Chapter 62. Transformer
Sometimes it's useful to perform some cmoputations on an NSCLDAQ event file and append data to parts of the event. Transformer provides the ability to add an arbitrary extension to fragments of event built data. It is a parallel program that runs on top of the communication framework we've been describing.
The program consists of an input stage, a set of parallel workers that can add extension to event fragments, a sorter that resorts the ouptut of the workers back into time stamp ordered events. The pipeline of input workers sorting and output are parallel processes as is each worker.
Transformer allows you to write the code needed to compute the extension for any single event fragment. This code does not have to be MPI or thread-aware though it must be thread-safe. This code is built into a shared library which is loaded by Transformer at run time.
The remainder of this chapter
Describes what your shared library must provide.
Describes how to build your shared library.
 | NOTE: |
|---|---|
The Transformer man page in 1daq desribes how to run the
program.
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62.1. User shared libraries for Transformer
The Transformer uses a set of parallel workers to compute the
actual extension. The workers receive blocks of events an
parcel them off to instances of a CBuiltRingItemExtender.
Each of those instances walks the event fragments in the event
providing the fragment's ring item to user code embedded in
a concrete subclass of
CBuiltRingItemExtender::CRingItemExtender.
The user code must implement the following methods:
virtual iovec operator()(pRingItem item);Takes the ring item of an event fragment as input and computes the extension for that event fragment. The return value is an
iovecSee the writev(2) man page for a description of that struct.If the length of the extension returned is zero, no extension will be added.
Refer to DataFormat.h(3daq) for information about the pRingItem data type.
virtual void free(iovec& extension);Since all extension data must remain valid until the entire event has been formatted, you may find that you need to use dynamic memory to hold your extensions. After an event has been formatted and sent to the sorter, this method is called once for each fragment passing in the iovec as
extensionthat was returned byoperator(). This method is expected to free any dynamic data that was created to hold that extension data.
In addition to a concrete extension class, your shared library must provide a factory method the Transforme uses to create instances of your class. Each worker gets its own class instance.