test_multiple_callbacks

Multiple callbacks example.

This example shows a Span created for a top-level operation, covering a set of asynchronous operations (representing callbacks), and have this Span finished when all of them have been executed.

Client.send() is used to create a new asynchronous operation (callback), and in turn every operation both restores the active Span, and creates a child Span (useful for measuring the performance of each callback).

Implementation details:

• For threading, a thread-safe counter is put in each Span to keep track of the pending callbacks, and call Span.finish() when the count becomes 0.
• For gevent, tornado, asyncio and contextvars the children coroutines representing the subtasks are simply yielded over, so no counter is needed.
• For tornado, the invoked coroutines do not set any active Span as doing so messes the used StackContext. So yielding over multiple coroutines is not supported.
• For contextvars, parent context is propagated to the children coroutines implicitly, manual context activation has been avoided.

threading implementation:

    def task(self, interval, parent_span):
        logger.info('Starting task')

        try:
            scope = self.tracer.scope_manager.activate(parent_span, False)
            with self.tracer.start_active_span('task'):
                time.sleep(interval)
        finally:
            scope.close()
            if parent_span._ref_count.decr() == 0:
                parent_span.finish()

asyncio implementation:

    async def task(self, interval, parent_span):
        logger.info('Starting task')

        with self.tracer.scope_manager.activate(parent_span, False):
            with self.tracer.start_active_span('task'):
                await asyncio.sleep(interval)

    # Invoke and yield over the corotuines.
    with self.tracer.start_active_span('parent'):
	tasks = self.submit_callbacks()
	await asyncio.gather(*tasks)