 2009 21:07:00 GMT

Unobtrusive JavaScript is a concept that’s been around for quite a while, but it’s now finally starting to make its way into the mainstream. The basic idea is that you should separate your JavaScript from your HTML – so instead of;

<a href="Default.aspx" onclick="alert('Sorry, that is unavailable'); return false;" >Click Here</a>

You should have something like this;

<a id="ClickHereLink" href="Default.aspx">Click Here</a>

... with some matching JavaScript in another file somewhere;

function pageLoad() {

var link = $get('ClickHereLink');

$addHandler(link, 'click', preventClick);

}

function preventClick() {

alert('Sorry, that is unavailable');

return false;

}

I’m not here to tell you why this is so great, but the best discussion and walkthrough of issues I’ve found is this article on onlinetools.org, so check it out if you’re interested.

How do I enforce it?

Recently I found myself cursing some code I’d written a long time ago, when I found it littered with onclick and onload handlers. It was frankly a pain to work out how it works. It was at this point that I thought “I should write something to shout at me when I write don’t write unobtrusive JavaScript”.

Coincidentally I have also been doing some work on HTML parsing, and I have to say I’ve found the XPath-like features in the HTML Agility Pack to be incredibly useful. If you’ve never used it I’d recommend taking a look.

It occurred to me that this was the perfect tool to do what I wanted – so I set about creating a simple MSBuild task that uses the HTML Agility Pack to scan ASPX and ASCX files for Obtrusive JavaScript – basically a form of static code analysis. If you’re interested in customising MSBuild, check this out.

The implementation is very simple;

· It is derived from Microsoft.Build.Utilities.Task.

· There is a property that is an array of ITaskItems, flagged with a [Required] attribute, as a way to pass in a list of files to check.

· There’s a hard-coded (for now) list of validation XPath expressions, with matching explanations.

· In the Execute method override, it loops through all the files.

· For each file, it opens the HTML and applies every XPath to it.

· If there are any matches for the XPath, it creates a build failure.

The key to success here is the simplicity with which the HTML Agility Pack lets me use XPath to find elements. The following is the basics of my code;

HtmlDocument doc = new HtmlDocument();

doc.Load(path);

HtmlNodeCollection results =

doc.DocumentNode.SelectNodes(

validator.SearchXPath);

Easy huh? The XPath expressions themselves mostly look like this;

//*[@onkeypress]

This will match any HTML element that has an onkeypress attribute... which of course means that the developer has specified a JavaScript event handler in the HTML, and broken the rules of unobtrusivitiy (hey, if that’s not a word I’m having first dibs on inventing it J).

Remembering that we’re actually scanning ASP.NET mark-up, not pure HTML, we can also check for ASP.NET attributes that actually create JavaScript event handlers in HTML too;

//*[@onclientclick]

Again, this is not difficult. The tricky one appears when you realise that it is valid to have OnClick in ASP.NET code! Of course, an <asp:button> can have an OnClick attribute referring to the server-side event handler. I wondered about saying “match any with an onclick that don’t have a runat attribute”, but this isn’t quite accurate (HtmlControls can have runat=”server” too remember).

In the end I went for explicitly excluding the ASP.NET controls that I knew were OK to use OnClick;

//*[@onclick and not(

name()='asp:button' or name()='asp:bulletedlist' or

name()='asp:imagebutton' or name()='asp:imagemap' or

name()='asp:linkbutton')]

Finally, I wanted to check that there was no JavaScript inside the page HTML itself... even between <script> tags. In other words, the only valid <script> tags should reference an external file, as follows;

[Of course, you could also use the <asp:ScriptManager> control if you wanted to; in fact I’d prefer it]

To achieve this I check that there is nothing other than whitespace between the start and end script tags;

//script[string-length(normalize-space(text()))>1]

... and that’s it! As usual, the source code for this MSBuild Task is attached to this post, along with a demonstration Web App.

Wiring up the Task

Now we have a simple MSBuild task, all that is left to do is add it to a build. I created a new ASP.NET Web Application, right clicked the project file and chose “unload”, and then edited the XML manually.

The only bit I needed to add was as follows;

<UsingTask

AssemblyFile="..\JsBuildTasks.dll"

TaskName="ValidateUnobtrusive" />

<ValidateUnobtrusive

SourceFiles="@(Content)" />

</Target>

First I import my task (I’ve copied a Release build of the assembly to the parent directory above my Web App project). Next, I override the BeforeBuild target, and add an instance of my ValidateUnobtrusive task. I’ve passed in an expansion of all the Content files in the project. The task filters for *.aspx and *.ascx at the moment – if you wanted to add *.html that would be trivial.

Save the file, and right click it to choose Reload Project. At this point Visual Studio 2008 warns you about a customised MSBuild project file – just choose “Load Project Normally” and you’re away.

From this point on, whenever you build a project containing Obtrusive JavaScript you’ll see errors like these;

Error Use of the 'onclick' JavaScript event on 'a' tag in markup causes Obtrusive Javascript. You should attach this handler in script instead.

Error Failed on 'script' tag . A JavaScript region containing something other than white space was found. You should link to related JavaScript resources files rather than embed script in a page.

Conclusion

I think this works really well so far (although I’m at the early stages of trialling it); if you try it do let me know how you get on. One of the particularly cool things (that I haven’t tried yet as I’m working isolated on my laptop most of the time right now, and I’m short of time) is that this could be wired up into a Continuous Integration build, under Team Build – I guess that would be a good plan if you didn’t like running the validation as part of your local build, or if it starts to perform badly due to the complexity or size of your pages.

I also think it raises other questions and possibilities. For example; how about some static analysis of “.js” files themselves? Could this help check for Directly Accessible Scripts, for example?

String.Unformat: I've created a monster

simonince — Thu, 09 Jul 2009 20:42:00 GMT

I don’t know if you’re the same, but when coding away I often find myself wishing for a String.Unformat function – call it the evil twin of String.Format. With String.Format I can build up strings like this;

var result = String.Format(

"http://{0}:{1}/{2}",

"localhost",

"12345",

"TestPage.aspx");

... which will return "http://localhost:12345/TestPage.aspx".

But what if I want to do the opposite; split out the contents of this URL, in one easy statement, much like String.Format? What if I could do this...?

string input = @"http://localhost:12345/TestPage.aspx";

object[] results = input.Unformat(@"http://{0}:{1}/{2}");

CollectionAssert.AreEquivalent(

new object[] { "localhost", "12345", "TestPage.aspx" },

results);

Of course something similar is already possible with Regular Expressions, but I find them much more unwieldy, they result in longer code, they’re overkill for what I’m trying to achieve, and they’re difficult to remember the syntax (oh, and even harder to get it right when coding late at night!). Take this equivalent example and judge for yourself;

string input = @"http://localhost:12345/TestPage.aspx";

string matchingformat = @"^http://(?<C1>.+):(?<C2>.+)/(?<C3>.+)$";

Regex expression = new Regex(matchingformat);

Match match = expression.Match(input);

Assert.AreEqual<object>("localhost", match.Groups["C1"].Value);

Assert.AreEqual<object>("12345", match.Groups["C2"].Value);

Assert.AreEqual<object>("TestPage.aspx", match.Groups["C3"].Value);

To get around this I have created my own String.Unformat function... all it does is take a simple format string and convert it into a regular expression by applying a number of transforms to it. And how do we do these transforms? With regular expressions, of course J

Now before we dive in here, I must make a point. This is not a replacement for regular expressions. It will be difficult to get the precision you need for anything complex – I’m only after a quick and simple solution to the basic case problem. Knowing this, read on...

How it does it

My sample code (and a bunch of demonstrations in the form of unit tests) is attached. Basically there are five phases to the code;

1. It escapes any special Regular Expression characters (e.g. ^, [, ], $ and so on) with a backslash so that they don’t accidentally affect the matching... they are assumed to be literal in the input string.

2. It replaces the {0} match syntax with regular expression (?<C1>.+) syntax.

3. It adds begin (^) and end ($) markers to the match string.

4. It performs the match.

5. It loops through the results, extracting matches and adding them to an object array to return.

Have a look in the attached and see what you think.

But...

The regular expressions I’m using are compiled where possible... but of course I’m applying multiple transforms (i.e. regular expressions) to a string just to do a simple match.

What does this mean? It means this will always be slower than if you just wrote a regular expression yourself (check out here and here if you want to)... so if performance really is that key to your code, or you’re doing this in a long loop, you might want to avoid it. If on the other hand you can see a great use for this when performance isn’t key – give it a go and let me know what you think!

I have also not had time to write a full suite of real unit tests, so if you find a bug shout up.

Conclusion

What do you think? Have I created a monster, or cooked up a treat?

After writing this I came across an interesting conversation here... it seems I have not been alone in my desires for String.Unformat.

foreach keyword or ForEach extension method?

simonince — Mon, 27 Apr 2009 18:21:00 GMT

Is this a question of taste, or something more serious? [Warning: this post won’t change your life J]

I found myself writing the following extension method yet again the other day;

public static void ForEach<T>(

this IEnumerable<T> source,

Action<T> action)

{

foreach (T item in source)

{

action(item);

}

This means I can write simple (contrived) code like the following;

entities.ForEach<ObjectDefinition>(o => o.DeleteDbRecord());

This will enumerate all my ObjectDefinition objects and call DeleteDbRecord on each. But why didn’t I just write it like this?

foreach (ObjectDefinition o in entities)

{

o.DeleteDbRecord();

}

I’ve raised this because some code I’ve seen (and written!) has reached a saturation point, where lambdas, extension methods, and other concepts have taken over - meaning the code looks more like it was written in LISP than C#.

But does that matter?

A quick search shows there are all sorts of discussions about this online, which is interesting. It is actually quite an old discussion now!

Types of Auditing

simonince — Thu, 16 Apr 2009 15:00:00 GMT

Auditing almost means something different to everyone I speak to... just a few things people mean when they say “our system has auditing” to me are;

Business Operation Auditing. This is recording when a business operation is performed, such as “Hire Employee”. They usually signify that a whole lot of work has been done across the system’s databases and services.

Data Operation Auditing. This refers to tracking individual changes to database tables. For example, this might involve recording that a user performed an operation that caused the Employee table’s row with identifier “5” to have the “Hired” flag set to “true”.

System Auditing. This is usually what I would call tracing or instrumentation rather than auditing – i.e. outputting detailed information about the running of the system. This often uses the TraceSource class, performance counters, or might be custom statistics recorded in a database/file about long running tasks – it can take many forms.

I’m sure this list is by no means complete – it isn’t intended to be. It’s just designed to make you think “what do they mean by auditing?” when you’re reading the requirements for the shiny new system you’re about to write. What is it you need to achieve?

I’m also hopeful that when I visit you one day in the future and say “what do you mean by auditing?”... you’ll realise I’m just trying to make sure we’re talking about the same thing J

My next couple of posts will cover one approach to Data Operation Auditing in the Entity Framework, so stay tuned...

* You might think there’s a rather large gap in the above... what about Security Auditing? Well, I’ve left it out deliberately as I see that as more of a vertical slice compared to the horizontal list that I’ve come up with. In other words, it’s just a slightly different way to different cut it – I’d categorise Security Auditing as either System or Business Operation auditing, depending upon your requirements, the technology you’re using for authentication and authorisation, and how you choose to implement it.

Procedural versus Set-Based SQL

simonince — Mon, 30 Mar 2009 12:22:00 GMT

During my day job I come across customers that are struggling to make a system perform to the expectations of their users, and they often see this as a SQL Server problem. This post is designed to demonstrate how much of a difference your approach to SQL tasks can make to the performance and scalability of your system. Of course, the real trick is to make sure you performance test thoroughly; a system can behave totally differently in a live environment to how it was seen to behave during development and system testing.

The Scenario

For this post I’ve tried to illustrate a common scenario with the attached SQL script. Very simply, we have Customers, Orders, OrderItems, and Products tables following a very familiar relationship pattern.

I’ve then invented a set of Stored Procedures to retrieve a list of “Big Orders”. The idea is that a “Big Order” is defined according to some rules, based on the date, size of the order, a substring in the order description, and so on.

There are three types of Stored Procedure, and they are all based on approaches that I’ve regularly seen in real implementations, and therefore hopefully illustrate my points well;

1. GetBigOrdersReselecting. This procedure also uses a function named IsBigOrder. Basically it retrieves a list of Order Ids to work with, and then loops through them calling IsBigOrder with each. IsBigOrder then selects the row it needs to work with using that ID, and determines if the order is “big”. If it is, GetBigOrdersReselecting adds this ID to the result set. This means the row is “reselected” multiple times – a very common approach.

2. GetBigOrdersCursor. This procedure opens a cursor and steps through all the orders determining if the order is “big”. If it is, it adds the ID to the result set using a temporary table. This approach is used in many, many systems.

3. GetBigOrdersSetBased. This procedure uses a single SELECT statement to fetch all the orders that are considered “big”... and is my preferred approach and the reason for this post.

The Tests

What I wanted to show was how each of these approaches behaves differently as the volume of data increases. Therefore I have used a Visual Studio 2008 Data Generation Plan to fill my tables with test data (with ratios Customer:Order:OrderItem of 1:5:15). I have then run each of the Stored Procedures and captured some statistics to look at.

I did this for two test data sets – those that included less than 15% of “big” orders (“Low Hit Ratio”), and those that included about 45% of “big” orders (“High Hit Ratio”). This is intended to reflect two different types of SQL – those that retrieve and process a large percentage of the data as a batch job might, and those that need a smaller percentage of the rows, like an OLTP system might.

It is at this point you must realise that this post is indicative only. It is actually pretty unscientific due to my time constraints. I’m running SQL 2008 on my laptop, with various programs open (e.g. Outlook), using randomly generated data that is different each time, without any sensible indexing strategy, without recompiling stored procs between runs, and so on. In other words, I believe the figures show a valid pattern, but don’t go on the individual millisecond results. Just take the core message on board.

The Results

Let’s get to the point then, and look at how long each of the three approaches took to run (in seconds) for increasing volumes of records in the Order table.

	50	250	500	1,000	2,500	5,000	10,000	25,000	50,000	100,000
Set based High Hit %	0.000	0.000	0.017	0.017	0.046	0.047	0.126	0.237	0.453	0.686
Cursor High Hit %	0.000	0.013	0.030	0.046	0.140	0.313	0.437	1.007	2.074	3.687
Reselect High Hit %	0.017	0.127	0.406	1.507	8.727	5.193	20.073
Set based Low Hit %	0.000	0.000	0.013	0.017	0.033	0.064	0.046	0.186	0.343	0.513
Cursor Low Hit %	0.000	0.013	0.017	0.030	0.077	0.173	0.280	0.734	1.413	2.530
Reselect Low Hit %	0.017	0.127	0.406	1.490	8.917	5.157	19.747

Note I didn’t bother running the 25,000+ runs for the “reselect” approach. There is also an interesting anomaly in there (I wish I had time to investigate!), in that “reselect” seems to temporarily improve around the 5,000 record mark. That could be for all sorts of reasons but at least you know I’m not faking my results J

Anyway, I think two graphs illustrate the results best; the first includes the “reselect” procedure results, the second excludes it so that we can see more detail of the other two.

Including Reselect

Excluding Reselect

You can clearly see a few things from this;

1. The Set-based approach is faster than any other approach for anything but the smallest amounts of data.

2. The Cursor approach gets a lot slower quickly (fairly proportionally, at least).

3. The Reselect approach is unpredictable, and much, much slower than either of the other two approaches. It almost spirals out of control quickly - with very few records it is probably in the unacceptable performance category for an OLTP system.

It is important to understand that as these queries run slower and slower so they will also have a disproportionately bad effect on any multi-user system. There is more chance that multiple connections will be executing queries against the same data at the same time, more memory will probably be in use, the processor will be busier, and your disks might well be more active. This increases the chances of deadlocks and more. Therefore the wrong approach to SQL statements not only compromises the performance of your queries, but also the scalability of your concurrent queries, and ultimately the stability of your system.

Remember that my example just uses SELECTs, but we could easily be looking at UPDATE statements too.

Implementation

So how is this set-based approach implemented for complex logic you have traditionally used procedural code for? Quite easily actually, after a bit of practice - complex rules can often be expressed using a combination of AND/OR, CASE, and more. Have a look at the HAVING clause of my set-based query;

SELECT

... SQL removed...

HAVING

(

o.OrderDate < GETDATE() AND

(

SUM(oi.NumberOfItems * p.Price) > 100 AND

CASE SUBSTRING(o.OrderDescription, 0, 4)

WHEN '011' THEN 1

WHEN '012' THEN 1

ELSE 0

END = 1

) OR

(

SUM(oi.NumberOfItems * p.Price) <= 100 AND

SUM(oi.NumberOfItems) > 3

)

) OR

(

o.OrderDate >= GETDATE() AND

SUM(oi.NumberOfItems * p.Price) > 150

)

This is by no means perfect. I could replace SUBSTRING with LIKE, I haven’t tuned it by looking at the query plan at all, and I should consider adding some indices; I might even choose to cache some data using an indexed view, and more. But without any of this work it still runs much faster than the other options, and I think you can see from my HAVING clause how easy it is to express the logic.

I’m trying to keep this post as brief as possible, so I won’t go into approaches to writing this set-based logic – but one key point is to ensure you use mutually exclusive OR clauses (so you might duplicate some lines – such as my use of o.OrderDate), and don’t forget to test, test, test. In fact, if you’re replacing procedural code you might consider automating running both queries against the same data and comparing the results.

Summary

This is one of the easiest summaries I’ve ever written...

Whenever you’re writing SQL always start out with writing set-based statements. It may look harder at first, but it gets easier once you’ve done it a few times, and I firmly believe the effort up front pays dividends when you come to scaling your application. Set-based SQL is also far easier to tune than procedural logic using the usual SQL tricks of changing indexes, partitioning, and so on.

There are always exceptions to rules of thumb, and I’m sure you’ll have a scenario where something can’t or shouldn’t be done in a set-based way... but just be absolutely sure that’s the case. Usually it comes down to data that must be dealt with in an ordered manner, perhaps with a running cumulative total or similar.

If you try reworking some SQL into a set-based approach I’d love to hear what you find; whether it is huge performance improvements, or otherwise – feel free to comment here.

Are you using the WCSF?

simonince — Fri, 05 Dec 2008 17:10:00 GMT

Have you been using the Web Client Software Factory? Or have you tried it and not adopted it? Or have you used some of the bundle content but not the whole factory?

Michael Puleio has just started a thread on codeplex requesting feedback on who is using the factory, and talks about it briefly in this post.

Combine that with Blaine’s WCSF Roadmap post and you can see p&p are already thinking about the next version of guidance for ASP.NET applications. Now is the chance to comment on what really matters to you, and what would help you in your day-to-day job.

AJAX Script Patterns: Service Agent

simonince — Fri, 05 Dec 2008 00:45:00 GMT

I’ve been thinking for a while about how people tend to build AJAX applications, as there seems to be something a lot of people have forgotten. This post examines how applying a pattern you probably know very well to AJAX could help – and leaves it to you to decide whether this is useful. Let me know what you think.

Setting the Scene

In your average application architecture, you’ll generally find a “Resource Access” layer that contains Service Agents (also known as Service Gateways) for accessing remote web services. What are these for? Well, I think the MSDN documentation on the pattern sums it up nicely in the Problem statement;

“How do you decouple the details of fulfilling the contract responsibilities defined by the [remote web] service from the rest of your application?”

The answer is of course that you create a class to manage the interaction with your service and hide the inner workings and contract implementations from the rest of your code. This class and any supporting entities make up your Service Agent.

AJAX: Why so Different?

With the growing popularity of AJAX, and the great features available in Microsoft AJAX, more and more web sites use script to call services hosted back on the ASP.NET server to load data, and dynamically update the page with DOM operations.

So how come so many sites have forgotten what we learnt years ago? This script calls a web service – and this web service forms a part of a larger system. It may change – the contracts, the implementation, the name, the location... Consider that these web services may also be shared between AJAX, Silverlight, ASP.NET, WPF, and/or other technologies as clients. So each of these could require tweaks to your shared web service (even if they’re using a different endpoint that uses a different binding, chances are the contracts and implementation are shared).

Microsoft AJAX helps us by auto-generating the web service proxy and related entities for calling a script-enabled web service. But if you change the web service, this changes the generated proxy, and you could find many pages in your web site suddenly need JavaScript changes to work correctly again. We don’t get the rich compile-time support in JavaScript that we get with .NET languages, so you might not find these problems until testing, or worse, live.

Therefore I see two options for handling this;

1. You could create a reusable ASP.NET AJAX server control that wraps up the web service calls and UI manipulation. This means you’ve just got one place to make changes, so is a great solution.

2. If the service is used differently in each place, perhaps it just doesn’t suit being a control. In which case, I propose creating a script-based Service Agent.

A Simple Example

To demonstrate this, I’ve created the bare minimum of a simple WCF service to be called from AJAX;

[ServiceContract(

Namespace =

"http://samples.microsoft.com/jslayering")]

[AspNetCompatibilityRequirements(

RequirementsMode =

AspNetCompatibilityRequirementsMode.Allowed)]

public class PersonService

{

[OperationContract]

public GetPersonResponse GetPerson(

GetPersonRequest request)

{

Implementation

}

[MessageContract]

public class GetPersonRequest

{

[MessageBodyMember]

public int Identifier { get; set; }

}

[MessageContract]

public class GetPersonResponse

{

[MessageBodyMember]

public bool Found { get; set; }

[MessageBodyMember]

public Person Person { get; set; }

}

[DataContract]

public class Person

{

[DataMember]

public int Identifier { get; set; }

[DataMember]

public string Name { get; set; }

[DataMember]

public int Age { get; set; }

}

You can see I’ve also chosen to use Message Contracts... we’ll see why in a moment J

All that happens in this service is the client requests a Person’s details, and they either get Found=true and a Person entity back, or Found=false. In my implementation this is hard-coded and randomised, so excuse the grim code.

Creating a Service Agent

The first thing we need is a JavaScript entity to hold person details in. This is because the Person class above is part of the contract of the web service – so if we change it, the JavaScript entity generated by Microsoft AJAX will change too... so we need to protect the rest of our JavaScript code from this. Let’s see my simple entity;

/// <reference name="MicrosoftAjax.js"/>

/// <reference path="~/PersonService.svc" />

Type.registerNamespace("ServiceAgents");

ServiceAgents.Person = function(id, name, age) {

this._id = id;

this._name = name;

this._age = age;

}

ServiceAgents.Person.prototype = {

get_id: function() {

return this._id;

get_name: function() {

return this._name;

get_age: function() {

return this._age;

}

ServiceAgents.Person.registerClass(

'ServiceAgents.Person');

if (typeof (Sys) !== 'undefined')

Sys.Application.notifyScriptLoaded();

* Note the use of the <reference> tag so I get intellisense in Visual Studio 2008!

This is the simplest of objects, and just stored the three fields found on a Person entity. Next, we need a Service Agent. I’ve chosen to create this as a component (inherited from Sys.Component) mainly because I might want to add event handlers and more to it another time, so having it as a managed disposable object is potentially useful.

/// <reference name="MicrosoftAjax.js"/>

/// <reference path="~/PersonService.svc" />

/// <reference path="Person.js" />

Type.registerNamespace("ServiceAgents");

ServiceAgents.PersonServiceAgent = function() {

ServiceAgents.PersonServiceAgent.

initializeBase(this);

}

ServiceAgents.PersonServiceAgent.prototype = {

initialize: function() {

ServiceAgents.PersonServiceAgent.

callBaseMethod(this, 'initialize');

dispose: function() {

ServiceAgents.PersonServiceAgent.

callBaseMethod(this, 'dispose');

GetPerson: function(identifier, callback) {

var success = Function.createDelegate(

this, this._onSuccess);

var failure = Function.createDelegate(

this, this._onFailure);

// call the web service

samples.microsoft.com.jslayering.PersonService.

GetPerson(identifier, success, failure, callback);

_onSuccess: function(result, context) {

// check we got a result back

if (!result.Found)

this._onFailure({ get_message: function()

{ return 'Person not found'; } },

context);

else {

// translate to our Person entity

var returnentity = new ServiceAgents.Person(

result.Person.Identifier,

result.Person.Name,

result.Person.Age);

// call the callback that was passed in

context(returnentity);

}

_onFailure: function(result, context) {

alert('A call to the Person Service failed: '

+ result.get_message());

}

ServiceAgents.PersonServiceAgent.registerClass(

'ServiceAgents.PersonServiceAgent', Sys.Component);

// ensure an instance of this component is created

// when the page is initialising

if (typeof (Sys) !== 'undefined') {

Sys.Application.add_init(

function() {

$create(

ServiceAgents.PersonServiceAgent,

{ id: 'PersonSA' },

null, null, null);

});

Sys.Application.notifyScriptLoaded();

}

There are four things to note in this component;

1. The only public method we’ve added is GetPerson. This literally sets up some call-backs and passes the call on to the auto-generated script proxy.

2. When the call succeeds, we check a result was found (using the Found property on the Message Contract) and if it was, we create a new instance of our own Person entity to pass back, by calling the supplied call-back method.

3. If there’s a failure we handle it; I could have let the caller do this, but this component is for illustrative purposes only... it is by no means perfect and I wanted to keep it quite brief!

4. We ensure an instance of our component is created when the page initialises, so that page script can easily consume it.

Using the Service Agent

Making use of this component is easy – we need to import the relevant services and scripts, and then get a reference to it using the $find method, and call it! First I register the scripts with a ScriptManager. We still need to point out on every page that we want to import the service, and now have to specify our Service Agent scripts too;

<asp:ScriptManager

ID="PageScriptManager"

runat="server">

<asp:ServiceReference

Path="~/PersonService.svc" />

</Services>

<asp:ScriptReference

Path="~/Scripts/Person.js" />

<asp:ScriptReference

Path="~/Scripts/PersonServiceAgent.js" />

</Scripts>

</asp:ScriptManager>

Then I’ve got the following script on my page;

function getPerson() {

var id = $get('RequestIdentifierTextBox').value;

var serviceAgent = $find('PersonSA');

serviceAgent.GetPerson(id, displayPerson);

// prevent post-back as we've handled it client side

return false;

}

function displayPerson(result) {

$get('IdentifierLabel').innerHTML = result.get_id();

$get('NameTextBox').value = result.get_name();

$get('AgeTextBox').value = result.get_age();

}

... which drives a page that looks like this;

Breaking the Application

Well this is not worthwhile unless we can demonstrate a tangible benefit. Imagine that code similar to that above is scattered throughout our web site, and we have many pages to maintain. Now imagine that our web service needs to change. Maybe the XML namespace is updated... or maybe we decide to rip out all of our Message Contracts (a bit contrived, but it demonstrates what we’re talking about!). My service becomes;

[ServiceContract(

Namespace =

"http://samples.microsoft.com/jslayering")]

[AspNetCompatibilityRequirements(

RequirementsMode =

AspNetCompatibilityRequirementsMode.Allowed)]

public class PersonService

{

[OperationContract]

public Person GetPerson(int identifier)

{

Implementation

}

Immediately our auto-generated proxy changes and the JavaScript code breaks. There’s no Found property anymore, we just get a null Person back if there is no match. And we can’t access a Person property on the Message Contract because it isn’t there... we’ve got a Person instead.

Obviously applying these changes site-wide would be a real headache. But for us, it’s easy. We just change a few lines! We’ll just tweak our _onSuccess call-back in the Service Agent;

_onSuccess: function(result, context) {

// note we only check for a result, not result.Found

if (!result)

this._onFailure({ get_message: function()

{ return 'Person not found'; } },

context);

else {

// note the changes when building our return entity –

// no ".Person." message contract syntax

var returnentity =

new ServiceAgents.Person(

result.Identifier,

result.Name,

result.Age);

context(returnentity);

}

Hey presto! Our site works again, and you can leave the office at a sensible time!

Conclusion

Obviously this is extra coding effort to put together a Service Agent, but do you see the benefits? Is it worthwhile? What about other patterns? How about View-Presenter, for example? Do they apply to JavaScript? Do you use this kind of approach already?

I’d love to know what you think, and what you do right now... feel free to comment or blog back at me.

The attached code should demonstrate all this – there are “-before” and “-after” web sites, both containing implementations of this page with a Service Agent and using direct calls against the auto-generated proxy.

In the “after” site I’ve removed the Message Contracts and tweaked the Service Agent... but I haven’t fixed the direct proxy calls so you can see how it breaks.

Enjoy...

Sufficient Architecture

simonince — Thu, 16 Oct 2008 14:16:00 GMT

“Sufficient Architecture? What does that mean?!”, I hear you say!! Bear with me.

Over a beer this week I had a good conversation with a colleague about the principles of architecture and design, and as a result I feel compelled to recount something that I have tried to adhere to for a long time. I know some people will disagree with what I’m saying, so if that’s you, shout up and let me know your thoughts.

When designing a new system I always have two principles in mind (and I’ll probably use the term “design” to refer to both Architecture and Design throughout this post, as it applies to both);

1. Design and Build only what you need today.

2. Design knowing that you’ll need something else tomorrow.

Let me elaborate a little.

Design and Build only what you need today

What I mean by this is that you should not try and anticipate the future. You shouldn’t add methods to classes “because in the future I might need to X”, and you shouldn’t complicate your architecture if there’s no need.

A good example that I discussed with a colleague is adding a Service Layer to an application; perhaps putting all business logic behind web services, and calling those from ASP.NET. I firmly believe you should add a Service Layer if you have good reasons for having one. If you need to scale out components separately, or perform asynchronous processing, or there are hosting prerequisites, or you have other applications that need to communicate with your application, etc – go ahead and add one.

However, if at this point you don’t have any good reason – don’t add one!! If you think an application might need to communicate with yours in the future, remember it is only that – “might”. You cannot anticipate accurately what it might need – you’d get contracts and message exchange patterns wrong, you’d be increasing the attackable surface area of your application, and you’d be complicating your development, maintenance, and deployment.

You should always build to meet the requirements.

What I don’t mean is that you should make your application rubbish – for example, having an application with a good User Experience should always be a requirement. But having one that enables the user to change the layout of your pages – that might or might not be a requirement. If it isn’t a requirement, don’t spend extra time designing and building it.

If you’re convinced that something should be done but it isn’t a requirement, you need to revisit your requirements (a great example of this is instrumentation – so often missed from non-functional requirements!). If you don’t revisit the requirements, you (the techie) are making up your own requirements, and that becomes very dangerous territory.

Design knowing that you’ll need something else tomorrow

The problem with only building what you need today is that we all know systems change and evolve, need maintaining, upgrading, or fixing. Building only what you need today compliments this though – as I’m sure regular readers know I firmly believe simplicity is something you should strive for in all your systems. It keeps learning curves shallow, and therefore helps keep maintenance and enhancement costs down. Building only what you need helps keep your application simple.

But we must also acknowledge that we know your system is probably going to change. This means you need to consider good architectural and design practices, use well known successful patterns (for example a UI pattern like MVP or MVC), write automated tests to validate your code (both now and when you’re enhancing it later, as they then become great regression tests), use clear naming conventions, ensure you can monitor your system, appropriately componentise the code, and so on.

When you design the API for your application, ask yourself something – “if in 5 years time someone comes to change this, will they understand what my code is doing?”.

Designing your application well means that if we need to add a Service Layer in the future (or perhaps just expose a small part of your logic for another application) the task of estimating, designing, and building these changes is not as complex as it could have been, and it is entirely appropriate that the effort is expended on doing this once you know you need to.

The argument against

What I am also not saying is “build applications as quickly as possible”, or “just databind a data grid directly to your database”. Please don’t just build a bad application, say “it meets requirements”, and blame me! J The term Rapid Application Development scares the life out of me – and I’m certainly not aiming at that! This is why the second principle exists. Your non-functional requirements should describe the need for the system to be maintainable, scalable, supportable, etc – and we as developers and architects know this means following good practices.

So what do you think?

The small print: Do remember that the examples in this post are primarily targeted at a single system, living in a fairly isolated world. In particular, my comments about Service Layers could be misinterpreted if you’re building an application that is part of an Enterprise’s Service Oriented Architecture – but of course you should have requirements that dictate how it should participate in the SOA, and what systems must integrate with your new application, so the principles hold true.

Lamba Expressions to the Rescue!

simonince — Mon, 08 Sep 2008 19:30:00 GMT

Maybe I’m biased because he’s on the same team as me (ahem, sorry for the plug J), but I really liked Stuart Leeks’ post on improving the “Include” method on ObjectQuery<T>.

It’s another example, like the MVC Framework he mentions, of how lamba expressions can be used in the real world to add type safety and easier programming constructs.

Make sure you check out his post here.

p&p Sharepoint Guidance

simonince — Thu, 28 Aug 2008 14:53:00 GMT

Via Blaine’s post (that gives quite a lot of information on what is included and what their plans are), the patterns & practices Sharepoint Guidance is starting to take shape, with a drop to codeplex having arrived this week. Be sure to check it out – this is an area that is becoming more and more popular, so it’s a good chance to get to know the guidance early on.

The drop includes a Reference Implementation so download it and compare it to how you’re working!

The “Service Interface” Pattern

simonince — Mon, 25 Aug 2008 20:06:00 GMT

I am constantly surprised when speaking with people how few have heard of or use the “Service Interface” pattern. It is actually a very straightforward pattern, is very little work to use, but brings such practical, quantifiable, visible, benefits I think it has to be one of my all time favourites. The Web Service Software Factory embodies a particularly good implementation of the pattern, too, so be sure to check that out. In fact, I’d go so far as to say that the easiest way to get familiar with this pattern is to read this post (obviously J) and then try the factory.

There’s already a bunch of documentation out there, including the pattern description on MSDN (although the corresponding implementation advice is a little dated due to the advent of things like WCF). The Web Service Software Factory documentation also describes the pattern, adding some more information and a sample to the MSDN docs.

The basics

What on earth is this pattern then? Well basically it is there to keep the “surface area” of a web service independent from the Business Logic. In exactly the same way as you would partition your User Interface from your Business Logic, it is wise to partition your Service Interface.

Separating these concerns means that your logic can evolve in implementation without breaking consumers of your service (remember that contracts between services are quite brittle in many cases), and also to evolve the communication mechanisms and formats your service uses without affecting your logic. Makes a lot of sense, huh?!

The Service Interface is also the ideal place to do things like message logging, exception shielding, versioning, and so on. All of these concepts are highly recommended for consideration before you deploy the first version of your service.

Pattern Components

There are a number of components that work together to form a well structured Service Interface. These components are depicted below;

I’m sure you’ve noticed that I don’t like to draw and talk too much without dropping down to see what this really means in code. So taking each component, one at a time, let’s see what they are, what they do, and what the code looks like. I’ll use WCF in this example, but the sample concepts can apply to other technologies.

Data Contracts

Data Contracts are the reusable entities used in service communications. They are similar to business entities, but are specifically used for over-the-wire transmission. Think of similarities to how Fowler’s description of Data Transfer Objects is implemented.

[DataContract(

Name="Employee",

Namespace="http://samples.microsoft.com/contracts/2008/08")]

public class Employee

{

[DataMember]

public string Name { get; set; }

[DataMember]

public int PayrollNumber { get; set; }

}

This shows a simple Data Contract. Note that this is an entity – all the data it contains is intrinsically related, in that it describes properties of an Employee. The DataContract and DataMember attributes are used to decorate the class for the DataContractSerializer’s benefit.

Do also pay attention to the fact that I’ve used the Namespace parameter of the DataContract attribute – this is an absolutely key step in ensuring your versioning strategy is up to scratch. I should really have also implemented IExtensibleDataContract, as per Craig’s post on Versioning WCF Services. I won’t cover this in detail but do make sure you read up on this before diving in.

Message Contracts

So Data Contracts define entities – but our service doesn’t deal in entities! It deals in messages. Therefore Message Contracts group together Data Contracts to form meaningful and useful messages.

Usually each operation (i.e. method on a service) has two Message Contracts – a Request and a Response. One Way operations obviously have no Response message, but you get the picture!

[MessageContract(

IsWrapped=true,

WrapperName="AddEmployeeRequest",

WrapperNamespace="http://samples.microsoft.com/contracts/2008/08")]

public class AddEmployeeRequest

{

[MessageBodyMember]

public int DepartmentIdentifier { get; set; }

[MessageBodyMember]

public Employee Employee { get; set; }

[MessageBodyMember]

public Employee Manager { get; set; }

}

[MessageContract(

IsWrapped=true,

WrapperName="AddEmployeeResponse",

WrapperNamespace="http://samples.microsoft.com/contracts/2008/08")]

public class AddEmployeeResponse

{

[MessageBodyMember]

public bool Success { get; set; }

[MessageBodyMember]

public string FailureMessage { get; set; }

}

Here we see I’ve introduced an “AddEmployee” operation that our service will provide. I always use the “Request” and “Response” naming suffix convention to ensure that the intended use of these messages is clear. Message Contracts are also not reused like Data Contracts are; they are inextricably bound to an operation, and exist purely to define that operation and to allow versioning strategies to exist.

Note that we use the MessageContract and MessageBodyMember attributes to decorate our contract, and that again I have used the Namespace parameter to help with versioning. I always use “IsWrapped=true” to ensure maximum compatibility (see Using Message Contracts).

Members of the Message Contract can be either Data Contracts or primitive types – the example shows the use of both our Employee contract and an Int32.

Service Contract

The Service Contract brings together our Message Contracts (and therefore implicitly our Data Contracts) to use them to define operations, and to group operations together to form a service.

[ServiceContract(

Namespace="http://samples.microsoft.com/contracts/2008/08")]

public interface IEmpoyeeManagement

{

[OperationContract(

Action="http://samples.microsoft.com/contracts/2008/08/ É

IEmployeeManagement/AddEmployee")]

AddEmployeeResponse AddEmployee(AddEmployeeRequest request);

}

Note that in this case we’re using a .NET interface, not a class. We’re also using the ServiceContract and OperationContract attributes to decorate the interface. The Namespace and Action parameters to these attributes are used for versioning as usual.

Here you can see that my Request and Response messages are tied together by the Operation Contract definition for AddEmployee.

Service Configuration

The Service Configuration consists of many different things. I would like to define it as things that can be added to a service to customise the behaviour without requiring code or contracts to be changed... but unfortunately that isn’t quite true. For example, changing the InstanceContextMode can have a big effect on how you write your service. I guess this means that some specific areas of configuration and implementation are actually quite closely coupled, for good reason.

Anyway, configuration of a service includes things like the web.config settings (under <system.serviceModel>), and the use of the ServiceBehavior attribute. My service is so basic all I have in configuration is the default WCF web.config settings, so I’ll not bore you with that here!

Service Implementation

Well this one is pretty obvious, right? This is the meat of my service – the point of all these contracts, right? It’s the logic?

Nope.

I don’t mean to be so ruthless with this statement, but I just think it is important to understand. A service implementation is still part of the Service Interface – and it is purely responsible for translating between wire-format entities and business entities, maybe applying custom authorisation, message logging, exception shielding, and so on. It is almost an “adapter” layer that maps incoming messages onto the correct business logic – I firmly believe a service implementation should be very thin as a result of this, delegating most work to the business layer. The service implementation is not the business logic itself.

Let’s see a simple example implementation then;

public class EmployeeManagementService : IEmpoyeeManagement

{

public AddEmployeeResponse AddEmployee(AddEmployeeRequest request)

{

try

{

ITranslator<Employee, Person> translator =

Container.Get<ITranslator<Employee, Person>>();

Logic.SavePerson(

translator.Translate(request.Employee),

translator.Translate(request.Manager),

request.DepartmentIdentifier);

AddEmployeeResponse response = new AddEmployeeResponse();

response.Success = true;

return response;

}

catch (BusinessLogicException exception)

{

AddEmployeeResponse response = new AddEmployeeResponse();

response.Success = false;

response.FailureMessage =

ErrorMapper.SanitiseErrorMessage(exception);

return response;

}

catch (Exception exception)

{

SystemFailure failure =

ErrorMapper.SanitiseFailure(exception);

throw new FaultException<SystemFailure>(failure);

}

We can see our service implementation is a class that implements our service contract, and there is very little else of note other than the method code itself. So let’s inspect the method code and see what we find.

Firstly we have some entity translation. It is really common for a service implementation to need to translate from a Data Contract to a Business Entity, and therefore a simple yet consistent approach to this has been born. Consider the following implementations;

public interface ITranslator<From, To>

{

To Translate(From from);

}

public class EmployeeToPersonTranslator :

ITranslator<Employee, Person>,

ITranslator<Person, Employee>

{

public Person Translate(Employee from) ..

public Employee Translate(Person from) ..

}

These translators can easily be registered with a Service Locator (or we could use Dependency Injection or Dependency Resolution) as in my example, and retrieved with a call to a container.

Next we see our service implementation calls into some business logic, passing in translated entities and any additional data from the request message that is needed. Finally it constructs a success message and passes it back.

The other interesting part of this code is what happens in event of an error. My implementation is primarily designed to demonstrate a few different approaches, but the important point is that no error is returned to the caller without being “sanitised” first – i.e. I am applying the concepts of Exception Shielding. This ensures private data about my service (for example connection strings) is never exposed to the caller.

I’ve shown a few different approaches – using a return value in the response message, or using a Fault Contract (just a Data Contract used to communicate a fault) by throwing a new FaultException<T>. Fault Contracts should really be used in WCF if there is an unexpected error, but I do think there are some valid circumstances to use a “roll your own” response message; perhaps this is a topic for another post.

One point to note is that using this Fault Contract requires me to decorate my service contract with a FaultContract attribute;

[ServiceContract(

Namespace="http://samples.microsoft.com/contracts/2008/08")]

public interface IEmpoyeeManagement

{

[FaultContract(typeof(SystemFailure)]

[OperationContract(

Action="http://samples.microsoft.com/contracts/2008/08/ É

IEmployeeManagement/AddEmployee")]

AddEmployeeResponse AddEmployee(AddEmployeeRequest request);

}

Summary

Well that’s it – I hope this has helped you see how Service Interface fits together and what some of the benefits are. I’ve found that this approach helps with versioning, but also it helps to encourage a message-exchange approach to design instead of RPC (Remote Procedure Call).

What I mean by this is that services become designed to have course-grained interfaces, avoiding chatty calls. RPC is often seen like Java’s RMI – just calling methods on an object that happens to be physically located somewhere else – and this leads to forgetting about the costs of crossing service boundaries. Defining Data Contracts helps maximise reuse, and defining Message Contracts helps decrease granularity.

So now you love Service Interface just as much as I do, have a read of Don’s post regarding Message Contracts, and see what you think. I’m a firm believer in always using Message Contracts as I’m sure is obvious from this post, but make up your own mind.

Strongly Typed Primitives

simonince — Fri, 15 Aug 2008 16:32:00 GMT

No, I’m not talking about NEANDERTHALS (there’s a joke in there somewhere), but rather a technique that a colleague of mine, Josh Twist, has recently blogged about – “Avoiding Primitive Obsession to tip developers into the pit of success”. I’d always seen a pit as symbolic of failure, so I’m pretty pleased there’s such a thing as a pit of success – improves my odds massively J

I’ve drawn attention to this because it is an approach I discovered a few years back, and I love it. I just use a slightly different implementation to Josh;

public class Command

{

#region Structure

private string _name;

private Command(string name)

{

_name = name;

}

public string Name

{

get

{

return _name;

}

#endregion

#region Instances

public static Command Profile = new Command("Profile");

public static Command WebExecute = new Command("WebExecute");

#endregion

}

You can see that I use a single class to encapsulate both the behaviour of the Command class and the static instances of a Command.

I can’t really think of a convincing reason why one of these would be better over the other to be honest – I guess Josh’s approach means you could have different collections of commands (WebCommands, WindowsCommands, SomeOtherCommands), but I’ve never needed to do that. Any other reasons you can think of?

Still – read Josh’s post and see what you think. This kind of pattern is so simple yet so effective at eliminating mistypes, increasing code readability, and improving refactoring (just try deleting a command type and watch that compiler pick up everywhere you need to make a change!).

Good stuff.

WCSF Application Architecture 7: Remote Logic with WCF Services

simonince — Fri, 18 Jul 2008 18:55:00 GMT

This article is part of a series;

· WCSF Application Architecture 1: Introduction

· WCSF Application Architecture 2: Application Controller

· WCSF Application Architecture 3: Model View Presenter

· WCSF Application Architecture 4: Environment Abstraction

· WCSF Application Architecture 5: Defining Modules

· WCSF Application Architecture 6: Structuring Modules

Unfortunately it is also the worst named post in the series J

Introduction

It is a common requirement for customers to want to split web applications across physically separate tiers, usually using some kind of services technology. This post discusses some approaches to this using the Web Client Software Factory.

Although I focus on using WCF services, many of the concepts apply to ASMX and other technologies too – I have just chosen WCF as I believe it to be the best way to write services in the current .NET Framework environment.

* Note: Chances are I’m going to use the term Web Services a lot in this post. What I really mean when I say that is any kind of remote service – pretty much anything that can be written with WCF, for example.

Food for Thought

So first things first – let’s clear up some common myths.

1: SOA != Services

Just because your application uses Web Services (or any other kind of remote service), that doesn’t make it Service Oriented. It could still be just an app that uses Web Services. The current Wikipedia definition of SOA starts with a nice succinct statement about this;

“(SOA) is ... where functionality is grouped around business processes and packaged as interoperable services ... SOA separates functions into ... services ... which are made accessible over a network in order that they can be combined and reused in the production of business applications”.

In other words, if you’re writing a Web Service and Web Application as an inextricably linked pair that make up a single system, chances are you’re not doing SOA. SOA is an Enterprise Architecture, not an Application Architecture (although some applications will be built on the Enterprise Architecture by aggregating and reusing business services, and hence some apps are validly referred to as Service Oriented).

What this means to me is two things

1. Be pragmatic about what rules you follow when you are writing a completely isolated internal application.

2. Ask yourself this: “Why are we using a Web Service?” If your answer is “Because our company policy is that all new apps must be Service Oriented” (or something similar) I would recommend stepping back, and thinking deeply for a short while. How is adding a Web Service making your application Service Oriented? Is it really adding any value? If so, great – there are many valid reasons... but make sure you know what they are!

2: Services cost performance

Using a remote service is expensive (in terms of processing time, for example). If you previously had a method call that looked like this;

View.Orders = myBusinessLogic.GetOrders(user, startDate, endDate);

... and you move the implementation of GetOrders to be a remote Web Service, you will notice a significant performance degradation. Raw performance and scalability are often at odds like this! Suddenly we must serialize user information and dates, locate the remote service and open a channel, pass the data across a potentially unreliable network, spin up a service instance, deserialize the information, perform the business logic call that used to exist on the client, serialize the results, pass them back across the network, deserialize the results and turn them into a .NET list, and finally close the channel to the remote service. Phew. Now all that might be pretty fast in .NET, but it will be slower than an in-process method call.

Having said that, if the logic you must execute is expensive itself, you may find that removing load from the client Web Server and scaling it out to another physical tier improves the scalability of your system. I’m not saying don’t use services here – I just want you to be realistic about it!

3: RPC != Messaging

Because services add overheads, it is important to adopt a message exchange approach rather than a remote procedure call approach. What does this mean? Basically it comes down to two things;

1. Granularity. RPC consists of chatty, frequent calls between client and service, very quickly making it difficult to scale your system. When you think RPC, think of many systems written with Remoting or Java’s RMI. Message exchanges are courser grained and minimise the amount of calls and proportion of overheads involved.

2. Programming Model. I tend to find that RPC encourages the sharing of concepts internal to a service with the outside world. Use a message oriented approach makes you think about defining the contracts, abstracting where necessary, and encapsulating behaviour appropriately. Watch for a future post on the Service Interface pattern when I get a chance!

Candidate Architectures

So now that we’ve thought a little about what we want to avoid, let’s see a couple of approaches and discuss their characteristics.

1: Baseline

First up is our baseline architecture; this is what future examples will evolve from, and represents a basic WCSF application (ignoring splits across modules) without any kind of service layer.

Note that I’ve decided to expose my business logic here using a WCSF application service (not a web service). This communicates with my “model” (basically the business logic), which in turns talks to various data access classes (e.g. DbAccess). I hope that’s pretty straightforward.

2: Split Tiers

I nearly named this “RPC approach” because I feel that it encourages an RPC style of programming. Components above the dotted line are in my WCSF Web Application, and those below it are in the WCF Web Service.

What we have done here is just move the logic into a Web Service, and then change the WCSF application service that previously exposed business logic to purely act as a facade to the remote Web Service – in other words, it is a Service Agent or Service Proxy. This is a key concept – a Web Service is a resource, so if you are communicating with a Web Service directly in a class, you’re writing a resource (or data) access component.

The problem with this approach, in my opinion, is that this encourages developers to just call methods as though they were local to the web application, potentially leading to an RPC style system with very chatty communications and a serious scalability problem. Often people don’t realise that they are effectively writing an application that has no business logic layer (the Web Application)!! This is obviously contrary to well understood wisdom nowadays, and looking at the diagram above, I hope you see how unnatural it looks.

This lack of separation can also lead to changes in the service contracts affecting the UI. This is by no means ideal.

3: Remote Services

My preferred approach is shown below;

Here you can see that our missing logic layer has made a triumphant return! This might seem subtle but I think it completely changes class responsibilities and how your developers perceive the system. The logic in the Web Application is responsible for deciding when to go to a web service, or when to use a local cache, how to co-ordinate calls between different resources (you might have multiple web services!), when to enforce security or other policy, and so on. This then leaves your Resource Access layer able to dedicate itself to providing a real shield from the inner workings of the service calls.

As I’m sure regular readers have noticed, I’m a big fan of talking in concrete examples, so let’s see some (simplified) code that demonstrates this approach. Firstly, the Presenter and implied View;

public class MyPresenter<IMyView>

{

private ILogicService _logicService;

public MyPresenter([ServiceDependency] ILogicService logicService)

{

_logicService = logicService;

}

public void OnRetrieveOrders()

{

List<OrderEntity> results =

_logicService.FetchOpenOrdersForCurrentUser(

View.StartDate, View.EndDate);

View.Orders = results;

}

The key here is that the presenter is using a WCSF service (badly named as ILogicService) to fetch orders for display to the user. These orders are represented by an “OrderEntity” entity. Next, let’s see an example of an ILogicService implementation;

public class LogicService : ILogicService

{

private IAuthenticationService _authenticationService;

private IEntityTranslatorService _entityTranslatorService;

public LogicService(

[ServiceDependency] IAuthenticationService

authenticationService,

[ServiceDependency] IEntityTranslatorService

entityTranslatorService)

{

_authenticationService = authenticationService;

_entityTranslatorService = entityTranslatorService;

}

public List<OrderEntity> FetchOpenOrdersForCurrentUser(

DateTime startDate,

DateTime endDate)

{

OrderServiceProxy proxy = null;

try

{

proxy = new OrderServiceProxy();

proxy.Open();

proxy.LogOnService(CERTIFICATE_DETAILS);

string orderStatusCode =

proxy.GetOrderStatusCode(OrderStatus.Open);

List<Order> orders = proxy.GetOrders(

_authenticationService.CurrentUserName,

orderStatusCode,

startDate,

endDate);

List<OrderEntity> results =

_entityTranslatorService.Translate<Order, OrderEntity>(orders);

proxy.Close();

return results;

}

catch (Exception)

{

if (proxy != null)

proxy.Abort();

throw;

}

There are a few really important points to make here;

1. I have chosen to abstract environmental issues, such as authentication, away into a framework service. See my previous post on Environment Abstraction for more information on this.

2. All the management of the interaction with the service proxy is kept out of the Presenter, and instead is handled by our WCSF application service.

3. I am performing Entity Translation between the “Order” and “OrderEntity” objects. The former is actually part of the definition of the Order Service’s Service Contract, in that it defines how the remote service is communicated with. The latter is a Business Entity defined by my WCSF module. Therefore it is important that I expose the Business Entity not the proxy communication entity further up my architecture.

4. The service performs some limited Orchestration – it calls a number of methods on the service to achieve what is seen by the Presenter as a single business operation. Obviously I would prefer a very course grained service design, but I’ve done this to illustrate that this is the right place to perform any required orchestration (potentially across multiple services) and workflow.

5. The disposal of the proxy is done correctly. A “using” block is not the best way to deal with WCF proxies – see Avoiding Problems with the Using Statement.

6. This is also the right place for rich error handling, publishing, and compensation – please don’t follow my poor example in this respect J

7. I haven’t used the rich implementation of the Service Interface pattern I would usually, because constructing the messages would increase the volume of sample code and detract from the point. I’ll probably post about Service Interfaces soon, so watch this space!

The final important point is that this separation of business logic, resource access, entity translation, etc is likely to be repeated by the Order Service.

Potential Gotchas

When using WCF proxies there are a number of things worth knowing about. The first is the best way to use a “using” block, mentioned above.

Secondly, remember that a WCSF service is a singleton – so if you cache the WCF proxy you will be using the same one for every call, no matter which end user is browsing your site. No probs... oh, hang on! But what if you’re using a session-full binding? That would mean all your WCF calls would be using the same session, no matter which end user the call was for! This could certainly be a problem – so be very careful about this.

Thirdly, if you are caching your WCF proxy remember that a faulted proxy cannot be used again. So if your backend service fails and then comes back up, and you’re caching the proxy forever and for every user, your web application will be unusable until you cause it to flush the cache (e.g. by restarting the App Pool). The correct way to handle a faulted proxy is by adding the following code when creating it;

proxy = new OrderServiceProxy();

((IOrderServiceChannel)proxy).Faulted +=

new EventHandler(OrderService_Faulted);

In the event handler you can then react in some way – perhaps by Aborting the proxy and replacing it with a fresh one. This is obviously pretty academic for my example code above, but if you are caching proxies, consider this carefully. Also, if you don’t have a business logic layer, you might find it more difficult to deal with this kind of situation!

Finally, I’ve used the class generated by “Add Service Reference” in Visual Studio. It is actually pretty slow to new up one of these for every business operation when compared to using a cached ChannelFactory. In .NET 3.5 this is less true, but I’d recommend this post comparing the two approaches.

Summary

There are many issues I’ve not covered here – security architecture, for example (Trusted Subsystem versus Delegation), and many, many, many, more. But I hope this has gotten you started on how to structure your code, freeing you up to investigate some of the more subtle complexities. Good luck!

And finally... a Summary of Summaries

This is the last post in my series on Web Client Software Factory Application Architecture. That doesn’t mean I’ll not post about WCSF again of course, just that we have now covered plenty, from the basic patterns involved to some more “fluffy” concepts of designing modules or service layers. I hate television series that have no end (wow, it is seriously difficult not to name and shame here, that’s how much I hate it!), so I’ll take my own advice and finish on a high J

Remember how much I haven’t covered – there is plenty of information out there, from the WCSF community site, landing page, other bloggers, obviously in the factory documentation, and so on. And if you’re blogging about it, do let me know!

WCSF Application Architecture 6: Structuring Modules

simonince — Fri, 04 Jul 2008 13:13:00 GMT

This article is part of a series;

· WCSF Application Architecture 1: Introduction

· WCSF Application Architecture 2: Application Controller

· WCSF Application Architecture 3: Model View Presenter

· WCSF Application Architecture 4: Environment Abstraction

· WCSF Application Architecture 5: Defining Modules

Introduction

This post is intended to address some common scenarios when using the Web Client Software Factory, in particular relating to how modules should be structured internally, and how they should relate to each other... as with all my other posts, this is based on what I’ve done and what I’ve seen others do – I’m sure there are other ways out there, so please feel free to chip in if you have something to say, suggest, or disagree with.

I’m going to present architectural candidates for you to consider, and discuss why you might choose to use them, which I hope should give you ideas about how they might apply to your situation.

So, get comfortable, and read on!

For consistency with my previous post in this series, I will assume that initially we have some business modules (Customers, Accounts, and Orders) and a foundation module (Infrastructure). I’ll pick and choose from these to illustrate my points as we go.

Model 1: Classic

I’ve named this model “Classic” as it is pretty much the basic use of the WCSF as it was intended;

Note: An arrow indicates a reference – so Customers has a project reference to Customers.Interfaces. The coloured bars indicate responsibilities for each project.

This diagram shows us a number of things;

1. The WCSF provides an out-of-the-box option to define an “interfaces” project for each module, in which the public “contract” of the module is defined. An “Interfaces” project contains contracts (i.e. for services a module exposes) and entities.

2. The Web Site contains the Web Pages and User Controls for a module.

3. The implementation projects (what I call the module projects that are not suffixed with “.interfaces”) contain the full remaining layers of the architectural stack, from UI (presenters etc) through business logic, to resource access.

4. The Web Site may reference both interface and implementation projects.

5. The implementation projects may reference their own or other modules’ “interfaces” projects. This allows modules to take dependencies on services exposed by other modules in a loosely coupled way.

6. As shown by the X marks, modules should not reference each other in any other way. Doing so would cause their implementations to be tightly coupled, and can cause circular references – which would be really bad news.

I’m a big believer in showing some code to demonstrate how this works, so let’s take a simple example and consider the following;

namespace Customers.Interfaces

{

public class Customer

{

public int Id { get; set; }

public string Name { get; set; }

}

public interface ICustomerService

{

List<Customer> GetCustomers();

}

namespace Customers

{

public class CustomerService : ICustomerService

{

public List<Customer> GetCustomers()

{

// implementation

}

Here we see the Customer entity and the ICustomerService contract are defined in the Customers.Interfaces project, and a real implementation of CustomerService in the implementation project. Obviously there would be IView definitions and Presenters in the implementation module too, and ASPX pages etc in the web site, but these are not relevant to this example.

Model 2: Shared Entities

Model 1 is in many ways the ideal – but the problem is life is often more complicated than this! The first thing you’re likely to encounter is when two modules need to share the same entity. So consider what happens if I define the following (slightly contrived) contract in my Orders.Interfaces project;

namespace Orders.Interfaces

{

public class OrderDetails

{

public string OrderReference { get; set; }

}

public interface IOrderService

{

void PlaceOrder(Customer customer, OrderDetails order);

}

Looks great huh? Ah, but Customer is defined in Customers.Interfaces... I could add a reference from Orders.Interfaces to Customers.Interfaces, but how long would it be before I needed to do the same in reverse and had a circular reference?

This leads me to a piece of advice;

Avoid having references between “.interfaces” projects

Also, remember that in my previous article we discovered that it is good practice to ensure that one and only one module owns access to any particular data source. So we shouldn’t create two different customers entities. So what do we do?

This diagram has way too many arrows on it to indicate acceptable references, but I wanted to provide a complete view, so I apologise! Basically, then, this boils down to creating a separate shared project in which you put common entities and contracts. Entities that only relate to a single module can stay in that module’s interfaces project.

So in our example, this becomes as follows;

namespace Shared

{

public class Customer

{

public int Id { get; set; }

public string Name { get; set; }

}

namespace Customers.Interfaces

{

public interface ICustomerService

{

List<Customer> GetCustomers();

}

namespace Customers

{

public class CustomerService : ICustomerService

{

public List<Customer> GetCustomers()

{

// implementation

}

namespace Orders.Interfaces

{

public class OrderDetails

{

public string OrderReference { get; set; }

}

public interface IOrderService

{

void PlaceOrder(Customer customer, OrderDetails order);

}

You see that the Customer entity has moved out, and now we have no circular reference concerns.

Model 3: Layered Modules

I have historically always separated my application layers by project – I find this helps keep the layering clear, and keeps the volume of code in any one project to a manageable level. The WCSF does not do this by default – but it doesn’t prevent you from doing it! In fact, some of the reference implementations split modules in a similar way;

This diagram shows that I have added two new standard C# projects to the solution, and named them Customers.Data and Customers.Logic (note I have omitted the Orders functionality here just to keep the diagram simple). My layers are now kept nice and separate. Each layer may reference the layer below, but not vice-versa.

Now there is an interesting point to make from this diagram – my Logic and Data projects have no reference to the Customers.Interfaces project. This means they cannot deal with the “Customer” entity!

I actually think this is a matter of choice – it depends on how much you wish to ensure your layers are loosely coupled. Using the approach I have taken, you would need to use an adapter or entity translation pattern in the Customers implementation project. Here, the UI facing classes (such as presenters or services) would translate between the outward facing entities defined in Customers.Interfaces, and those used internally by the module (perhaps database-focused entities). The services in the Customers module would simply act as a facade to real logic, contained in the Logic project.

The truth is, it can be a bit of a pain taking this approach, so you may wish to compromise – but the point is, it needs a few moments of thought. Choose an approach that you feel comfortable with and stick to it throughout your solution.

Model 4: Split Modules

An alternative approach to bringing extra clarity to your layering is to physically split up your modules;

Note: I have excluded the Web Site from this diagram, as I’m sure you’re getting the point!

Here you can see that we have Customers and Orders business modules, without matching interfaces projects. These only contain UI code, allowing you to keep the projects really focused. We then have two foundation modules – CustomersLogic and OrdersLogic (not very well named, I admit!). These modules expose services to the UI modules to allow them to access the “model” (as Model-View-Presenter) that is our business logic. The reference between the UI code and the business logic is loosely coupled as it goes via the foundation modules’ interfaces projects. If you really wanted, you could take this one step further and split the foundation modules into more projects as we discussed in the Layered Modules approach.

This is indeed very similar to the Layered Modules approach, except that it gives you an opportunity to deploy business logic separately to your user interface. This can have big benefits for team working (e.g. different teams working on the UI and business logic), or for deployment – perhaps your UI works for a number of systems, with different business logic plugged in behind?

Model 5: Split Master Logic Module

All of these approaches so far have one thing in common; they expect you to be able to vertically partition your code to some greater or lesser extent. In reality, this is the hardest bit of getting your WCSF implementation right – what parts of the system are really independent from each-other?

In particular, if you are migrating an existing system you may well find all of your business logic and data access is so tightly related, interlinked, and interdependent, that you cannot split it into modules without all sorts of circular references, shared entities, and so on. You could try, and end up with spaghetti dependencies between modules – even if it is loosely coupled, too many dependencies on each-other’s services is not a good sign. Or maybe you just happen to be building a small system that only really has one single area of functionality?

So all this means you could well end up with one single module, immediately losing some of the benefits to team working, componentisation, etc, that come with modularisation. Right?

Well, this is where the Split Master Logic Module approach comes in. Split your UI into modules – because that is often pretty easy - and then keep all your back end logic in a single foundation module that exposes services to the rest of your application;

I must make it clear that with this model you are missing out on the benefits of the complete vertical slice through your architecture you would get with a purist approach, but it does still have a place I believe.

This allows you to work in a loosely coupled way, modularising your UI (including independent deployment of modules if you wish), yet maximising the reuse of any existing business logic you have – it could be as simple as surfacing existing code as WCSF application services. You of course still have all the flexibility of a loosely coupled architecture too, so should you wish to migrate your existing code to a new, more modular architecture, just plug-and-play different service implementations as time goes by.

Conclusion

Well, that’s it – a lightning fast tour of some approaches to splitting your modules up. Have I missed an approach? Have you come up with something better? Do you hate one of the approaches above? As always, shout up and let me know.

The one approach I haven’t discussed is using a Service Layer to physically split the UI from a logic tier. Watch this space and I’ll try to post some thoughts on this soon.

Finally, I threw a few comments into the text above that might have got you thinking... if so, check out;

· The Adapter pattern

· Entity Translation – best explained by Don Smith, in context of the Web Service Software Factory but relevant to us too!

· My thoughts on LINQ solution structures (aimed at a WPF app, but some relevance to the web)

Enjoy!