Home > C++, uml > DataLoggerThread


The figure below models a program in which a pipeline of worker threads communicate with each other via message passing. The accordion thingies ‘tween the threads are message queues that keep the threads loosely coupled and prevent message bursts from overwhelming  downstream threads.

During the process of writing one of these multi-threaded programs to handle bursty, high rate, message streams, I needed a way to periodically extract state information from each thread so that I could “see” and evaluate what the hell was happening inside the system during runtime. Thus, I wrote a generic “Data Logger” thread and added periodic state reporting functionality to each worker thread to round out the system:

Because the reporting frequency is low (it’s configurable for each worker thread and the default value is once every 5 seconds) and the state report messages are small, I didn’t feel the need to provide a queue per worker thread – YAGNI.

The figure below shows a more detailed design model of the data logging facility in the form of a bent” UML class diagram. Upon construction, each DataLoggerThread object can be configured to output state messages to a user named disk file and/or the global console during runtime. The rate at which a DataLoggerThread object “pops” state report messages from its input queue is also configurable.

The DataLoggerThread class provides two different methods of access to user code at runtime:

void DataLoggerThread::record_txt_block(const Data&)


void DataLoggerThread::operator<<(const Data&).

Objects of the DataLoggerThread class run in their own thread of execution – transparently in the background to mainline user code. On construction, each object instance creates a mutex-protected, inter-thread queue and auto-starts its own thread of operation behind the scenes. On destruction, the object gracefully self-terminates. During runtime, each DataLoggerThread object polls its input queue and formats/writes the queue entries to the global console (which is protected from simultaneous, multiple thread access by a previously developed CoutMonitor class) and/or to a user-named disk log file. The queue is drained of all entries on each (configurable,) periodic activation by the underlying (Boost) threads library.

DataLoggerThread objects pre-pend a “milliseconds since midnight” timestamp to each log entry just prior to being pushed onto the queue and a date-time stamp is pre-pended to each user supplied filespec so that file access collisions don’t occur between multiple instances of the class.

That’s all I’m gonna disclose for now, but that’s OK because every programmer who writes soft, real-time, multi-threaded code has their own homegrown contraption, no?

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