447 lines
17 KiB
ReStructuredText
447 lines
17 KiB
ReStructuredText
Log contexts
|
|
============
|
|
|
|
.. contents::
|
|
|
|
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.util.logcontext`` 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:
|
|
|
|
.. code:: python
|
|
|
|
from synapse.util import logcontext # omitted from future snippets
|
|
|
|
def handle_request(request_id):
|
|
request_context = logcontext.LoggingContext()
|
|
|
|
calling_context = logcontext.LoggingContext.current_context()
|
|
logcontext.LoggingContext.set_current_context(request_context)
|
|
try:
|
|
request_context.request = request_id
|
|
do_request_handling()
|
|
logger.debug("finished")
|
|
finally:
|
|
logcontext.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:
|
|
|
|
.. code:: python
|
|
|
|
def handle_request(request_id):
|
|
with logcontext.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:
|
|
|
|
.. code:: python
|
|
|
|
@defer.inlineCallbacks
|
|
def handle_request(request_id):
|
|
with logcontext.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.
|
|
|
|
.. code:: python
|
|
|
|
@defer.inlineCallbacks
|
|
def fun():
|
|
logger.debug("starting")
|
|
yield do_some_stuff() # just like this
|
|
|
|
d = more_stuff()
|
|
result = yield d # also fine, of course
|
|
|
|
defer.returnValue(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 content. (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
|
|
``logcontext.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:
|
|
|
|
.. code:: python
|
|
|
|
# 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:
|
|
|
|
.. code:: python
|
|
|
|
@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 ``logcontext.make_deferred_yieldable``, which just does the
|
|
boilerplate for you, so the above could be written:
|
|
|
|
.. code:: python
|
|
|
|
def sleep(seconds):
|
|
return logcontext.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:
|
|
|
|
.. code:: python
|
|
|
|
@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:
|
|
|
|
.. code:: python
|
|
|
|
@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 ``logcontext.preserve_fn``, 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:
|
|
|
|
.. code:: python
|
|
|
|
@defer.inlineCallbacks
|
|
def do_request_handling():
|
|
yield foreground_operation()
|
|
|
|
logcontext.preserve_fn(background_operation)()
|
|
|
|
# this will now be logged against the request context
|
|
logger.debug("Request handling complete")
|
|
|
|
XXX: I think ``preserve_context_over_fn`` is supposed to do the first option,
|
|
but the fact that it does ``preserve_context_over_deferred`` on its results
|
|
means that its use is fraught with difficulty.
|
|
|
|
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``:
|
|
|
|
.. code:: python
|
|
|
|
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:
|
|
|
|
.. code:: python
|
|
|
|
@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
|
|
``logcontext.preserve_fn`` almost certainly makes more sense, so that
|
|
``operation1`` and ``operation2`` are both logged against the original
|
|
logcontext. This looks like:
|
|
|
|
.. code:: python
|
|
|
|
@defer.inlineCallbacks
|
|
def do_request_handling():
|
|
d1 = logcontext.preserve_fn(operation1)()
|
|
d2 = logcontext.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:
|
|
|
|
.. code:: python
|
|
|
|
def handle_request(request_id):
|
|
with logcontext.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:
|
|
|
|
.. code:: python
|
|
|
|
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
|
|
``logcontext._PreservingContextDeferred``:
|
|
|
|
.. code:: python
|
|
|
|
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:
|
|
|
|
.. code:: python
|
|
|
|
@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.
|