In Host Integration Server 2000, we added the ability to load balance LU 6.2 (APPC) sessions across connections defined under a SNA Server service.

This feature is automatically enabled when you create duplicate Remote APPC LUs across multiple connections defined under a SNA Server service.

The following is an example of this type of configuration:

Connection (PU) Name Remote APPC LU Alias Remote APPC LU Name
HOST1 CICS1 APPN.CICS1
HOST2 CICS1 APPN.CICS1
HOST3 CICS1 APPN.CICS1

In order to support load balancing across multiple connections within the same SNA Server service, the SNA Server service logically treats the duplicate Remote APPC LUs as a single Remote APPC LU.

In other words, the three APPN.CICS1 LUs shown above are treated as a single Remote APPC LU called CICS1 (APPN.CICS1).

If you were to open 15 sessions that use the Remote APPC LU CICS1, the SNA Server service will internally distribute the sessions across the three connections such that you’d get 5 sessions on each connection.

Considerations when using this feature

1) Session Limits

Since the duplicate Remote APPC LUs are treated as a single Remote APPC LU, the session limits negotiated during the CNOS (Change Number of Sessions) exchange are shared by the Remote APPC LUs across the multiple connections.

As an example, let’s assume that you are using the #BATCH mode and that the CNOS exchange resulted in a parallel session limit of 75. This would mean that the total session limit that would be available over an APPC triplet (Local APPC LU/Remote APPC LU/APPC Mode) that used the #BATCH mode and Remote APPC LU CICS1 would be 75 across the three connections. You WILL NOT get 75 sessions across each host connection when using Remote APPC LU CICS1.

In effect, the session limits are shared across the duplicate Remote APPC LUs. In this example, you will see that you would get a maximum of 25 sessions on each of the three host connections.

2) Automatic Activation Limit

The number of automatically activated sessions specified in the APPC Mode definition will also be shared across the connections. For example, if the APPC Mode has an automatic activation limit of 1 specified, one session will be activated on one of the defined Remote APPC LUs on one of the host connections. Using the example above, there will be one automatically activated session on one of the Remote APPC LUs (CICS1) defined on one of the host connections (e.g. HOST1). You won’t see 3 automatically activated sessions (one on each of the three example host connections).

If you have 3 instances of the Remote APPC LU defined on three different host connections and you want one auto-activated session for each of these, the APPC Mode would have to have an Automatic Activation Limit of 3.

3) SNA Server service doesn’t return Session Limits Reached Error in OPEN LU62 RSP Error messages on HIS 2000 (including SP1 and SP2) and HIS 2004 (including SP1).

Note: This was corrected in HIS 2006.

When an APPC application starts, the HIS “client” DMOD (dynamic access module) sends an OPEN LU62 REQUEST message to the SNA Server service where the Remote APPC LU is defined to request a LU 6.2 session over the specified APPC triplet.

If an LU 6.2 session is not already available (already bound), the SNA Server service will return an OPEN LU62 RSP error message with a corresponding Err2 error message. The Err2 messages are used to determine which SNA Server service is most likely to be able to provide the requested LU 6.2 session.

When testing with a non-duplicated Remote APPC LU, the SNA Server service will return an Err2 error of 0x0604 (Session limits reached for the requested Local APPC LU/Remote APPC LU/Mode) when the parallel session limit has been reached.

For example, if the parallel session limit for the #BATCH mode is 9, the SNA Server service will return an OPEN LU62 RSP error message with Err2:0604 when it receives a new session request if there are 9 active sessions over the requested APPC triplet.

If you do this same test when there are duplicate Remote APPC LUs defined on a SNA Server service so that the sessions are load balanced across the duplicate Remote APPC LUs and host connections, you won't see the same results.

In my tests, the SNA Server service (on HIS 2000 and HIS 2004) will return an OPEN LU62 RSP error message with Err2:0002 (CNOS has been done, but no sessions are currently active) in the case where you have actually reached the specified parallel session limit.

For example, if the parallel session limit for the #BATCH mode is 9, the SNA Server service will return an OPEN LU62 RSP error message with Err2:0002 if there are 9 active sessions over the requested APPC triplet. In this case, you would see that there are 3 active sessions across the three host connections that the Remote APPC LU is defined on.