zulip/docs/subsystems/dependencies.md

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.

Provisioning

We refer to "provisioning" as the process of installing and configuring the dependencies of a Zulip development environment. It's done using 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 have 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.

One case where we apply added scrutiny to third-party dependencies is JS libraries. They are a particularly important concern because we want to keep the Zulip webapp's JS bundle small, so that Zulip continues to load quickly on systems with low network bandwidth. We'll look at large JS libraries with much greater scrutiny for whether their functionality justifies their size than Python dependencies, since an extra 50KB of code usually doesn't matter in the backend, but does in JavaScript.

System packages

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 usually means 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 few places:

• For production, in our puppet configuration, puppet/zulip/, using the Package and SafePackage directives.
• For development, in APT_DEPENDENCIES in tools/lib/provision.py.
• The packages needed to build a Zulip virtualenv, in VENV_DEPENDENCIES 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 pgroonga postgres extension, used by our full-text search.

Python packages

We manage Python packages via the Python-standard requirements.txt 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 scripts/lib/setup_venv.py

• Using pip to manage dependencies. This is standard in the Python ecosystem, and means we only need to record a list of versions in a requirements.txt file to declare what we're using. Since we have a few different installation targets, we maintain several requirements.txt format files in the requirements/ directory (e.g. dev.txt for development, prod.txt for production, docs.txt for ReadTheDocs, common.txt for the vast majority of packages common to prod and development, etc.). We use pip install --no-deps to 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.txt files. 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 requirements/ directory.
• 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 requirements.txt file (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, 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.
• Pinning versions of indirect dependencies. We "pin" or "lock" the versions of our indirect dependencies files with tools/update-locked-requirements (powered by pip-compile). What this means is that we have some "source" requirements files, like requirements/common.txt, 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 compile to generate the prod_lock.txt and dev_lock.txt 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 requirements/README.md.
• 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.py to ensure that our Django code always uses the correct virtualenv as well.

JavaScript and other frontend packages

We use the same set of strategies described for Python dependencies for most of our JavaScript dependencies, so we won't repeat the reasoning here.

• In a fashion very analogous to the Python codebase, scripts/lib/node_cache.py manages cached node_modules directories in /srv/zulip-npm-cache. Each is named by its hash, computed by the generate_sha1sum_node_modules function. scripts/lib/clean-npm-cache handles garbage-collection.
• We use yarn, a pip-like tool for JavaScript, to download most JavaScript dependencies. Yarn talks to standard the [npm][] repository. We use the standard package.json file to declare our direct dependencies, with sections for development and production. Yarn takes care of pinning the versions of indirect dependencies in the yarn.lock file; yarn upgrade updates the yarn.lock files.
• tools/update-prod-static. This process is discussed in detail in the static asset pipeline article, but we don't use the node_modules directories 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_modules directory 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 JavaScript dependencies that we have copied into the main Zulip repository under static/third, often with patches. These date from an era before npm existed. 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 installer for yarn, modified to support installing to a path that is not the current user's home directory).

• nvm has its own system for installing each version of node at its own path, which we use, though we install a /usr/local/bin/node wrapper to access the desired version conveniently and efficiently (nvm has a lot of startup overhead).
• install-yarn.sh is configured to install yarn at /srv/zulip-yarn. We don't do anything special to try to manage multiple versions of yarn.

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.

Emoji

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 node_modules directories, with scripts/lib/clean-emoji-cache 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 emoji infrastructure.

Translations data

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 static/locale/language*.json using manage.py compilemessages, which extends the default Django implementation of that tool.

Pygments data

The list of languages supported by our markdown syntax highlighting comes from the pygments package. tools/setup/build_pygments_data is responsible for generating static/generated/pygments_data.js so that our JavaScript markdown processor has access to the supported list.

Authors data

Zulip maintains data on the developers who have contributed the most to the current version of Zulip in the /about page. These data are fetched using the GitHub API with tools/update-authors-json. In development, it just returns some basic test data to avoid adding load to GitHub's APIs unnecessarily; it's primarily run as part of building a release tarball.

Modifying provisioning

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 tools/lib/provision.py.
• 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).