Travis CI

The Zulip server uses Travis CI for its continuous integration. This page documents useful tools and tips to know about when using Travis CI and debugging issues with it.


The overall goal of our Travis CI setup is to avoid regressions and minimize the total time spent debugging Zulip. We do that by trying to catch as many possible future bugs as possible, while minimizing both latency and false positives, both of which can waste a lot of developer time. There are a few implications of this overall goal:

  • If a test is failing nondeterministically in Travis CI, we consider that to be an urgent problem.
  • If the tests become a lot slower, that is also an urgent problem.
  • Everything we do in CI should also have a way to run it quickly (under 1 minute, preferably under 3 seconds), in order to iterate fast in development. Except when working on the Travis CI configuration itself, a developer should never have to repeatedly wait 10 minutes for a full Travis run to iteratively debug something.


The main Travis configuration file is .travis.yml. The specific test suites we have are listed in the matrix section, which has a matrix of Python versions and test suites ($TEST_SUITE). We’ve configured it to use a few helper scripts for each job:

  • tools/travis/setup-$TEST_SUITE: The script that sets up the test environment for that suite (E.g., installing dependencies).
    • For the backend and frontend suites, this is a thin wrapper around tools/provision, aka the development environment provision script.
    • For the production suite, this is a more complicated process because of all the packages Travis installs. See the comments in tools/travis/setup-production for details.
  • tools/travis/$TEST_SUITE: The script that runs the actual test suite.

The main purpose of the distinction between the two is that if the setup-backend job fails, Travis CI will report it as the suite having “Errored” (grey in their emails), whereas if the backend job fails, it’ll be reported as “Failed” failure (red in their emails). Note that Travis CI’s web UI seems to make no visual distinction between these.

An important detail is that Travis CI will by default hide most phases other than the actual test; you can see this easily by looking at the line numbers in the Travis CI output. There are actually a bunch of phases (e.g. the project’s setup job, downloading caches near the beginning, uploading caches at the end, etc.), and if you’re debugging our configuration, you’ll want to look at these closely.

Useful debugging tips and tools

  • Zulip uses the ts tool to log the current time on every line of the output in our Travis CI scripts. You can use this output to determine which steps are actually consuming a lot of time.
  • For performance issues, this statistics tool can give you test runtime history data that can help with determining when a performance issue was introduced and whether it was fixed. Note you need to click the “Run” button for it to do anything.
  • You can sign up your personal repo for Travis CI so that every remote branch you push will be tested, which can be helpful when debugging something complicated.

Performance optimizations


An important element of making Travis CI perform effectively is caching the provisioning of a Zulip development environment. In particular, we cache the following across jobs:

  • Python virtualenvs
  • node_modules directories
  • Built/downloaded emoji sprite sheets and data

This has a huge impact on the performance of running tests in Travis CI; without these caches, the average test time would be several times longer.

We have designed these caches carefully (they are also used in production and the Zulip development environment) to ensure that each is named by a hash of its dependencies, so Zulip should always be using the same version of dependencies it would have used had the cache not existed. In practice, bugs are always possible, so be mindful of this possibility.

A consequence of this caching is that test jobs for branches which modify package.json, requirements/, and other key dependencies will be significantly slower than normal, because they won’t get to benefit from the cache.

Uninstalling packages

In the production suite, we run apt-get upgrade at some point (effectively, because the Zulip installer does). This carries a huge performance cost in Travis CI, because (1) they don’t keep their test systems up to date and (2) literally everything is installed in their build workers (e.g. several copies of Postgres, Java, MySQL, etc.).

In order to make Zulip’s tests performance reasonably well, we uninstall (or mark with apt-mark hold) many of these dependencies that are irrelevant to Zulip in tools/travis/setup-production.