499 lines
18 KiB
ReStructuredText
499 lines
18 KiB
ReStructuredText
Log Contexts
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============
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.. contents::
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To help track the processing of individual requests, synapse uses a
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'log context' to track which request it is handling at any given moment. This
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is done via a thread-local variable; a ``logging.Filter`` is then used to fish
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the information back out of the thread-local variable and add it to each log
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record.
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Logcontexts are also used for CPU and database accounting, so that we can track
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which requests were responsible for high CPU use or database activity.
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The ``synapse.logging.context`` module provides a facilities for managing the
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current log context (as well as providing the ``LoggingContextFilter`` class).
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Deferreds make the whole thing complicated, so this document describes how it
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all works, and how to write code which follows the rules.
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Logcontexts without Deferreds
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-----------------------------
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In the absence of any Deferred voodoo, things are simple enough. As with any
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code of this nature, the rule is that our function should leave things as it
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found them:
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.. code:: python
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from synapse.logging import context # omitted from future snippets
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def handle_request(request_id):
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request_context = context.LoggingContext()
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calling_context = context.LoggingContext.current_context()
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context.LoggingContext.set_current_context(request_context)
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try:
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request_context.request = request_id
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do_request_handling()
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logger.debug("finished")
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finally:
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context.LoggingContext.set_current_context(calling_context)
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def do_request_handling():
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logger.debug("phew") # this will be logged against request_id
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LoggingContext implements the context management methods, so the above can be
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written much more succinctly as:
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.. code:: python
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def handle_request(request_id):
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with context.LoggingContext() as request_context:
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request_context.request = request_id
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do_request_handling()
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logger.debug("finished")
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def do_request_handling():
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logger.debug("phew")
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Using logcontexts with Deferreds
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--------------------------------
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Deferreds — and in particular, ``defer.inlineCallbacks`` — break
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the linear flow of code so that there is no longer a single entry point where
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we should set the logcontext and a single exit point where we should remove it.
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Consider the example above, where ``do_request_handling`` needs to do some
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blocking operation, and returns a deferred:
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.. code:: python
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@defer.inlineCallbacks
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def handle_request(request_id):
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with context.LoggingContext() as request_context:
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request_context.request = request_id
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yield do_request_handling()
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logger.debug("finished")
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In the above flow:
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* The logcontext is set
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* ``do_request_handling`` is called, and returns a deferred
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* ``handle_request`` yields the deferred
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* The ``inlineCallbacks`` wrapper of ``handle_request`` returns a deferred
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So we have stopped processing the request (and will probably go on to start
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processing the next), without clearing the logcontext.
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To circumvent this problem, synapse code assumes that, wherever you have a
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deferred, you will want to yield on it. To that end, whereever functions return
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a deferred, we adopt the following conventions:
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**Rules for functions returning deferreds:**
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* If the deferred is already complete, the function returns with the same
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logcontext it started with.
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* If the deferred is incomplete, the function clears the logcontext before
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returning; when the deferred completes, it restores the logcontext before
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running any callbacks.
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That sounds complicated, but actually it means a lot of code (including the
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example above) "just works". There are two cases:
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* If ``do_request_handling`` returns a completed deferred, then the logcontext
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will still be in place. In this case, execution will continue immediately
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after the ``yield``; the "finished" line will be logged against the right
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context, and the ``with`` block restores the original context before we
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return to the caller.
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* If the returned deferred is incomplete, ``do_request_handling`` clears the
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logcontext before returning. The logcontext is therefore clear when
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``handle_request`` yields the deferred. At that point, the ``inlineCallbacks``
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wrapper adds a callback to the deferred, and returns another (incomplete)
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deferred to the caller, and it is safe to begin processing the next request.
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Once ``do_request_handling``'s deferred completes, it will reinstate the
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logcontext, before running the callback added by the ``inlineCallbacks``
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wrapper. That callback runs the second half of ``handle_request``, so again
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the "finished" line will be logged against the right
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context, and the ``with`` block restores the original context.
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As an aside, it's worth noting that ``handle_request`` follows our rules -
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though that only matters if the caller has its own logcontext which it cares
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about.
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The following sections describe pitfalls and helpful patterns when implementing
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these rules.
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Always yield your deferreds
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---------------------------
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Whenever you get a deferred back from a function, you should ``yield`` on it
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as soon as possible. (Returning it directly to your caller is ok too, if you're
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not doing ``inlineCallbacks``.) Do not pass go; do not do any logging; do not
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call any other functions.
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.. code:: python
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@defer.inlineCallbacks
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def fun():
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logger.debug("starting")
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yield do_some_stuff() # just like this
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d = more_stuff()
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result = yield d # also fine, of course
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return result
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def nonInlineCallbacksFun():
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logger.debug("just a wrapper really")
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return do_some_stuff() # this is ok too - the caller will yield on
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# it anyway.
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Provided this pattern is followed all the way back up to the callchain to where
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the logcontext was set, this will make things work out ok: provided
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``do_some_stuff`` and ``more_stuff`` follow the rules above, then so will
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``fun`` (as wrapped by ``inlineCallbacks``) and ``nonInlineCallbacksFun``.
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It's all too easy to forget to ``yield``: for instance if we forgot that
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``do_some_stuff`` returned a deferred, we might plough on regardless. This
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leads to a mess; it will probably work itself out eventually, but not before
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a load of stuff has been logged against the wrong context. (Normally, other
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things will break, more obviously, if you forget to ``yield``, so this tends
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not to be a major problem in practice.)
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Of course sometimes you need to do something a bit fancier with your Deferreds
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- not all code follows the linear A-then-B-then-C pattern. Notes on
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implementing more complex patterns are in later sections.
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Where you create a new Deferred, make it follow the rules
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---------------------------------------------------------
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Most of the time, a Deferred comes from another synapse function. Sometimes,
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though, we need to make up a new Deferred, or we get a Deferred back from
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external code. We need to make it follow our rules.
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The easy way to do it is with a combination of ``defer.inlineCallbacks``, and
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``context.PreserveLoggingContext``. Suppose we want to implement ``sleep``,
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which returns a deferred which will run its callbacks after a given number of
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seconds. That might look like:
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.. code:: python
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# not a logcontext-rules-compliant function
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def get_sleep_deferred(seconds):
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d = defer.Deferred()
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reactor.callLater(seconds, d.callback, None)
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return d
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That doesn't follow the rules, but we can fix it by wrapping it with
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``PreserveLoggingContext`` and ``yield`` ing on it:
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.. code:: python
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@defer.inlineCallbacks
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def sleep(seconds):
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with PreserveLoggingContext():
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yield get_sleep_deferred(seconds)
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This technique works equally for external functions which return deferreds,
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or deferreds we have made ourselves.
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You can also use ``context.make_deferred_yieldable``, which just does the
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boilerplate for you, so the above could be written:
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.. code:: python
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def sleep(seconds):
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return context.make_deferred_yieldable(get_sleep_deferred(seconds))
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Fire-and-forget
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---------------
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Sometimes you want to fire off a chain of execution, but not wait for its
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result. That might look a bit like this:
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.. code:: python
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@defer.inlineCallbacks
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def do_request_handling():
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yield foreground_operation()
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# *don't* do this
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background_operation()
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logger.debug("Request handling complete")
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@defer.inlineCallbacks
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def background_operation():
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yield first_background_step()
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logger.debug("Completed first step")
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yield second_background_step()
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logger.debug("Completed second step")
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The above code does a couple of steps in the background after
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``do_request_handling`` has finished. The log lines are still logged against
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the ``request_context`` logcontext, which may or may not be desirable. There
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are two big problems with the above, however. The first problem is that, if
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``background_operation`` returns an incomplete Deferred, it will expect its
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caller to ``yield`` immediately, so will have cleared the logcontext. In this
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example, that means that 'Request handling complete' will be logged without any
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context.
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The second problem, which is potentially even worse, is that when the Deferred
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returned by ``background_operation`` completes, it will restore the original
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logcontext. There is nothing waiting on that Deferred, so the logcontext will
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leak into the reactor and possibly get attached to some arbitrary future
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operation.
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There are two potential solutions to this.
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One option is to surround the call to ``background_operation`` with a
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``PreserveLoggingContext`` call. That will reset the logcontext before
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starting ``background_operation`` (so the context restored when the deferred
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completes will be the empty logcontext), and will restore the current
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logcontext before continuing the foreground process:
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.. code:: python
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@defer.inlineCallbacks
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def do_request_handling():
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yield foreground_operation()
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# start background_operation off in the empty logcontext, to
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# avoid leaking the current context into the reactor.
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with PreserveLoggingContext():
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background_operation()
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# this will now be logged against the request context
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logger.debug("Request handling complete")
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Obviously that option means that the operations done in
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``background_operation`` would be not be logged against a logcontext (though
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that might be fixed by setting a different logcontext via a ``with
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LoggingContext(...)`` in ``background_operation``).
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The second option is to use ``context.run_in_background``, which wraps a
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function so that it doesn't reset the logcontext even when it returns an
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incomplete deferred, and adds a callback to the returned deferred to reset the
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logcontext. In other words, it turns a function that follows the Synapse rules
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about logcontexts and Deferreds into one which behaves more like an external
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function — the opposite operation to that described in the previous section.
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It can be used like this:
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.. code:: python
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@defer.inlineCallbacks
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def do_request_handling():
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yield foreground_operation()
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context.run_in_background(background_operation)
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# this will now be logged against the request context
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logger.debug("Request handling complete")
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Passing synapse deferreds into third-party functions
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----------------------------------------------------
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A typical example of this is where we want to collect together two or more
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deferred via ``defer.gatherResults``:
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.. code:: python
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d1 = operation1()
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d2 = operation2()
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d3 = defer.gatherResults([d1, d2])
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This is really a variation of the fire-and-forget problem above, in that we are
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firing off ``d1`` and ``d2`` without yielding on them. The difference
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is that we now have third-party code attached to their callbacks. Anyway either
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technique given in the `Fire-and-forget`_ section will work.
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Of course, the new Deferred returned by ``gatherResults`` needs to be wrapped
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in order to make it follow the logcontext rules before we can yield it, as
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described in `Where you create a new Deferred, make it follow the rules`_.
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So, option one: reset the logcontext before starting the operations to be
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gathered:
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.. code:: python
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@defer.inlineCallbacks
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def do_request_handling():
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with PreserveLoggingContext():
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d1 = operation1()
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d2 = operation2()
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result = yield defer.gatherResults([d1, d2])
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In this case particularly, though, option two, of using
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``context.preserve_fn`` almost certainly makes more sense, so that
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``operation1`` and ``operation2`` are both logged against the original
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logcontext. This looks like:
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.. code:: python
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@defer.inlineCallbacks
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def do_request_handling():
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d1 = context.preserve_fn(operation1)()
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d2 = context.preserve_fn(operation2)()
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with PreserveLoggingContext():
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result = yield defer.gatherResults([d1, d2])
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Was all this really necessary?
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------------------------------
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The conventions used work fine for a linear flow where everything happens in
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series via ``defer.inlineCallbacks`` and ``yield``, but are certainly tricky to
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follow for any more exotic flows. It's hard not to wonder if we could have done
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something else.
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We're not going to rewrite Synapse now, so the following is entirely of
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academic interest, but I'd like to record some thoughts on an alternative
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approach.
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I briefly prototyped some code following an alternative set of rules. I think
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it would work, but I certainly didn't get as far as thinking how it would
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interact with concepts as complicated as the cache descriptors.
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My alternative rules were:
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* functions always preserve the logcontext of their caller, whether or not they
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are returning a Deferred.
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* Deferreds returned by synapse functions run their callbacks in the same
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context as the function was orignally called in.
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The main point of this scheme is that everywhere that sets the logcontext is
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responsible for clearing it before returning control to the reactor.
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So, for example, if you were the function which started a ``with
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LoggingContext`` block, you wouldn't ``yield`` within it — instead you'd start
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off the background process, and then leave the ``with`` block to wait for it:
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.. code:: python
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def handle_request(request_id):
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with context.LoggingContext() as request_context:
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request_context.request = request_id
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d = do_request_handling()
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def cb(r):
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logger.debug("finished")
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d.addCallback(cb)
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return d
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(in general, mixing ``with LoggingContext`` blocks and
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``defer.inlineCallbacks`` in the same function leads to slighly
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counter-intuitive code, under this scheme).
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Because we leave the original ``with`` block as soon as the Deferred is
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returned (as opposed to waiting for it to be resolved, as we do today), the
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logcontext is cleared before control passes back to the reactor; so if there is
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some code within ``do_request_handling`` which needs to wait for a Deferred to
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complete, there is no need for it to worry about clearing the logcontext before
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doing so:
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.. code:: python
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def handle_request():
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r = do_some_stuff()
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r.addCallback(do_some_more_stuff)
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return r
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— and provided ``do_some_stuff`` follows the rules of returning a Deferred which
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runs its callbacks in the original logcontext, all is happy.
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The business of a Deferred which runs its callbacks in the original logcontext
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isn't hard to achieve — we have it today, in the shape of
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``context._PreservingContextDeferred``:
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.. code:: python
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def do_some_stuff():
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deferred = do_some_io()
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pcd = _PreservingContextDeferred(LoggingContext.current_context())
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deferred.chainDeferred(pcd)
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return pcd
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It turns out that, thanks to the way that Deferreds chain together, we
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automatically get the property of a context-preserving deferred with
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``defer.inlineCallbacks``, provided the final Defered the function ``yields``
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on has that property. So we can just write:
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.. code:: python
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@defer.inlineCallbacks
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def handle_request():
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yield do_some_stuff()
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yield do_some_more_stuff()
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To conclude: I think this scheme would have worked equally well, with less
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danger of messing it up, and probably made some more esoteric code easier to
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write. But again — changing the conventions of the entire Synapse codebase is
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not a sensible option for the marginal improvement offered.
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A note on garbage-collection of Deferred chains
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-----------------------------------------------
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It turns out that our logcontext rules do not play nicely with Deferred
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chains which get orphaned and garbage-collected.
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Imagine we have some code that looks like this:
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.. code:: python
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listener_queue = []
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def on_something_interesting():
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for d in listener_queue:
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d.callback("foo")
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@defer.inlineCallbacks
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def await_something_interesting():
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new_deferred = defer.Deferred()
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listener_queue.append(new_deferred)
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with PreserveLoggingContext():
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yield new_deferred
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Obviously, the idea here is that we have a bunch of things which are waiting
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for an event. (It's just an example of the problem here, but a relatively
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common one.)
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Now let's imagine two further things happen. First of all, whatever was
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waiting for the interesting thing goes away. (Perhaps the request times out,
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or something *even more* interesting happens.)
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Secondly, let's suppose that we decide that the interesting thing is never
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going to happen, and we reset the listener queue:
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.. code:: python
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def reset_listener_queue():
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listener_queue.clear()
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So, both ends of the deferred chain have now dropped their references, and the
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deferred chain is now orphaned, and will be garbage-collected at some point.
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Note that ``await_something_interesting`` is a generator function, and when
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Python garbage-collects generator functions, it gives them a chance to clean
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up by making the ``yield`` raise a ``GeneratorExit`` exception. In our case,
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that means that the ``__exit__`` handler of ``PreserveLoggingContext`` will
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carefully restore the request context, but there is now nothing waiting for
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its return, so the request context is never cleared.
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To reiterate, this problem only arises when *both* ends of a deferred chain
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are dropped. Dropping the the reference to a deferred you're supposed to be
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calling is probably bad practice, so this doesn't actually happen too much.
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Unfortunately, when it does happen, it will lead to leaked logcontexts which
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are incredibly hard to track down.
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