synapse/docs/log_contexts.rst

Log Contexts

To help track the processing of individual requests, synapse uses a 'log context' to track which request it is handling at any given moment. This is done via a thread-local variable; a logging.Filter is then used to fish the information back out of the thread-local variable and add it to each log record.

Logcontexts are also used for CPU and database accounting, so that we can track which requests were responsible for high CPU use or database activity.

The synapse.logging.context module provides a facilities for managing the current log context (as well as providing the LoggingContextFilter class).

Deferreds make the whole thing complicated, so this document describes how it all works, and how to write code which follows the rules.

Logcontexts without Deferreds

In the absence of any Deferred voodoo, things are simple enough. As with any code of this nature, the rule is that our function should leave things as it found them:

from synapse.logging import context         # omitted from future snippets

def handle_request(request_id):
    request_context = context.LoggingContext()

    calling_context = context.LoggingContext.current_context()
    context.LoggingContext.set_current_context(request_context)
    try:
        request_context.request = request_id
        do_request_handling()
        logger.debug("finished")
    finally:
        context.LoggingContext.set_current_context(calling_context)

def do_request_handling():
    logger.debug("phew")  # this will be logged against request_id

LoggingContext implements the context management methods, so the above can be written much more succinctly as:

def handle_request(request_id):
    with context.LoggingContext() as request_context:
        request_context.request = request_id
        do_request_handling()
        logger.debug("finished")

def do_request_handling():
    logger.debug("phew")

Using logcontexts with Deferreds

Deferreds — and in particular, defer.inlineCallbacks — break the linear flow of code so that there is no longer a single entry point where we should set the logcontext and a single exit point where we should remove it.

Consider the example above, where do_request_handling needs to do some blocking operation, and returns a deferred:

@defer.inlineCallbacks
def handle_request(request_id):
    with context.LoggingContext() as request_context:
        request_context.request = request_id
        yield do_request_handling()
        logger.debug("finished")

In the above flow:

• The logcontext is set
• do_request_handling is called, and returns a deferred
• handle_request yields the deferred
• The inlineCallbacks wrapper of handle_request returns a deferred

So we have stopped processing the request (and will probably go on to start processing the next), without clearing the logcontext.

To circumvent this problem, synapse code assumes that, wherever you have a deferred, you will want to yield on it. To that end, whereever functions return a deferred, we adopt the following conventions:

Rules for functions returning deferreds:

• If the deferred is already complete, the function returns with the same logcontext it started with.
• If the deferred is incomplete, the function clears the logcontext before returning; when the deferred completes, it restores the logcontext before running any callbacks.

That sounds complicated, but actually it means a lot of code (including the example above) "just works". There are two cases:

• If do_request_handling returns a completed deferred, then the logcontext will still be in place. In this case, execution will continue immediately after the yield; the "finished" line will be logged against the right context, and the with block restores the original context before we return to the caller.

• If the returned deferred is incomplete, do_request_handling clears the logcontext before returning. The logcontext is therefore clear when handle_request yields the deferred. At that point, the inlineCallbacks wrapper adds a callback to the deferred, and returns another (incomplete) deferred to the caller, and it is safe to begin processing the next request.

  Once do_request_handling's deferred completes, it will reinstate the logcontext, before running the callback added by the inlineCallbacks wrapper. That callback runs the second half of handle_request, so again the "finished" line will be logged against the right context, and the with block restores the original context.

As an aside, it's worth noting that handle_request follows our rules -though that only matters if the caller has its own logcontext which it cares about.

The following sections describe pitfalls and helpful patterns when implementing these rules.

Always yield your deferreds

Whenever you get a deferred back from a function, you should yield on it as soon as possible. (Returning it directly to your caller is ok too, if you're not doing inlineCallbacks.) Do not pass go; do not do any logging; do not call any other functions.

@defer.inlineCallbacks
def fun():
    logger.debug("starting")
    yield do_some_stuff()       # just like this

    d = more_stuff()
    result = yield d            # also fine, of course

    return result

def nonInlineCallbacksFun():
    logger.debug("just a wrapper really")
    return do_some_stuff()      # this is ok too - the caller will yield on
                                # it anyway.

Provided this pattern is followed all the way back up to the callchain to where the logcontext was set, this will make things work out ok: provided do_some_stuff and more_stuff follow the rules above, then so will fun (as wrapped by inlineCallbacks) and nonInlineCallbacksFun.

It's all too easy to forget to yield: for instance if we forgot that do_some_stuff returned a deferred, we might plough on regardless. This leads to a mess; it will probably work itself out eventually, but not before a load of stuff has been logged against the wrong context. (Normally, other things will break, more obviously, if you forget to yield, so this tends not to be a major problem in practice.)

Of course sometimes you need to do something a bit fancier with your Deferreds - not all code follows the linear A-then-B-then-C pattern. Notes on implementing more complex patterns are in later sections.

Where you create a new Deferred, make it follow the rules

Most of the time, a Deferred comes from another synapse function. Sometimes, though, we need to make up a new Deferred, or we get a Deferred back from external code. We need to make it follow our rules.

The easy way to do it is with a combination of defer.inlineCallbacks, and context.PreserveLoggingContext. Suppose we want to implement sleep, which returns a deferred which will run its callbacks after a given number of seconds. That might look like:

# not a logcontext-rules-compliant function
def get_sleep_deferred(seconds):
    d = defer.Deferred()
    reactor.callLater(seconds, d.callback, None)
    return d

That doesn't follow the rules, but we can fix it by wrapping it with PreserveLoggingContext and yield ing on it:

@defer.inlineCallbacks
def sleep(seconds):
    with PreserveLoggingContext():
        yield get_sleep_deferred(seconds)

This technique works equally for external functions which return deferreds, or deferreds we have made ourselves.

You can also use context.make_deferred_yieldable, which just does the boilerplate for you, so the above could be written:

def sleep(seconds):
    return context.make_deferred_yieldable(get_sleep_deferred(seconds))

Fire-and-forget

Sometimes you want to fire off a chain of execution, but not wait for its result. That might look a bit like this:

@defer.inlineCallbacks
def do_request_handling():
    yield foreground_operation()

    # *don't* do this
    background_operation()

    logger.debug("Request handling complete")

@defer.inlineCallbacks
def background_operation():
    yield first_background_step()
    logger.debug("Completed first step")
    yield second_background_step()
    logger.debug("Completed second step")

The above code does a couple of steps in the background after do_request_handling has finished. The log lines are still logged against the request_context logcontext, which may or may not be desirable. There are two big problems with the above, however. The first problem is that, if background_operation returns an incomplete Deferred, it will expect its caller to yield immediately, so will have cleared the logcontext. In this example, that means that 'Request handling complete' will be logged without any context.

The second problem, which is potentially even worse, is that when the Deferred returned by background_operation completes, it will restore the original logcontext. There is nothing waiting on that Deferred, so the logcontext will leak into the reactor and possibly get attached to some arbitrary future operation.

There are two potential solutions to this.

One option is to surround the call to background_operation with a PreserveLoggingContext call. That will reset the logcontext before starting background_operation (so the context restored when the deferred completes will be the empty logcontext), and will restore the current logcontext before continuing the foreground process:

@defer.inlineCallbacks
def do_request_handling():
    yield foreground_operation()

    # start background_operation off in the empty logcontext, to
    # avoid leaking the current context into the reactor.
    with PreserveLoggingContext():
        background_operation()

    # this will now be logged against the request context
    logger.debug("Request handling complete")

Obviously that option means that the operations done in background_operation would be not be logged against a logcontext (though that might be fixed by setting a different logcontext via a with LoggingContext(...) in background_operation).

The second option is to use context.run_in_background, which wraps a function so that it doesn't reset the logcontext even when it returns an incomplete deferred, and adds a callback to the returned deferred to reset the logcontext. In other words, it turns a function that follows the Synapse rules about logcontexts and Deferreds into one which behaves more like an external function — the opposite operation to that described in the previous section. It can be used like this:

@defer.inlineCallbacks
def do_request_handling():
    yield foreground_operation()

    context.run_in_background(background_operation)

    # this will now be logged against the request context
    logger.debug("Request handling complete")

Passing synapse deferreds into third-party functions

A typical example of this is where we want to collect together two or more deferred via defer.gatherResults:

d1 = operation1()
d2 = operation2()
d3 = defer.gatherResults([d1, d2])

This is really a variation of the fire-and-forget problem above, in that we are firing off d1 and d2 without yielding on them. The difference is that we now have third-party code attached to their callbacks. Anyway either technique given in the Fire-and-forget section will work.

Of course, the new Deferred returned by gatherResults needs to be wrapped in order to make it follow the logcontext rules before we can yield it, as described in Where you create a new Deferred, make it follow the rules.

So, option one: reset the logcontext before starting the operations to be gathered:

@defer.inlineCallbacks
def do_request_handling():
    with PreserveLoggingContext():
        d1 = operation1()
        d2 = operation2()
        result = yield defer.gatherResults([d1, d2])

In this case particularly, though, option two, of using context.preserve_fn almost certainly makes more sense, so that operation1 and operation2 are both logged against the original logcontext. This looks like:

@defer.inlineCallbacks
def do_request_handling():
    d1 = context.preserve_fn(operation1)()
    d2 = context.preserve_fn(operation2)()

    with PreserveLoggingContext():
        result = yield defer.gatherResults([d1, d2])

Was all this really necessary?

The conventions used work fine for a linear flow where everything happens in series via defer.inlineCallbacks and yield, but are certainly tricky to follow for any more exotic flows. It's hard not to wonder if we could have done something else.

We're not going to rewrite Synapse now, so the following is entirely of academic interest, but I'd like to record some thoughts on an alternative approach.

I briefly prototyped some code following an alternative set of rules. I think it would work, but I certainly didn't get as far as thinking how it would interact with concepts as complicated as the cache descriptors.

My alternative rules were:

• functions always preserve the logcontext of their caller, whether or not they are returning a Deferred.
• Deferreds returned by synapse functions run their callbacks in the same context as the function was orignally called in.

The main point of this scheme is that everywhere that sets the logcontext is responsible for clearing it before returning control to the reactor.

So, for example, if you were the function which started a with LoggingContext block, you wouldn't yield within it — instead you'd start off the background process, and then leave the with block to wait for it:

def handle_request(request_id):
    with context.LoggingContext() as request_context:
        request_context.request = request_id
        d = do_request_handling()

    def cb(r):
        logger.debug("finished")

    d.addCallback(cb)
    return d

(in general, mixing with LoggingContext blocks and defer.inlineCallbacks in the same function leads to slighly counter-intuitive code, under this scheme).

Because we leave the original with block as soon as the Deferred is returned (as opposed to waiting for it to be resolved, as we do today), the logcontext is cleared before control passes back to the reactor; so if there is some code within do_request_handling which needs to wait for a Deferred to complete, there is no need for it to worry about clearing the logcontext before doing so:

def handle_request():
    r = do_some_stuff()
    r.addCallback(do_some_more_stuff)
    return r

— and provided do_some_stuff follows the rules of returning a Deferred which runs its callbacks in the original logcontext, all is happy.

The business of a Deferred which runs its callbacks in the original logcontext isn't hard to achieve — we have it today, in the shape of context._PreservingContextDeferred:

def do_some_stuff():
    deferred = do_some_io()
    pcd = _PreservingContextDeferred(LoggingContext.current_context())
    deferred.chainDeferred(pcd)
    return pcd

It turns out that, thanks to the way that Deferreds chain together, we automatically get the property of a context-preserving deferred with defer.inlineCallbacks, provided the final Defered the function yields on has that property. So we can just write:

@defer.inlineCallbacks
def handle_request():
    yield do_some_stuff()
    yield do_some_more_stuff()

To conclude: I think this scheme would have worked equally well, with less danger of messing it up, and probably made some more esoteric code easier to write. But again — changing the conventions of the entire Synapse codebase is not a sensible option for the marginal improvement offered.

A note on garbage-collection of Deferred chains

It turns out that our logcontext rules do not play nicely with Deferred chains which get orphaned and garbage-collected.

Imagine we have some code that looks like this:

listener_queue = []

def on_something_interesting():
    for d in listener_queue:
        d.callback("foo")

@defer.inlineCallbacks
def await_something_interesting():
    new_deferred = defer.Deferred()
    listener_queue.append(new_deferred)

    with PreserveLoggingContext():
        yield new_deferred

Obviously, the idea here is that we have a bunch of things which are waiting for an event. (It's just an example of the problem here, but a relatively common one.)

Now let's imagine two further things happen. First of all, whatever was waiting for the interesting thing goes away. (Perhaps the request times out, or something even more interesting happens.)

Secondly, let's suppose that we decide that the interesting thing is never going to happen, and we reset the listener queue:

def reset_listener_queue():
    listener_queue.clear()

So, both ends of the deferred chain have now dropped their references, and the deferred chain is now orphaned, and will be garbage-collected at some point. Note that await_something_interesting is a generator function, and when Python garbage-collects generator functions, it gives them a chance to clean up by making the yield raise a GeneratorExit exception. In our case, that means that the __exit__ handler of PreserveLoggingContext will carefully restore the request context, but there is now nothing waiting for its return, so the request context is never cleared.

To reiterate, this problem only arises when both ends of a deferred chain are dropped. Dropping the the reference to a deferred you're supposed to be calling is probably bad practice, so this doesn't actually happen too much. Unfortunately, when it does happen, it will lead to leaked logcontexts which are incredibly hard to track down.