# Sysplex

Sysplex is a set of z/OS systems that communicate with each other through certain multisystem hardware components and software services.

The architectural operations of Substation ES comply with the Sysplex environment but do not use the Sysplex features. For example, the Sysplex load balancing features are redundant for Substation ES, because load balancing is achieved within the configuration and operations of Substation ES communications together with the Enterprise Message Service architecture.

Enterprise Message Service supports the message queue and broadcast topic messaging structures, which are represented here in simplistic terms:

- Queues are used for higher latency applications. A message queue is exactly as it sounds: a series of messages held in a queue. Queues can be persistent or nonpersistent.
- Topics are generally used for broadcast, administrative, and low latency applications. A topic is a type of message that is broadcast to all the clients who are listening to the particular topic or who have expressed interest in listening to a particular topic (known as durable subscribers). Topics are always nonpersistent, except in the case of a durable subscriber, where the message is kept until the subscriber consumes the message.

Note: For more information about queues and topics, see the appropriate Enterprise Message Service documentation.

Substation ES supports these messaging techniques, which are activated by dynamic configuration. However, if multiple Substation ES instances share the same configuration file (repository), note the following considerations:

- Queues

  Where multiple Substation ES instances are picking up messages from a single queue, each message is consumed only once by an instance of Substation ES.

- Topics

  Topics differ from queues in that a message to a topic is processed by all of the consumers currently active in all connected Substation ES instances. Where there are multiple BESs and each must process the same message, that practice is obviously advantageous. However, where there is a single CICSPlex, the same message might be processed multiple times.

  The group construct within Substation ES provides a way for a single connection service to consume topic messages in a shared configuration environment.

Where there is a Sysplex and a CICSPlex across multiple LPARs, a common approach is to have multiple Substation ES instances processing messages with a common configuration file. You can configure these Substation ES instances in two ways: both active, each processing only its own messages; or one running and the other a standby, configured but not running. In case of a failure, the standby processes the messages that were destined for the failed Substation ES instance.

During a failover or restart of a Substation ES instance, messages that are produced through topics are not lost if the Substation ES consumer service is registered as a durable subscriber. Topic consumers that are not durable receive messages only if they are enabled to receive them.

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