IBM Tivoli NetView for z/OS Programming: Pipes - IBM notice

Introduction and Concepts
The LOCATE stage is a filter. LOCATE examines the messages from its input
stream, and searches for those containing a specified string. The messages that
match are written to the output stream; those that do not match are discarded or
passed to a secondary output stream, if one is connected.
Filters perform many functions of general use. For example, they select messages
based on the content of the message or on the position of the message in the
stream flowing through the pipeline.
Understanding NetView Pipelines
12 Programming: Pipes
When you issue the PIPE command, NetView pipelines check the spelling and
syntax of all the stage specifications. If spelling and syntax are correct, pipeline
processing begins. Otherwise, the pipeline is stopped and a nonzero return code is
generated.
How a Pipeline Begins
After processing begins, the PIPE command decides which stage to run and when
to run it. It is not a matter of turning on all stages at once or of turning on one
stage and running it to completion before starting the next stage. For the most
part, the processing resembles the plumbing pipeline described earlier. That is:
v Data begins flowing from a source through a device driver. At this point, no
subsequent stages are active.
v The device driver passes the data to the next stage, a filter, perhaps. The driver
then gets more data. At this point only the driver and the filter are active.
v Data flows from stage to stage, activating each stage as it goes.
v Soon, the entire pipeline is active with a flow of data just as a plumbing pipeline
is active with a flow of water.
v Ultimately, data begins to leave the pipeline through a device driver and the
source of data will be exhausted.
v As the last bits of data flow through the pipeline, stages disconnect from their
input and output streams as they become inactive.
v After all the stages have disconnected, the pipeline ends.
By operating in this fashion, a pipeline can process an extremely large volume of
data without having to keep the entire volume in storage. However, some stages
need to read all the data before they can begin processing messages. For example,
the COLLECT command must collect all the messages from the input stream
before writing the messages as one multiline write-to-operator message (MLWTO)
to its output stream.
How a Pipeline Ends
Each stage uses its own rules to determine when (and whether) to disconnect. For
many stages, a disconnect from one side causes the stage to disconnect from the
other side. Some stages (TOSTRING, for example) examine the message stream to
determine when to disconnect the output stream.
Usually, a pipeline continues to process as long as any stages are connected.
A pipeline ends when all of its stages end. A stage ends when one of the following
events occurs:
v The stage completes its function.
v The stage detects an unrecoverable error.