Planning for Fast Path terminals

To have a Fast Path-capable system, specify Fast Path support on the FPCTRL macro.

IMS systems with a very high transaction rate use Fast Path's expedited message handler (EMH) facility. EMH is a performance option that expedites message processing by imposing restrictions on message lengths and segmentation. LU 6.2 terminals can use EMH; no definitions or specifications are required.

With Fast Path, an EMH buffer is acquired from the EMH buffer pool when the first eligible input message is received for a Fast Path transaction. The buffer remains allocated to the terminal for future use until the session terminates or the user signs off. If an EMH buffer is allocated to the terminal, it is reused if it meets the requirements of the next input call. If the message requires a larger buffer, Fast Path swaps it for a larger one.

If, at entry to IMS, an input message satisfies all of the following criteria, it is edited in a Fast Path EMH buffer instead of in a full-function message queue buffer.
  • Terminal is not FES capable
  • Terminal is not the MTO
  • Input is single segment
  • Input is non-MFS edited
  • Input is not an IMS command
  • Execution of at least one Fast Path transaction is scheduled using Fast Path

Exception: If the message is not a Fast Path transaction, the message is moved to a full-function message queue buffer.

When a message with all of these characteristics is received, an EMH buffer is allocated to the terminal. The EMH buffer remains allocated to the terminal until either the user signs off or the session terminates. The buffer is used repeatedly for Fast Path transactions until a larger EMH buffer is required for special applications. When a larger buffer is needed, the current EMH buffer is swapped for the larger-size EMH buffer. The smaller EMH buffer is released and returned to the EMH pool.

The EMHL parameter in the IMS control region initialization (also known as the startup parameter) specifies the default EMH buffer size for all Fast Path transactions for all Fast Path-eligible terminals. A Fast Path transaction can specify an EMH buffer that is larger than the default. Specify larger application EMH buffer sizes on the APPLCTN or TRANSACT stage_1 macros by using the FPATH=size parameter when you generate the system. If the EMH buffer pool is exhausted, message DFS3971 is sent to the input terminal.

Buffers are extracted from the EMH buffer pool, which expands and contracts dynamically. The size of the pool depends on the number of terminals that are concurrently entering Fast Path transactions and the buffer sizes that are required to satisfy each request.

You can say FPATH=No or Yes on the APPLCTN and TRANSACT macros, or you can specify FPATH=size, where size is the EMH buffer size that is required to run the transaction. FPATH=size implies FPATH=Yes. The minimum EMH buffer size is 12 bytes, and the maximum is 30720 bytes.

MSDBs are available to ETO terminals, unless they have terminal-related keys.

The OPTIONS keyword on the TERMINAL macro or ETO descriptor is used to:
  • Declare FORCRESP or TRANRESP so that Fast Path-eligible terminals operate in response mode
  • Specify PAGDEL for automatic page deletion where appropriate

You can use non-VTAM® terminals to enter Fast Path transactions. For non-VTAM terminals, the LINE macro must indicate response mode by using the RESP=TERM parameter.