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# 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:
```python
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:
```python
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)
```
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