Provisioning and third-party dependencies¶
Zulip is a large project, with well over 100 third-party dependencies, and managing them well is essential to the quality of the project. In this document, we discuss the various classes of dependencies that Zulip has, and how we manage them. Zulip’s dependency management has some really nice properties:
- Fast provisioning. When switching to a different commit in the Zulip project with the same dependencies, it takes under 10 seconds to re-provision a working Zulip development environment after switching. If there are new dependencies, one only needs to wait to download the new ones, not all the pre-existing dependencies.
- Consistent provisioning. Every time a Zulip development or production environment is provisioned/installed, it should end up using the exactly correct versions of all major dependencies.
- Low maintenance burden. To the extent possible, we want to avoid manual work and keeping track of things that could be automated. This makes it easy to keep running the latest versions of our various dependencies.
The purpose of this document is to detail all of Zulip’s third-party dependencies and how we manage their versions.
We refer to “provisioning” as the process of installing and
configuring the dependencies of a Zulip development environment. It’s
tools/provision, and the output is conveniently logged by
var/log/provision.log to help with debugging. Provisioning makes
use of a lot of caching. Some of those caches are not immune to being
corrupted if you mess around with files in your repository a lot. We
tools/provision --force to (still fairly quickly) rerun most
steps that would otherwise have been skipped due to caching.
In the Vagrant development environment,
vagrant provision will run
the provision script;
vagrant up will boot the machine, and will
also run an initial provision the first time only.
Philosophy on adding third-party dependencies¶
In the Zulip project, we take a pragmatic approach to third-party
dependencies. Overall, if a third-party project does something well
that Zulip needs to do (and has an appropriate license), we’d love to
use it rather than reinventing the wheel. If the third-party project
needs some small changes to work, we prefer to make those changes and
contribute them upstream. When the upstream maintainer is slow to
respond, we may use a fork of the dependency until the code is merged
upstream; as a result, we usually have a few packages in
requirements.txt that are installed from a GitHub URL.
What we look for in choosing dependencies is whether the project is well-maintained. Usually one can tell fairly quickly from looking at a project’s issue tracker how well-managed it is: a quick look at how the issue tracker is managed (or not) and the test suite is usually enough to decide if a project is going to be a high-maintenance dependency or not. That said, we do still take on some smaller dependencies that don’t have a well-managed project, if we feel that using the project will still be a better investment than writing our own implementation of that project’s functionality. We’ve adopted a few projects in the past that had a good codebase but whose maintainer no longer had time for them.
For the third-party services like Postgres, Redis, Nginx, and RabbitMQ
that are documented in the
architecture overview, we rely on the
versions of those packages provided alongside the Linux distribution
on which Zulip is deployed. Because Zulip
only supports Ubuntu in production, this
apt, though we do support
other platforms in development. Since
we don’t control the versions of these dependencies, we avoid relying
on specific versions of these packages wherever possible.
The exact lists of
apt packages needed by Zulip are maintained in a
- For production, in our puppet configuration,
puppet/zulip/, using the
- For development, in
- The packages needed to build a Zulip virtualenv, in
scripts/lib/setup_venv.py. These are separate from the rest because (1) we may need to install a virtualenv before running the more complex scripts that, in turn, install other dependencies, and (2) because that list is shared between development and production.
We maintain a PPA (personal package archive) with some packages
unique to Zulip (e.g the
tsearch_extras postgres extension) and
backported versions of other dependencies (e.g.
camo, to fix a buggy
init script). Our goal is to shrink or eliminate this PPA where
possible by getting issues addressed in the upstream distributions.
We also rely on the
pgroonga PPA for the
extension, used by our full-text search.
We manage Python packages via the Python-standard
system and virtualenvs, but there’s a number of interesting details
about how Zulip makes this system work well for us that are worth
highlighting. The system is largely managed by the code in
pipto manage dependencies. This is standard in the Python ecosystem, and means we only need to record a list of versions in a
requirements.txtfile to declare what we’re using. Since we have a few different installation targets, we maintain several
requirements.txtformat files in the
common.infor the vast majority of packages common to prod and development, etc.). We use
pip install --no-depsto ensure we only install the packages we explicitly declare as dependencies.
- virtualenv with pinned versions. For a large application like
Zulip, it is important to ensure that we’re always using consistent,
predictable versions of all of our Python dependencies. To ensure
this, we install our dependencies in a virtualenv that contains
only the packages and versions that Zulip needs, and we always pin
exact versions of our dependencies in our
requirements.txtfiles. We pin exact versions, not minimum versions, so that installing Zulip won’t break if a dependency makes a buggy release. A side effect is that it’s easy to debug problems caused by dependency upgrades, since we’re always doing those upgrades with an explicit commit updating the
- Pinning versions of indirect dependencies. We “pin” or “lock”
the versions of our indirect dependencies files with
pip-compile). What this means is that we have some “source” requirements files, like
requirements/common.in, that declare the packages that Zulip depends on directly. Those packages have their own recursive dependencies. When adding or removing a dependency from Zulip, one simply edits the appropriate “source” requirements files, and then runs
tools/update-locked-requirements. That tool will use
pip compileto generate the locked requirements files like
dev.txtetc files that explicitly declare versions of all of Zulip’s recursive dependencies. For indirect dependencies (i.e. dependencies not explicitly declared in the source requirements files), it provides helpful comments explaining which direct dependency (or dependencies) needed that indirect dependency. The process for using this system is documented in more detail in
- Caching of virtualenvs and packages. To make updating the
dependencies of a Zulip installation efficient, we maintain a cache
of virtualenvs named by the hash of the relevant
scripts/lib/hash_reqs.py). These caches live under
/srv/zulip-venv-cache/<hash>. That way, when re-provisioning a development environment or deploying a new production version with the same Python dependencies, no downloading or installation is required: we just use the same virtualenv. When the only changes are upgraded versions, we’ll use virtualenv-clone to clone the most similar existing virtualenv and then just upgrade the packages needed, making small version upgrades extremely efficient. And finally, we use
pip’s built-in caching to ensure that a specific version of a specific package is only downloaded once.
- Garbage-collecting caches. We have a tool,
scripts/lib/clean_venv_cache.py, which will clean old cached virtualenvs that are no longer in use. In production, the algorithm preserves recent virtualenvs as well as those in use by any current production deployment directory under
/home/zulip/deployments/. This helps ensure that a Zulip installation doesn’t leak large amounts of disk over time.
- Scripts. Often, we want a script running in production to use
the Zulip virtualenv. To make that work without a lot of duplicated
code, we have a helpful library,
scripts/lib/setup_path_on_import.py, which on import will put the currently running Python script into the Zulip virtualenv. This is called by
./manage.pyto ensure that our Django code always uses the correct virtualenv as well.
- In a fashion very analogous to the Python codebase,
/srv/zulip-npm-cache. Each is named by its hash, computed by the
- We use yarn, a
package.jsonfile to declare our direct dependencies, with sections for development and production. Yarn takes care of pinning the versions of indirect dependencies in the
yarn upgradeupdates the
tools/update-prod-static. This process is discussed in detail in the static asset pipeline article, but we don’t use the
node_modulesdirectories directly in production. Instead, static assets are compiled using our static asset pipeline and it is the compiled assets that are served directly to users. As a result, we don’t ship the
node_modulesdirectory in a Zulip production release tarball, which is a good thing, because doing so would more than double the size of a Zulip release tarball.
- Checked-in packages. In contrast with Python, we have a few
static/third, often with patches. These date from an era before
npmexisted. It is a project goal to eliminate these checked-in versions of dependencies and instead use versions managed by the npm repositories.
Node and Yarn¶
These are installed by
scripts/lib/install-node (which in turn uses
the standard third-party
nvm installer to download
node and pin
its version) and
scripts/lib/third/install-yarn.sh (the standard
yarn, modified to support installing to a path that is
not the current user’s home directory).
nvmhas its own system for installing each version of
nodeat its own path, which we use, though we install a
/usr/local/bin/nodewrapper to access the desired version conveniently and efficiently (
nvmhas a lot of startup overhead).
install-yarn.shis configured to install
/srv/zulip-yarn. We don’t do anything special to try to manage multiple versions of
Other third-party and generated files¶
In this section, we discuss the other third-party dependencies, generated code, and other files whose original primary source is not the Zulip server repository, and how we provision and otherwise maintain them.
Zulip uses the iamcal emoji data package for its emoji data
and sprite sheets. We download this dependency using
npm, and then
have a tool,
tools/setup/build_emoji, which reformats the emoji data
into the files under
static/generated/emoji. Those files are in
turn used by our markdown processor and
tools/update-prod-static to make Zulip’s emoji work in the various
environments where they need to be displayed.
Since processing emoji is a relatively expensive operation, as part of
optimizing provisioning, we use the same caching strategy for the
compiled emoji data as we use for virtualenvs and
scripts/lib/clean_emoji_cache.py responsible for
garbage-collection. This caching and garbage-collection is required
because a correct emoji implementation involves over 1000 small image
files and a few large ones. There is a more extended article on our
Zulip’s translations infrastructure generates
several files from the source data, which we manage similar to our
emoji, but without the caching (and thus without the
garbage-collection). New translations data is downloaded from
Transifex and then compiled to generate both the production locale
files and also language data in
manage.py compilemessages, which extends the default Django
implementation of that tool.
When making changes to Zulip’s provisioning process or dependencies, usually one needs to think about making changes in 3 places:
tools/lib/provision.py. This is the main provisioning script, used by most developers to maintain their development environment.
docs/development/dev-setup-non-vagrant.md. This is our “manual installation” documentation. Strategically, we’d like to move the support for more versions of Linux from here into
- Production. Our tools for compiling/generating static assets need
to be called from
tools/update-prod-static, which is called by
tools/build-release-tarball(for doing Zulip releases) as well as
tools/upgrade-zulip-from-git(for deploying a Zulip server off of master).