Using ENF event code 48 in system logger applications

System logger issues ENF event code 48 to broadcast status changes in the system logger address space, log streams, and coupling facility structures. Since these status changes can affect the outcome of system logger service requests, IBM® suggests that you use ENF event code 48 to receive notification of these status changes, using the ENFREQ service to listen for event 48. Note that your program must be authorized to use the ENFREQ service. Applications should issue the ENFREQ service to listen for event 48 before connecting to a log stream.

For example, suppose an IXGWRITE request fails with a return and reason code indicating that some system logger resource is unavailable, perhaps because the system logger address space has failed or because a structure rebuild is in progress for the coupling facility structure for the log stream. Before the application can resume issuing system logger service requests. it must listen for the event code 48 notification that the resource is available again.

In order to listen for ENF code 48 events, you must code an SRB-type listen exit for event code 48 events to scan the event 48 parameter list for status information on the system logger component, log streams, and coupling facility structures associated with log streams. The listen exit must be in place before system logger applications are activated.

If you use ENF event code 48 to receive information about system logger events, make sure that you take into account the asynchronous nature of the ENF exit. You might get notified of events out of sequence, being notified for instance, that a problem has been resolved before you get a return and reason code describing a problem.

For example, if you issue IXGWRITE to write data to the log stream while the coupling facility structure space allocated for the log stream is full, you might get an ENF 48 notification that the structure is no longer full before you get the return and reason code from IXGWRITE to say that the structure is full.

Applications that do not want to use ENF event code 48 or that are unauthorized and cannot use ENFREQ will still receive logger service return and reason codes indicating failure or resource shortages. These applications can simply set a timer and then retry the requests to see if the problem has resolved itself.

References:

See Writing an ENF event 48 listen exit for information on ENF 48 events, and coding your ENF event 48 listen exit.
See z/OS MVS Programming: Assembler Services Guide for guidance about using the ENFREQ macro.
See z/OS MVS Programming: Authorized Assembler Services Reference EDT-IXG for reference information on the ENFREQ macro.

Figure 1 shows an example of how to issue the ENFREQ service to listen for ENF event code 48 and specify a listen exit to analyze the event 48 parameter list:

Figure 1. Issuing ENFREQ to Listen for ENF Event Code 48

ENFREQ ACTION=LISTEN,
      CODE=ENFPC048,
      ESTBNME=THISMOD,
      EXITNME=LOGLISTEN,
      SRBEXIT=(R02),
      EOM=YES,
      DTOKEN=ENFREQ_DTOKEN,
      RETCODE=ENFREQ_RETCODE