APPC/IMS flood control

By default, APPC/IMS flood control is active and starts queuing incoming APPC transaction requests to 64-bit storage when the number of active APPC conversations reaches 5,000. If the flood condition worsens, APPC/IMS flood control stops all APPC input when the number of queued APPC requests in 64-bit storage reaches the default threshold of 10,000.

Except when all APPC input is stopped, APPC/IMS flood control does not apply to APPC requests that are used to submit IMS commands or to APPC requests that are received on back-end IMS systems in a shared-queues environment.

As the number of active and queued APPC requests nears these thresholds, IMS issues a warning message.

Modifying or disabling APPC/IMS flood control

You can modify or disable both the initial threshold for queuing requests to 64-bit storage and the secondary threshold that stops all APPC/IMS input by specifying the APPCMAXC=(31_bit_max,64_bit_max) parameter in the DFSDCxxx member in the IMS PROCLIB data set.

The 31_bit_max value defines the maximum number of active APPC conversations that IMS can process concurrently before IMS starts queuing new APPC transaction requests to 64-bit storage. A specification of 0 completely disables APPC/IMS flood control.

You can view the current 31_bit_max value by issuing the IMS type-1 command /DISPLAY A DC. If the displayed value is 0, APPC/IMS flood control is disabled.

The 64_bit_max defines the maximum number of APPC transaction requests that can be queued in 64-bit storage before IMS stops all APPC input from z/OS®. A specification of 0 disables the queuing of APPC requests in 64-bit storage.

You can clear all of the queued APPC requests in 64-bit storage by issuing the IMS type-1 command /PURGE APPC. The APPC conversation is rejected with the TP_Not_Available_No_Retry sense code.

When 64-bit queuing is disabled, if a flood condition occurs, the 31 bit maximum defines the threshold at which IMS stops all APPC input.

When APPC input is stopped

When IMS stops APPC input from z/OS, IMS does not itself reject incoming APPC requests, but rather issues a call to APPC/MVS™ to request that it stop sending any more APPC requests. Between the time that IMS issues the request and the time that APPC/MVS stops sending input, IMS can still receive APPC requests, so it is possible that the total number of APPC requests that are received or queued by APPC/IMS might exceed the defined maximum.

After APPC input is stopped, when the number of active APPC conversations in IMS 31-bit storage drops below 50% of the 31_bit_max value, IMS automatically requests a resumption of APPC input from APPC/MVS.

IMS issues DFS4157E when APPC input is stopped.

VTAM alternative to APPC/IMS flood control

In addition to or as an alternative to the APPC/IMS flood control measures, you can specify a session limit for an individual logical unit (LU) in the VTAM® ACB. VTAM stops sending messages to APPC/MVS after the session limit is reached. If only one LU is defined for an IMS system, the maximum number of active APPC requests is then the number of sessions that are defined in VTAM.