Zulip architectural overview
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In this overview, we’ll mainly discuss the core Zulip server and web application.
Usage assumptions and concepts
Zulip is a real-time team chat application meant to provide a great experience for a wide range of organizations, from companies to volunteer projects to groups of friends, ranging in size from a small team to 10,000s of users. It has hundreds of features both larger and small, and supports dedicated apps for iOS, Android, Linux, Windows, and macOS, all modern web browsers, several cross-protocol chat clients, and numerous dedicated Zulip API clients (e.g. bots).
A server can host multiple Zulip realms (organizations), each on its own (sub)domain. While most installations host only one organization, some such as zulip.com host thousands. Each organization is a private chamber with its own users, streams, customizations, and so on. This means that one person might be a user of multiple Zulip realms. The administrators of an organization have a great deal of control over who can register an account, what permissions new users have, etc. For more on security considerations and options, see the security model section and the Zulip Help Center.
Django and Tornado
Django is the main web application server; Tornado runs the server-to-client real-time push system. The app servers are configured by the Supervisor configuration (which explains how to start the server processes; see “Supervisor” below) and the nginx configuration (which explains which HTTP requests get sent to which app server).
Tornado is an asynchronous server and is meant specifically to hold open tens of thousands of long-lived (long-polling) connections – that is to say, routes that maintain a persistent connection from every running client. For this reason, it’s responsible for event (message) delivery, but not much else. We try to avoid any blocking calls in Tornado because we don’t want to delay delivery to thousands of other connections (as this would make Zulip very much not real-time). For instance, we avoid doing cache or database queries inside the Tornado code paths, since those blocking requests carry a very high performance penalty for a single-threaded, asynchronous server system. (In principle, we could do non-blocking requests to those services, but the Django-based database libraries we use in most of our codebase using don’t support that, and in any case, our architecture doesn’t require Tornado to do that).
The parts that are activated relatively rarely (e.g. when people type or click on something) are processed by the Django application server.
There is detailed documentation on the
real-time push and event queue system; most of
the code is in
nginx is the front-end web server to all Zulip traffic; it serves static
assets and proxies to Django and Tornado. It handles HTTP requests
according to the rules laid down in the many config files found in
zulip/puppet/zulip/files/nginx/zulip-include-frontend/app is the most
important of these files. It explains what happens when requests come in
In production, all requests to URLs beginning with
/static/are served from the corresponding files in
/home/zulip/prod-static/, and the production build process (
tools/build-release-tarball) compiles, minifies, and installs the static assets into the
prod-static/tree form. In development, files are served directly from
/static/in the Git repository.
/api/v1/events, i.e. the real-time push system, are sent to the Tornado server.
Requests to all other paths are sent to the Django app running via
By default (i.e. if
LOCAL_UPLOADS_DIRis set), nginx will serve user-uploaded content like avatars, custom emoji, and uploaded files. However, one can configure Zulip to store these in a cloud storage service like Amazon S3 instead.
Note that we do not use
nginx in the development environment, opting
for a simple Tornado-based proxy instead.
We use supervisord to start server processes, restart them automatically if they crash, and direct logging.
The config file is
is where Tornado and Django are set up, as well as a number of background
processes that process event queues. We use event queues for the kinds
of tasks that are best run in the background because they are
expensive (in terms of performance) and don’t have to be synchronous
— e.g., sending emails or updating analytics. Also see the queuing
memcached is used to cache database model
manage putting things into memcached, and invalidating the cache when
values change. The memcached configuration is in
puppet/zulip/files/memcached.conf. See our
caching guide to learn how this works in
Redis is used for a few very short-term data stores, primarily our rate-limiting system.
Redis is configured in
zulip/puppet/zulip/files/redis and it’s a
pretty standard configuration except for the last line, which turns off
# Zulip-specific configuration: disable saving to disk. save ""
People often wonder if we could replace memcached with Redis (or replace RabbitMQ with Redis, with some loss of functionality).
The answer is likely yes, but it wouldn’t improve Zulip. Operationally, our current setup is likely easier to develop and run in production than a pure Redis system would be. Meanwhile, the perceived benefit for using Redis is usually to reduce memory consumption by running fewer services, and no such benefit would materialize:
Our cache uses significant memory, but that memory usage would be essentially the same with Redis as it is with memcached.
All of these services have low minimum memory requirements, and in fact our applications for Redis and RabbitMQ do not use significant memory even at scale.
We would likely need to run multiple Redis services (with different configurations) in order to ensure the pure LRU use case (memcached) doesn’t push out data that we want to persist until expiry (Redis-based rate limiting) or until consumed (RabbitMQ-based queuing of deferred work).
RabbitMQ is a queueing system. Its config files live in
zulip/puppet/zulip/files/rabbitmq. Initial configuration happens in
We use RabbitMQ for queuing expensive work (e.g. sending emails triggered by a message, push notifications, some analytics, etc.) that require reliable delivery but which we don’t want to do on the main thread. It’s also used for communication between the application server and the Tornado push system.
Two simple wrappers around
pika (the Python RabbitMQ client) are in
zulip/zerver/lib/queue.py. There’s an asynchronous client for use in
Tornado and a more general client for use elsewhere. Most of the
processes started by Supervisor are queue processors that continually
pull things out of a RabbitMQ queue and handle them; they are defined
Also see the queuing guide.
PostgreSQL is the database that stores all persistent data, that is, data that’s expected to live beyond a user’s current session. Starting with Zulip 3.0, new Zulip installations will install modern PostgreSQL release rather than using the version included with the operating system.
In production, PostgreSQL is installed with a default configuration. The
directory that would contain configuration files
puppet/zulip/files/postgresql) has only a utility script and a custom
list of stopwords used by a PostgreSQL extension.
In a development environment, configuration of that PostgreSQL
extension is handled by
tools/postgresql-init-dev-db (invoked by
tools/provision). That file also manages setting up the
development PostgreSQL user.
tools/provision also invokes
to create the actual database with its schema.
Nagios is an optional component used for notifications to the system administrator, e.g., in case of outages.
zulip/puppet/zulip/manifests/nagios.pp installs Nagios plugins from
This component is intended to install Nagios plugins intended to be run
on a Nagios server; most of the Zulip Nagios plugins are intended to be
run on the Zulip servers themselves, and are included with the relevant
component of the Zulip server (e.g.
puppet/zulip/manifests/postgresql_backups.pp installs a few under
This section gives names for some of the elements in the Zulip UI used in Zulip development conversations. In general, our goal is to minimize the set of terminology listed here by giving elements self-explanatory names.
bankruptcy: When a user has been off Zulip for several days and has hundreds of unread messages, they are prompted for whether they want to mark all their unread messages as read. This is called “declaring bankruptcy” (in reference to the concept in finance).
chevron: A small downward-facing arrow next to a message’s timestamp, offering contextual options, e.g., “Reply”, “Mute [this topic]”, or “Link to this conversation”. To avoid visual clutter, the chevron only appears in the web UI upon hover.
ellipsis: A small vertical three dot icon (technically called as ellipsis-v), present in sidebars as a menu icon. It offers contextual options for global filters (All messages and Starred messages), stream filters and topics in left sidebar and users in right sidebar. To avoid visual clutter ellipsis only appears in the web UI upon hover.
huddle: What the codebase calls a “group private message”.
message editing: If the realm admin allows it, then after a user posts a message, the user has a few minutes to click “Edit” and change the content of their message. If they do, Zulip adds a marker such as “(EDITED)” at the top of the message, visible to anyone who can see the message.
realm: What the codebase calls an “organization” in the UI.
recipient bar: A visual indication of the context of a message or group of messages, displaying the stream and topic or private message recipient list, at the top of a group of messages. A typical 1-line message to a new recipient shows to the user as three lines of content: first the recipient bar, second the sender’s name and avatar alongside the timestamp (and, on hover, the star and the chevron), and third the message content. The recipient bar is or contains hyperlinks to help the user narrow.
star: Zulip allows a user to mark any message they can see, public or private, as “starred”. A user can easily access messages they’ve starred through the “Starred messages” link in the left sidebar, or use “is:starred” as a narrow or a search constraint. Whether a user has or has not starred a particular message is private; other users and realm admins don’t know whether a message has been starred, or by whom.
subject: What the codebase calls a “topic” in many places.