# Externally Initiated Request or Request/Reply

In the externally initiated request or request/reply scenario, a Substation ES is in each z/OS LPAR, and each Substation ES instance uses Enterprise Message Service messaging by means of queues; connections to CICS use the IBM EXCI facility.

Any active Substation ES instance can pick up messages from the server for processing. If a Substation ES instance or LPAR fails, a second Substation ES instance continues to process messages from the queues.

The following figure shows the externally initiated request or request/reply scenario:

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