You can include subflows in your message flows
in exactly the same way as you include built-in or user-defined nodes.
You
can also connect subflows to other nodes in the same way. You can define a
subflow once, and use it in more than one message flow (and even in more than
one message flow project), so a subflow can provide the following benefits:
- Reuse and reduced development time.
- A consistent way of achieving a particular function, and increased maintainability
of your message flows (consider a subflow as analogous to a programming macro,
or to inline code that is written once but used in many places).
- Flexibility. If you promote some or all of the properties of the nodes
in the subflow, you can tailor a subflow to a specific context (for example,
by updating the output queue or data source information).
However, remember that a subflow is not a single node, and its inclusion
increases the number of nodes in the message flow, which might affect its
performance.
Consider these examples of subflow use:
- You can define a subflow that provides a common sequence
of actions that applies to several message flows if an error is encountered;
for example, you might have a common error routine that writes the message
to a database through the Warehouse node, and puts it to a queue for processing
by an error recovery routine. The use of this routine in multiple message
flows, or in several places within one message flow, provides an efficient
and consistent use of resources and avoids reinventing such routines every
time an error is encountered.
- You might want to perform a common calculation on messages
that pass through several different message flows; for example, you might
access currency exchange rates from a database and apply these to calculate
prices in several different currencies. You can include the currency calculator
subflow in each of the message flows in which it is appropriate.
Use the Passthrough node to enable versioning of a subflow at run
time. The Passthrough node allows you to add a label to your message flow
or subflow. By combining this label with keyword replacement from your version
control system, you can identify which version of a subflow is included in
a deployed message flow. You can use this label for your own purposes. If
you have included the correct version keywords in the label, you can see the
value of the label:
- Stored in the broker archive (bar) file, using the mqsireadbar command
- As last deployed to a particular broker, on the properties of a deployed
message flow in the Message Brokers Toolkit
- In the run time, if you enable user trace for that message flow
The message that it propagates on its Out terminal is the same message
that it received on its In terminal; for example, if you develop an error
processing subflow to include in several message flows, you might want to
modify that subflow. However, you might want to introduce the modified version
initially to just a subset of the message flows in which it is included. Set
a value for the instance of the Passthrough node that identifies which version
of the subflow you have included.
The use of subflows is demonstrated
in the Error Handler sample and the Coordinated Request Reply sample.
The Error Handler sample uses a subflow to trap information about errors and
store the information in a database. The Coordinated Request Reply sample
uses a subflow to encapsulate the storage of the ReplyToQ and
ReplyToQMgr values in a WebSphere MQ message so that the processing logic
can be easily reused in other message flows and to allow alternative implementations
to be substituted.