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| -rw-r--r-- | synapse/logging/opentracing.py | 125 |
1 files changed, 125 insertions, 0 deletions
diff --git a/synapse/logging/opentracing.py b/synapse/logging/opentracing.py index 415040f5ee..3da33d7826 100644 --- a/synapse/logging/opentracing.py +++ b/synapse/logging/opentracing.py @@ -24,6 +24,131 @@ # this move the methods have work very similarly to opentracing's and it should only # be a matter of few regexes to move over to opentracing's access patterns proper. +""" +============================ +Using OpenTracing in Synapse +============================ + +Python-specific tracing concepts are at https://opentracing.io/guides/python/. +Note that Synapse wraps OpenTracing in a small module (this one) in order to make the +OpenTracing dependency optional. That means that the access patterns are +different to those demonstrated in the OpenTracing guides. However, it is +still useful to know, especially if OpenTracing is included as a full dependency +in the future or if you are modifying this module. + + +OpenTracing is encapsulated so that +no span objects from OpenTracing are exposed in Synapse's code. This allows +OpenTracing to be easily disabled in Synapse and thereby have OpenTracing as +an optional dependency. This does however limit the number of modifiable spans +at any point in the code to one. From here out references to `opentracing` +in the code snippets refer to the Synapses module. + +Tracing +------- + +In Synapse it is not possible to start a non-active span. Spans can be started +using the ``start_active_span`` method. This returns a scope (see +OpenTracing docs) which is a context manager that needs to be entered and +exited. This is usually done by using ``with``. + +.. code-block:: python + + from synapse.logging.opentracing import start_active_span + + with start_active_span("operation name"): + # Do something we want to tracer + +Forgetting to enter or exit a scope will result in some mysterious and grievous log +context errors. + +At anytime where there is an active span ``opentracing.set_tag`` can be used to +set a tag on the current active span. + +Tracing functions +----------------- + +Functions can be easily traced using decorators. There is a decorator for +'normal' function and for functions which are actually deferreds. The name of +the function becomes the operation name for the span. + +.. code-block:: python + + from synapse.logging.opentracing import trace, trace_deferred + + # Start a span using 'normal_function' as the operation name + @trace + def normal_function(*args, **kwargs): + # Does all kinds of cool and expected things + return something_usual_and_useful + + # Start a span using 'deferred_function' as the operation name + @trace_deferred + @defer.inlineCallbacks + def deferred_function(*args, **kwargs): + # We start + yield we_wait + # we finish + defer.returnValue(something_usual_and_useful) + +Operation names can be explicitly set for functions by using +``trace_using_operation_name`` and +``trace_deferred_using_operation_name`` + +.. code-block:: python + + from synapse.logging.opentracing import ( + trace_using_operation_name, + trace_deferred_using_operation_name + ) + + @trace_using_operation_name("A *much* better operation name") + def normal_function(*args, **kwargs): + # Does all kinds of cool and expected things + return something_usual_and_useful + + @trace_deferred_using_operation_name("Another exciting operation name!") + @defer.inlineCallbacks + def deferred_function(*args, **kwargs): + # We start + yield we_wait + # we finish + defer.returnValue(something_usual_and_useful) + +Contexts and carriers +--------------------- + +There are a selection of wrappers for injecting and extracting contexts from +carriers provided. Unfortunately OpenTracing's three context injection +techniques are not adequate for our inject of OpenTracing span-contexts into +Twisted's http headers, EDU contents and our database tables. Also note that +the binary encoding format mandated by OpenTracing is not actually implemented +by jaeger_client v4.0.0 - it will silently noop. +Please refer to the end of ``logging/opentracing.py`` for the available +injection and extraction methods. + +Homeserver whitelisting +----------------------- + +Most of the whitelist checks are encapsulated in the modules's injection +and extraction method but be aware that using custom carriers or crossing +unchartered waters will require the enforcement of the whitelist. +``logging/opentracing.py`` has a ``whitelisted_homeserver`` method which takes +in a destination and compares it to the whitelist. + +======= +Gotchas +======= + +- Checking whitelists on span propagation +- Inserting pii +- Forgetting to enter or exit a scope +- Span source: make sure that the span you expect to be active across a + function call really will be that one. Does the current function have more + than one caller? Will all of those calling functions have be in a context + with an active span? +""" + import contextlib import logging import re |