Addressing OLTP Solutions with CICS: The Transaction Server ... - Ibm

As the COMMAREA has an allowable maximum size of 32 KB, its use for storage
of the data returned from the DPL request would in turn limit to 32KB the
maximum output of the SQL call processed by the DB2 for MVS/ESA server.
Where more than 32 KB of data has to be returned, there are two possibilities:
filtering of the data returned to the CICS/ESA program to reduce the size of data
below 32 KB, or staging the data in a CICS resource such as temporary storage.
The data is then returned in 32 KB chunks until it has all been sent. The staging
method requires more complex application design and results in more network
traffic. Therefore we recommend that you use another of the possible ways of
accessing the DB2 for MVS/ESA database from a CICS for AIX region if you
anticipate that the size of returned data will exceed 32 KB.
7.4.5 Accessing DB2 for MVS/ESA Database through MQseries Products
106 CICS for AIX as the Transaction Server
Another means of enabling a CICS for AIX application to access a DB2 for
MVS/ESA database is through the use of Distributed Queue Management (DQM)
along with two different Message Queue (MQ) managers. Use this method
instead of using DB2 client/server connectivity as the transport vehicle.
IBM MQSeries products enable applications to communicate through messages
and queues. The communicating applications can be local on the same system,
or they can be distributed across a wide variety of IBM and non-IBM platforms.
A single programming interface, the Message Queueing Interface (MQI), is
consistent across all the supported platforms. MQSeries can transfer data with
assured delivery. Messages are not lost even in the event of system failures,
and there is no duplicate delivery of the same message. The communicating
applications do not have to be active at the same time. The sender can still
continue doing work without having to wait for the receiver. Message-driven
processing is an application style and facilitates the design of applications in
discrete functional blocks that are linked by messages. In this way, the units of
an application can run on different systems at different times, or they can act in
parallel. Faster application development is possible because the MQSeries
products shield the network complexity from the application.
In the local RISC System/6000 machine (containing the CICS for AIX region; see
Figure 34 on page 107). MQSeries for AIX is installed, and an instance of the
MQ manager is started up with two message channel agents (MCAs), one the
sender MCA, and the other, the receiver MCA. It is essential for CICS for AIX
and MQSeries to be installed on the same system when CICS applications make
MQI calls. The MQ manager must be started before the CICS for AIX region is
started.
In the remote S/390 machine, MQSeries for MVS/ESA is installed, and an
instance of the MQ manager is started up with one sender MCA and one
receiver MCA. The DB2 for MVS/ESA database server resides in this same
MVS/ESA system.
CICS for AIX and MQSeries for AIX communicate using the X/Open XA interface.
This requires an XAD entry to be configured using the switch load file supplied
by MQSeries for AIX.
The CICS application running in the CICS for AIX region makes a connection to
the local MQ manager using an MQCONN call to the MQI. The CICS application
then builds a message with the relevant SQL access information that it intends
to pass on to its partner user program in the remote MVS/ESA system. The
CICS application then sends the message by issuing an MQPUT call. The local