Using Eliot with Twisted

Eliot provides a variety of APIs to support integration with the Twisted networking framework.

Non-blocking Destinations

eliot.logwriter.ThreadedWriter is a logging destination that wraps a blocking destination and writes to it in a non-reactor thread. This is useful because it keeps the Twisted reactor from blocking, e.g. if you’re writing to a log file and the hard drive is overloaded. ThreadedWriter is a Twisted Service and starting it will call add_destination for you and stopping it will call remove_destination; there is no need to call those directly.

"""
Output an Eliot message to a log file using the threaded log writer.
"""
from __future__ import unicode_literals, print_function

from twisted.internet.task import react

from eliot.logwriter import ThreadedWriter
from eliot import Message, FileDestination


def main(reactor):
    print("Logging to example-eliot.log...")
    logWriter = ThreadedWriter(
        FileDestination(file=open("example-eliot.log", "ab")), reactor)

    # Manually start the service, which will add it as a
    # destination. Normally we'd register ThreadedWriter with the usual
    # Twisted Service/Application infrastructure.
    logWriter.startService()

    # Log a message:
    Message.log(value="hello", another=1)

    # Manually stop the service.
    done = logWriter.stopService()
    return done


if __name__ == '__main__':
    react(main, [])

If you want log rotation you can pass in an eliot.FileDestination wrapping one of the classes from twisted.python.logfile as the destination file.

Trial Integration

If you’re using Twisted’s trial program to run your tests you can redirect your Eliot logs to Twisted’s logs by calling eliot.twisted.redirectLogsForTrial(). This function will automatically detect whether or not it is running under trial. If it is then you will be able to read your Eliot logs in _trial_temp/test.log, where trial writes out logs by default. If it is not running under trial it will not do anything. In addition calling it multiple times has the same effect as calling it once. As a result you can simply call it in your package’s __init__.py and rely on it doing the right thing. Take care if you are separately redirecting Twisted logs to Eliot; you should make sure not to call redirectLogsForTrial in that case so as to prevent infinite loops.

Logging Failures

eliot.writeFailure is the equivalent of eliot.write_traceback, only for Failure instances:

from eliot import writeFailure

class YourClass(object):

    def run(self):
        d = dosomething()
        d.addErrback(writeFailure)

Actions and Deferreds

An additional set of APIs is available to help log actions when using Deferreds. To understand why, consider the following example:

from eliot import startAction

def go():
    action = startAction(action_type=u"yourapp:subsystem:frob")
    with action:
        d = Deferred()
        d.addCallback(gotResult, x=1)
        return d

This has two problems. First, gotResult is not going to run in the context of the action. Second, the action finishes once the with block finishes, i.e. before gotResult runs. If we want gotResult to be run in the context of the action and to delay the action finish we need to do some extra work, and manually wrapping all callbacks would be tedious.

To solve this problem you can use the eliot.twisted.DeferredContext class. It grabs the action context when it is first created and provides the same API as Deferred (addCallbacks and friends), with the difference that added callbacks run in the context of the action. When all callbacks have been added you can indicate that the action should finish after those callbacks have run by calling DeferredContext.addActionFinish. As you would expect, if the Deferred fires with a regular result that will result in success message. If the Deferred fires with an errback that will result in failure message. Finally, you can unwrap the DeferredContext and access the wrapped Deferred by accessing its result attribute.

from eliot import startAction
from eliot.twisted import DeferredContext

def go():
    with startAction(action_type=u"your_type").context() as action:
        d = DeferredContext(Deferred())
        # gotResult(result, x=1) will be called in the context of the action:
        d.addCallback(gotResult, x=1)
        # After gotResult finishes, finish the action:
        d.addActionFinish()
        # Return the underlying Deferred:
        return d.result