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 provide integration with Sentry to make it easier for very large installations like zulip.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) inzerver/logging_handlers.py
, and then send them to the administrator inzerver/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
inzerver/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
, which is our standard system for API code to return a JSON-format HTTP error response.
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.
Zulip’s optional Sentry integration will aggregate errors to show which users and realms are affected, any logging which happened prior to the exception, local variables in each frame of the exception, and the full request headers which triggered it.
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@zulip 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@zulip via ?)
2016-05-20 14:50:26.333 INFO [zr] 127.0.0.1 POST 302 37ms (db: 6ms/7q) /accounts/login/local/ (unauth@zulip 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 POST 200 12ms (db: 1ms/2q) (+start: 53ms) /api/v1/events/internal [1463769771:0/0] (8@zulip via internal)
2016-05-20 14:50:26.657 INFO [zr] 127.0.0.1 POST 200 10ms (+start: 8ms) /api/v1/events/internal [1463769771:0/0] (8@zulip via internal)
2016-05-20 14:50:26.959 INFO [zr] 127.0.0.1 GET 200 588ms (db: 26ms/21q) / [1463769771:0] (8@zulip 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.
Searching backend log files
Zulip comes with a tool, ./scripts/log-search
, to quickly search
through the main server.log
log file based on a number of different
axes – including client IP address, client user-id, request path,
response code. It can also search the NGINX logs, which provide
similar information.
Because often the requests to static resources, or to things like user avatars, are not as important, they are also filtered out of the output by default.
The output shows timestamp, request duration, client IP address, response code, and request method, hostname, and path; any property which is limited by the tool is not displayed, for conciseness:
zulip@example-prod:~/deployments/current$ ./scripts/log-search realm-name
22:30:36.593 1ms 2606:2800:220:1:248:1893:25c8:1946 302 GET /
22:30:42.508 366ms 2606:2800:220:1:248:1893:25c8:1946 200 GET /login/
23:18:30.977 1ms 93.184.216.34 302 GET /
23:18:31.286 132ms 93.184.216.34 200 GET /login/
23:18:48.094 20ms 93.184.216.34 200 GET /accounts/password/reset/
23:18:51.520 149ms 93.184.216.34 200 GET /login/
23:18:59.420 20ms 93.184.216.34 200 GET /accounts/password/reset/
23:19:02.929 1300ms 93.184.216.34 302 POST /accounts/password/reset/
23:19:03.056 93ms 93.184.216.34 200 GET /accounts/password/reset/done/
23:19:08.911 26ms 93.184.216.34 302 GET /accounts/password/reset/OA/b56jfp-bd80ee99b98e703456b3bdcd91892be2/
23:19:09.189 116ms 93.184.216.34 200 GET /accounts/password/reset/OA/set-password/
23:19:18.743 215ms 93.184.216.34 302 POST /accounts/password/reset/OA/set-password/
23:19:18.779 12ms 93.184.216.34 200 GET /accounts/password/done/
23:19:20.796 12ms 93.184.216.34 200 GET /accounts/login/
23:19:29.323 295ms 8@ 93.184.216.34 302 POST /accounts/login/
23:19:29.704 362ms 8@ 93.184.216.34 200 GET /
23:20:04.980 110ms 8@ 93.184.216.34 200 DELETE /json/users/me/subscriptions
zulip@example-prod:~/deployments/current$ ./scripts/log-search 2606:2800:220:1:248:1893:25c8:1946
22:30:36.593 1ms 302 GET https://realm-one.example-prod.example.com/
22:30:42.508 366ms 200 GET https://realm-one.example-prod.example.com/login/
See ./scripts/log-search --help
for complete documentation.
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 listens for the
error
event onwindow
, and 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 = require("./static/js/blueslip"); blueslip.get_log()
.
Blueslip supports several error levels:
throw new Error(…)
: 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.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
(andblueslip.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 toblueslip.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).