zulip/docs/subsystems/logging.md

Logging and Error reporting

Having a good system for logging error reporting is essential to making a large project like Zulip successful. Without reliable error reporting, one has to rely solely on bug reports from users in order to produce a working product.

Our goal as a project is to have zero known 500 errors on the backend and zero known JavaScript exceptions on the frontend. While there will always be new bugs being introduced, that goal is impossible without an efficient and effective error reporting framework.

We expect to in the future integrate a service like Sentry to make it easier for very large installations like zulipchat.com to manage their exceptions and ensure they are all tracked down, but our default email-based system is great for small installations.

Backend error reporting

The Django framework provides much of the infrastructure needed by our error reporting system:

• The ability to send emails to the server's administrators with any 500 errors, using the mail_admins function. We enhance these data with extra details (like what user was involved in the error) in zerver/logging_handlers.py, and then send them to the administrator in zerver/lib/error_notify.py (which also supports sending Zulips to a stream about production errors).
• The ability to rate-limit certain errors to avoid sending hundreds of emails in an outage (see _RateLimitFilter in zerver/lib/logging_util.py)
• A nice framework for filtering passwords and other important user data from the exception details, which we use in zerver/filters.py.
• Middleware for handling JsonableError, our system for allowing code anywhere in Django to report an API-facing json_error from anywhere in a view code path.

Since 500 errors in any Zulip server are usually a problem the server administrator should investigate and/or report upstream, we have this email reporting system configured to report errors by default.

Backend logging

Django's logging system uses the standard Python logging infrastructure. We have configured them so that logging.exception and logging.error get emailed to the server maintainer, while logging.warning will just appear in /var/log/zulip/errors.log. Lower log levels just appear in the main server log (as well as in the log for corresponding process, be it django.log for the main Django processes or the appropriate events_* log file for a queue worker).

Backend logging format

The main Zulip server log contains a line for each backend request. It also contains warnings, errors, and the full tracebacks for any Python exceptions. In production, it goes to /var/log/zulip/server.log; in development, it goes to the terminal where you run run-dev.py.

In development, it's good to keep an eye on the run-dev.py console as you work on backend changes, since it's a great way to notice bugs you just introduced.

In production, one usually wants to look at errors.log for errors since the main server log can be very verbose, but the main server log can be extremely valuable for investigating performance problems.

2016-05-20 14:50:22.056 INFO [zr] 127.0.0.1       GET     302 528ms (db: 1ms/1q) (+start: 123ms) / (unauth via ?)
[20/May/2016 14:50:22]"GET / HTTP/1.0" 302 0
2016-05-20 14:50:22.272 INFO [zr] 127.0.0.1       GET     200 124ms (db: 3ms/2q) /login/ (unauth via ?)
2016-05-20 14:50:26.333 INFO [zr] 127.0.0.1       POST    302  37ms (db: 6ms/7q) /accounts/login/local/ (unauth via ?)
[20/May/2016 14:50:26]"POST /accounts/login/local/ HTTP/1.0" 302 0
2016-05-20 14:50:26.538 INFO [zr] 127.0.0.1       GET     200  12ms (db: 1ms/2q) (+start: 53ms) /api/v1/events [1463769771:0/0] (cordelia@zulip.com via internal)
2016-05-20 14:50:26.657 INFO [zr] 127.0.0.1       GET     200  10ms (+start: 8ms) /api/v1/events [1463769771:0/0] (cordelia@zulip.com via internal)
2016-05-20 14:50:26.959 INFO [zr] 127.0.0.1       GET     200 588ms (db: 26ms/21q) / [1463769771:0] (cordelia@zulip.com via website)

The format of this output is:

• Timestamp
• Log level
• Logger name, abbreviated as "zr" for these Zulip request logs
• IP address
• HTTP method
• HTTP status code
• Time to process
• (Optional perf data details, e.g. database time/queries, memcached time/queries, Django process startup time, markdown processing time, etc.)
• Endpoint/URL from zproject/urls.py
• "email via client" showing user account involved (if logged in) and the type of client they used ("web", "Android", etc.).

The performance data details are particularly useful for investigating performance problems, since one can see at a glance whether a slow request was caused by delays in the database, in the markdown processor, in memcached, or in other Python code.

One useful thing to note, however, is that the database time is only the time spent connecting to and receiving a response from the database. Especially when response are large, there can often be a great deal of Python processing overhead to marshall the data from the database into Django objects that is not accounted for in these numbers.

Blueslip frontend error reporting

We have a custom library, called blueslip (named after the form used at MIT to report problems with the facilities), that takes care of reporting JavaScript errors. In production, this means emailing the server administrators (though the setting controlling this, BROWSER_ERROR_REPORTING, is disabled by default, since most problems are unlikely to be addressable by a system administrator, and it's very hard to make JavaScript errors not at least somewhat spammy due to the variety of browser versions and sets of extensions that someone might use). In development, this means displaying a highly visible overlay over the message view area, to make exceptions in testing a new feature hard to miss.

• Blueslip is implemented in static/js/blueslip.js.
• In order to capture essentially any error occurring in the browser, blueslip does the following:
  • Wraps every function passed into $.ready(), i.e., every on-webapp-startup method used by Zulip.
  • Wraps every jQuery AJAX request handler used by Zulip.
  • Wraps every function passed into $.on(), i.e. all event handlers declared in Zulip.
  • Declares a default browser exception handler.
  • Has methods for being manually triggered by Zulip JavaScript code for warnings and assertion failures.
• Blueslip keeps a log of all the notices it has received during a browser session, and includes them in reports to the server, so that one can see cases where exceptions chained together. You can print this log from the browser console using blueslip.get_log().

Blueslip supports several error levels:

• blueslip.fatal: For fatal errors that cannot be easily recovered from. We try to avoid using it, since it kills the current JS thread, rather than returning execution to the caller. Unhandled exceptions in our JS code are treated like blueslip.fatal.
• blueslip.error: For logging of events that are definitely caused by a bug and thus sufficiently important to be reported, but where we can handle the error without creating major user-facing problems (e.g. an exception when handling a presence update).
• blueslip.warn: For logging of events that are a problem but not important enough to send an email about in production. They are, however, highlighted in the JS console in development.
• blueslip.log (and blueslip.info): Logged to the JS console in development and also in the blueslip log in production. Useful for data that might help discern what state the browser was in during an error (e.g. whether the user was in a narrow).
• blueslip.debug: Similar to blueslip.log, but are not printed to the JS console in development.

Frontend performance reporting

In order to make it easier to debug potential performance problems in the critically latency-sensitive message sending code pathway, we log and report to the server the following whenever a message is sent:

• The time the user triggered the message (aka the start time).
• The time the send_message response returned from the server.
• The time the message was received by the browser from the get_events protocol (these last two race with each other).
• Whether the message was locally echoed.
• If so, whether there was a disparity between the echoed content and the server-rendered content, which can be used for statistics on how effective our local echo system is.

The code is all in zerver/lib/report.py and static/js/sent_messages.js.

We have similar reporting for the time it takes to narrow / switch to a new view:

• The time the action was initiated
• The time when the updated message feed was visible to the user
• The time when the browser was idle again after switching views (intended to catch issues where we generate a lot of deferred work).