You may have heard by now that the report processing engine of Reporting Services 2008 works significantly different underneath the covers, compared to previous releases. While I plan to dive into more technical details in future postings, I wanted to provide a few examples upfront explaining why you as a report author or end user might care:
So what is the main architectural difference with on-demand report processing?
Unlike the RS 2005 processing engine that processed the report as a snapshot evaluating all expressions exactly once upfront (including textbox values, style expressions, etc.), the RS 2008 on-demand processing engine pre-computes and persists only certain invariants, such as grouping, sorting, filtering, aggregates, and report/group variables. All other expressions (style expressions, textbox values) are evaluated on-demand based on the invariants, at the time a rendering extension requests a specific page containing those values. Hence, you could also call it rendering-driven on-demand processing engine.
You probably now think, thanks that’s nice; and where is the catch? J
Well, most expressions in a report, particularly textbox values and style expressions, are evaluated on-demand in Reporting Services 2008 as you navigate to a particular page. If you go there multiple times, it might be re-evaluated again. Try this to see the effect of on-demand processing:
If you do not want this on-demand behavior in particular cases (e.g. time-dependent calculations, custom code, or expensive calculations requiring caching), you should use so-called Report Variables or Group Variables, and then reference the variable value in the report (e.g. =Variables!V1.Value). Variables are covered in more detail in a previous posting. If it is just about consistent time stamps, you should instead use =Globals!ExecutionTime, regardless of the version of Reporting Services (more info on global built-in collections is available on MSDN).
Among other topics, future postings in the on-demand processing series will also provide report design tips to better leverage the new processing engine in RS 2008.
PingBack from http://blogs.msdn.com/robertbruckner/archive/2008/07/20/Using-group-variables-in-reporting-services-2008-for-custom-aggregation.aspx
Note: this posting provides an overview of when to consider using Report Variables and/or Group Variables
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