40 WebSphere Information Integrator Q Replication: Fast Track Implementation Scenarios
2.4 Best practices considerations
Q Capture, Replication queue map, Q Apply, and WebSphere MQ provide a
number of parameters to optimize the operation and performance of the Q
replication environment. Each of the main parameters associated with these
components is discussed in the following sections. A brief description of each
parameter function, and its key considerations and best practices
recommendations is provided. Since latency is a critical criterion in the selection
of Q replication as the technology to address business requirements, it is
specifically addressed here.
2.4.1 Q Capture considerations
The Q Capture program’s operating parameters govern the way that it starts, the
amount of memory that it uses, which queue manager it connects to, and how
frequently it commits messages to queues, amongst other things. The
parameters are read from the IBMQREP_CAPPARMS table when Q Capture is
started or re-initialized. One can specify temporary runtime values for some
parameters when the program is started, and also while the program is running.
The main parameters of interest discussed here are COMMIT_INTERVAL,
This parameter specifies how often, in milliseconds, a Q Capture program
commits transactions to WebSphere MQ. By default, a Q Capture program waits
500 milliseconds (a half second) between commits. At each interval, the Q
Capture program issues an MQCMIT call. This signals the WebSphere MQ
queue manager to make messages that were placed on send queues available
to the Q Apply program or other user applications. All of the DB2 transactions
that are grouped within an MQCMIT call are considered to be a WebSphere MQ
unit of work, or transaction. Typically, each WebSphere MQ transaction contains
several DB2 transactions.
The default COMMIT_INTERVAL value is 500 milliseconds. Committing
messages more frequently requires more CPU usage and I/O overhead.
Lowering the commit interval value is not likely to improve throughput, but will
potentially improve latency. If you cannot increase the amount of memory
(MEMORY_LIMIT) allocated for the Q Capture, then you can reduce the commit
interval. Reducing the commit interval flushes messages from memory more
often and prevents the Q Capture program from spilling transactions to disk.
Raising the commit interval reduces CPU usage, which may be appropriate in a
low volume environment, but can increase latency.

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