mirror of https://github.com/zulip/zulip.git
341 lines
16 KiB
Markdown
341 lines
16 KiB
Markdown
Zulip architectural overview
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============================
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Key Codebases
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-------------
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The core Zulip application is at
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[<https://github.com/zulip/zulip>](https://github.com/zulip/zulip) and
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is a web application written in Python 3.x and using the Django framework. That
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codebase includes server-side code and the web client, as well as Python API
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bindings and most of our integrations with other services and applications (see
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[the directory structure guide](../overview/directory-structure.md)).
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[Zulip Mobile](https://github.com/zulip/zulip-mobile) is the official
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mobile Zulip client supporting both iOS and Android, written in
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JavaScript with React Native, and
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[Zulip Desktop](https://github.com/zulip/zulip-electron) is the
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official Zulip desktop client for macOS, Linux, and Windows.
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We also maintain several separate repositories for integrations and
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other glue code: a
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[Hubot adapter](https://github.com/zulip/hubot-zulip); integrations
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with [Phabricator](https://github.com/zulip/phabricator-to-zulip),
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[Jenkins](https://github.com/zulip/zulip-jenkins-plugin),
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[Puppet](https://github.com/matthewbarr/puppet-zulip),
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[Redmine](https://github.com/zulip/zulip-redmine-plugin), and
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[Trello](https://github.com/zulip/trello-to-zulip);
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[node.js API bindings](https://github.com/zulip/zulip-node); and our
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[full-text search PostgreSQL extension](https://github.com/zulip/tsearch_extras).
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We use [Transifex](https://www.transifex.com/zulip/zulip/) to do
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translations.
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In this overview, we'll mainly discuss the core Zulip server and web
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application.
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Usage assumptions and concepts
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------------------------------
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Zulip is a real-time web-based chat application meant for companies and
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similar groups ranging in size from a small team to more than a thousand
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users. It features real-time notifications, message persistence and
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search, public group conversations (*streams*), private streams,
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private one-on-one and group conversations, inline image previews, team
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presence/buddy lists, a rich API, Markdown message support, and numerous
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integrations with other services. The maintainer team aims to support
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users who connect to Zulip using dedicated iOS, Android, Linux, Windows,
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and macOS clients, as well as people using modern web browsers or
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dedicated Zulip API clients.
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A server can host multiple Zulip *realms* (organizations) at the same
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domain, each of which is a private chamber with its own users,
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streams, customizations, and so on. This means that one person might
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be a user of multiple Zulip realms. The administrators of a realm can
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choose whether to allow anyone to register an account and join, or
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only allow people who have been invited, or restrict registrations to
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members of particular groups (using email domain names or corporate
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single-sign-on login for verification). For more on security
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considerations, see [the security model section](../production/security-model.md).
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The Zulip "All messages" screen is like a chronologically ordered inbox;
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it displays messages, starting at the oldest message that the user
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hasn't viewed yet (for more on that logic, see [the guide to the
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pointer and unread counts](../subsystems/pointer.md)). The "All messages" screen displays
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the most recent messages in all the streams a user has joined (except
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for the streams they've muted), as well as private messages from other
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users, in strict chronological order. A user can *narrow* to view only
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the messages in a single stream, and can further narrow to focus on a
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*topic* (thread) within that stream. Each narrow has its own URL. The
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user can quickly see what conversation they're in -- the stream and
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topic, or the names of the user(s) they're private messaging with
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-- using *the recipient bar* displayed atop each conversation.
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Zulip's philosophy is to provide sensible defaults but give the user
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fine-grained control over their incoming information flow; a user can
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mute topics and streams, and can make fine-grained choices to reduce
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real-time notifications they find irrelevant.
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Components
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----------
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### Django and Tornado
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Zulip is primarily implemented in the
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[Django](https://www.djangoproject.com/) Python web framework. We
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also make use of [Tornado](http://www.tornadoweb.org) for the
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real-time push system.
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Django is the main web application server; Tornado runs the
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server-to-client real-time push system. The app servers are configured
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by the Supervisor configuration (which explains how to start the server
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processes; see "Supervisor" below) and the nginx configuration (which
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explains which HTTP requests get sent to which app server).
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Tornado is an asynchronous server and is meant specifically to hold
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open tens of thousands of long-lived (long-polling or websocket)
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connections -- that is to say, routes that maintain a persistent
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connection from every running client. For this reason, it's
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responsible for event (message) delivery, but not much else. We try to
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avoid any blocking calls in Tornado because we don't want to delay
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delivery to thousands of other connections (as this would make Zulip
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very much not real-time). For instance, we avoid doing cache or
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database queries inside the Tornado code paths, since those blocking
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requests carry a very high performance penalty for a single-threaded,
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asynchronous server system. (In principle, we could do non-blocking
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requests to those services, but the Django-based database libraries we
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use in most of our codebase using don't support that, and in any case,
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our architecture doesn't require Tornado to do that).
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The parts that are activated relatively rarely (e.g. when people type or
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click on something) are processed by the Django application server. One
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exception to this is that Zulip uses websockets through Tornado to
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minimize latency on the code path for **sending** messages.
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There is detailed documentation on the
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[real-time push and event queue system](../subsystems/events-system.md); most of
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the code is in `zerver/tornado`.
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#### HTML templates, JavaScript, etc.
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Zulip's HTML is primarily implemented using two types of HTML
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templates: backend templates (powered by the [Jinja2][] template
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engine used for logged-out ("portico") pages and the webapp's base
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content) and frontend templates (powered by [Handlebars][]) used for
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live-rendering HTML from JavaScript for things like the main message
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feed.
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For more details on the frontend, see our documentation on
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[translation](../translating/translating.md),
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[templates](../subsystems/html-templates.md),
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[directory structure](../overview/directory-structure.md), and
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[the static asset pipeline](../subsystems/front-end-build-process.md).
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[Jinja2]: http://jinja.pocoo.org/
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[Handlebars]: http://handlebarsjs.com/
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### nginx
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nginx is the front-end web server to all Zulip traffic; it serves static
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assets and proxies to Django and Tornado. It handles HTTP requests
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according to the rules laid down in the many config files found in
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`zulip/puppet/zulip/files/nginx/`.
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`zulip/puppet/zulip/files/nginx/zulip-include-frontend/app` is the most
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important of these files. It explains what happens when requests come in
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from outside.
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- In production, all requests to URLs beginning with `/static/` are
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served from the corresponding files in `/home/zulip/prod-static/`,
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and the production build process (`tools/build-release-tarball`)
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compiles, minifies, and installs the static assets into the
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`prod-static/` tree form. In development, files are served directly
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from `/static/` in the git repository.
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- Requests to `/json/events`, `/api/v1/events`, and `/sockjs` are
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sent to the Tornado server. These are requests to the real-time push
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system, because the user's web browser sets up a long-lived TCP
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connection with Tornado to serve as [a channel for push
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notifications](https://en.wikipedia.org/wiki/Push_technology#Long_polling).
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nginx gets the hostname for the Tornado server via
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`puppet/zulip/files/nginx/zulip-include-frontend/upstreams`.
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- Requests to all other paths are sent to the Django app via the UNIX
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socket `unix:/home/zulip/deployments/uwsgi-socket` (defined in
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`puppet/zulip/files/nginx/zulip-include-frontend/upstreams`). We use
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`zproject/wsgi.py` to implement uWSGI here (see
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`django.core.wsgi`).
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- By default (i.e. if `LOCAL_UPLOADS_DIR` is set), nginx will serve
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user-uploaded content like avatars, custom emoji, and uploaded
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files. However, one can configure Zulip to store these in a cloud
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storage service like Amazon S3 instead.
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### Supervisor
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We use [supervisord](http://supervisord.org/) to start server processes,
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restart them automatically if they crash, and direct logging.
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The config file is
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`zulip/puppet/zulip/templates/supervisor/zulip.conf.template.erb`. This
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is where Tornado and Django are set up, as well as a number of background
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processes that process event queues. We use event queues for the kinds
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of tasks that are best run in the background because they are
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expensive (in terms of performance) and don't have to be synchronous
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--- e.g., sending emails or updating analytics. Also see [the queuing
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guide](../subsystems/queuing.md).
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### memcached
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memcached is used to cache database model
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objects. `zerver/lib/cache.py` and `zerver/lib/cache_helpers.py`
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manage putting things into memcached, and invalidating the cache when
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values change. The memcached configuration is in
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`puppet/zulip/files/memcached.conf`. See our
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[caching guide](../subsystems/caching.md) to learn how this works in
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detail.
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### Redis
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Redis is used for a few very short-term data stores, such as in the
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basis of `zerver/lib/rate_limiter.py`, a per-user rate limiting scheme
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[example](http://blog.domaintools.com/2013/04/rate-limiting-with-redis/)),
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and the [email-to-Zulip
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integration](https://zulipchat.com/integrations/doc/email).
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Redis is configured in `zulip/puppet/zulip/files/redis` and it's a
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pretty standard configuration except for the last line, which turns off
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persistence:
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# Zulip-specific configuration: disable saving to disk.
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save ""
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People often wonder if we could replace memcached with redis (or
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replace RabbitMQ with redis, with some loss of functionality).
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The answer is likely yes, but it wouldn't improve Zulip.
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Operationally, our current setup is likely easier to develop and run
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in production than a pure redis system would be. Meanwhile, the
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perceived benefit for using redis is usually to reduce memory
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consumption by running fewer services, and no such benefit would
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materialize:
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* Our cache uses significant memory, but that memory usage would be
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essentially the same with redis as it is with memcached.
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* All of these services have low minimum memory requirements, and in
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fact our applications for redis and RabbitMQ do not use significant
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memory even at scale.
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* We would likely need to run multiple redis services (with different
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configurations) in order to ensure the pure LRU use case (memcached)
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doesn't push out data that we want to persist until expiry
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(redis-based rate limiting) or until consumed (RabbitMQ-based
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queuing of deferred work).
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### RabbitMQ
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RabbitMQ is a queueing system. Its config files live in
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`zulip/puppet/zulip/files/rabbitmq`. Initial configuration happens in
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`zulip/scripts/setup/configure-rabbitmq`.
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We use RabbitMQ for queuing expensive work (e.g. sending emails
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triggered by a message, push notifications, some analytics, etc.) that
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require reliable delivery but which we don't want to do on the main
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thread. It's also used for communication between the application server
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and the Tornado push system.
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Two simple wrappers around `pika` (the Python RabbitMQ client) are in
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`zulip/zerver/lib/queue.py`. There's an asynchronous client for use in
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Tornado and a more general client for use elsewhere. Most of the
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processes started by Supervisor are queue processors that continually
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pull things out of a RabbitMQ queue and handle them; they are defined
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in `zerver/worker/queue_processors.py`.
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Also see [the queuing guide](../subsystems/queuing.md).
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### PostgreSQL
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PostgreSQL (also known as Postgres) is the database that stores all
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persistent data, that is, data that's expected to live beyond a user's
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current session.
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In production, Postgres is installed with a default configuration. The
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directory that would contain configuration files
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(`puppet/zulip/files/postgresql`) has only a utility script and a custom
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list of stopwords used by a Postgresql extension.
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In a development environment, configuration of that postgresql
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extension is handled by `tools/postgres-init-dev-db` (invoked by
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`tools/provision`). That file also manages setting up the
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development postgresql user.
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`tools/provision` also invokes `tools/do-destroy-rebuild-database`
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to create the actual database with its schema.
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### Thumbor and thumbnailing
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We use Thumbor, a popular open source thumbnailing server, to serve
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images (both for inline URL previews and serving uploaded image
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files). See [our thumbnailing docs](../subsystems/thumbnailing.md)
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for more details on how this works.
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### Nagios
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Nagios is an optional component used for notifications to the system
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administrator, e.g., in case of outages.
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`zulip/puppet/zulip/manifests/nagios.pp` installs Nagios plugins from
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`puppet/zulip/files/nagios_plugins/`.
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This component is intended to install Nagios plugins intended to be run
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on a Nagios server; most of the Zulip Nagios plugins are intended to be
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run on the Zulip servers themselves, and are included with the relevant
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component of the Zulip server (e.g.
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`puppet/zulip/manifests/postgres_common.pp` installs a few under
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`/usr/lib/nagios/plugins/zulip_postgres_common`).
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## Glossary
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This section gives names for some of the elements in the Zulip UI used
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in Zulip development conversations. Contributions to extend this list
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are welcome!
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* **chevron**: A small downward-facing arrow next to a message's
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timestamp, offering contextual options, e.g., "Reply", "Mute [this
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topic]", or "Link to this conversation". To avoid visual clutter,
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the chevron only appears in the web UI upon hover.
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* **huddle**: What the codebase calls a "group private message".
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* **message editing**: If the realm admin allows it, then after a user
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posts a message, the user has a few minutes to click "Edit" and
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change the content of their message. If they do, Zulip adds a
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marker such as "(EDITED)" at the top of the message, visible to
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anyone who can see the message.
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* **realm**: What the codebase calls an "organization" in the UI.
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* **recipient bar**: A visual indication of the context of a message
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or group of messages, displaying the stream and topic or private
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message recipient list, at the top of a group of messages. A
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typical 1-line message to a new recipient shows to the user as
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three lines of content: first the recipient bar, second the
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sender's name and avatar alongside the timestamp (and, on hover,
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the star and the chevron), and third the message content. The
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recipient bar is or contains hyperlinks to help the user narrow.
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* **star**: Zulip allows a user to mark any message they can see,
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public or private, as "starred". A user can easily access messages
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they've starred through the "Starred messages" link in the
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left sidebar, or use "is:starred" as a narrow or a search
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constraint. Whether a user has or has not starred a particular
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message is private; other users and realm admins don't know
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whether a message has been starred, or by whom.
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* **subject**: What the codebase calls a "topic" in many places.
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* **bankruptcy**: When a user has been off Zulip for several days and
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has hundreds of unread messages, they are prompted for whether
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they want to mark all their unread messages as read. This is
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called "declaring bankruptcy" (in reference to the concept in
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finance).
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