# Configuration Tips

The configuration tips provide information about how you can configure Enterprise Message Service servers, Substation ES address spaces, and so on to achieve a high availability environment.

A pair of Enterprise Message Service servers working in parallel provides a single Enterprise Message Service server view with no loss of service if either server fails. You can place the Enterprise Message Service servers on different subnets so that failure of one subnet does not cause loss of service.

Configure multiple Substation ES address spaces in different LPARs sharing the same service configuration, each listening to the same Enterprise Message Service queues. If Substation ES A gets a message from an Enterprise Message Service queue and does not respond with a reply, as would happen if Substation ES A or LPAR A fails, the Enterprise Message Service server retains the message on the queue and gives it to the next “listen” from Substation ES B. From the business view, there is no loss of service.

The CICS regions can be Application-Owning Regions (AORs), Terminal-Owning Regions (TORs) with AORs behind them, or TORs in a CICSPlex. The following are the interface scenarios:

- If Substation ES is interfacing with AOR CICS and the CICS address space fails (not the transaction, but the address space), Substation ES disables all the recipes associated with the CICS address space and stops listening to the queues defined in the recipes.

  If other recipes are in the Substation ES A or B define processing of a queue for another CICS address space, then that process continues to handle messages.

- If Substation ES is interfacing with TOR CICS or TOR in a CICSPlex, then you must build fault tolerance into the CICSPlex environments.
- Substation ES supports the notion of primary and alternate executions when services communicate with transactional management systems. When the primary system is unavailable, Substation ES automatically routes messages to the secondary systems. Once the primary system is available again, execution is routed through to the primary system again. This feature increases flexibility and reliability for the CICS or IMS interfaces and adds further dynamics to Substation ES high availability environment.

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