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Markdown implementation

Zulip uses a special flavor of Markdown/CommonMark for its message formatting. Our Markdown flavor is unique primarily to add important extensions, such as quote blocks and math blocks, and also to do previews and correct issues specific to the chat context. Beyond that, it has a number of minor historical variations resulting from its history predating CommonMark (and thus Zulip choosing different solutions to some problems) and based in part on Python-Markdown, which is proudly a classic Markdown implementation. We reduce these variations with every major Zulip release.

Zulip has two implementations of Markdown. The backend implementation at zerver/lib/markdown/ is based on Python-Markdown and is used to authoritatively render messages to HTML (and implements slow/expensive/complex features like querying the Twitter API to render tweets nicely). The frontend implementation is in JavaScript, based on marked.js (web/src/echo.js), and is used to preview and locally echo messages the moment the sender hits Enter, without waiting for round trip from the server. Those frontend renderings are only shown to the sender of a message, and they are (ideally) identical to the backend rendering.

The JavaScript Markdown implementation has a function, markdown.contains_backend_only_syntax, that is used to check whether a message contains any syntax that needs to be rendered to HTML on the backend. If markdown.contains_backend_only_syntax returns true, the frontend simply won't echo the message for the sender until it receives the rendered HTML from the backend. If there is a bug where markdown.contains_backend_only_syntax returns false incorrectly, the frontend will discover this when the backend returns the newly sent message, and will update the HTML based on the authoritative backend rendering (which would cause a change in the rendering that is visible only to the sender shortly after a message is sent). As a result, we try to make sure that markdown.contains_backend_only_syntax is always correct.

Testing

The Python-Markdown implementation is tested by zerver/tests/test_markdown.py, and the marked.js implementation and markdown.contains_backend_only_syntax are tested by web/tests/markdown.test.js.

A shared set of fixed test data ("test fixtures") is present in zerver/tests/fixtures/markdown_test_cases.json, and is automatically used by both test suites; as a result, it is the preferred place to add new tests for Zulip's Markdown system. Some important notes on reading this file:

expected_output is the expected output for the backend Markdown processor.
When the frontend processor doesn't support a feature and it should just be rendered on the backend, we set backend_only_rendering to true in the fixtures; this will automatically verify that markdown.contains_backend_only_syntax rejects the syntax, ensuring it will be rendered only by the backend processor.
When the two processors disagree, we set marked_expected_output in the fixtures; this will ensure that the syntax stays that way. If the differences are important (i.e. not just whitespace), we should also open an issue on GitHub to track the problem.
For mobile push notifications, we need a text version of the rendered content, since the APNS and GCM push notification systems don't support richer markup. Mostly, this involves stripping HTML, but there's some syntax we take special care with. Tests for what this plain-text version of content should be stored in the text_content field.

If you're going to manually test some changes in the frontend Markdown implementation, the easiest way to do this is as follows:

Log in to your development server.
Stop your Zulip server with Ctrl-C, leaving the browser open.
Compose and send the messages you'd like to test. They will be locally echoed using the frontend rendering.

This procedure prevents any server-side rendering. If you don't do this, backend will likely render the Markdown you're testing and swap it in before you can see the frontend's rendering.

If you are working on a feature that breaks multiple testcases, and want to debug the testcases one by one, you can add "ignore": true to any testcases in markdown_test_cases.json that you want to ignore. This is a workaround due to lack of comments support in JSON. Revert your "ignore" changes before committing. After this, you can run the frontend tests with tools/test-js-with-node markdown and backend tests with tools/test-backend zerver.tests.test_markdown.MarkdownTest.test_markdown_fixtures.

Changing Zulip's Markdown processor

First, you will likely find these third-party resources helpful:

Python-Markdown is the Markdown library used by Zulip as a base to build our custom Markdown syntax upon.
Python's XML ElementTree is the part of the Python standard library used by Python Markdown and any custom extensions to generate and modify the output HTML.

When changing Zulip's Markdown syntax, you need to update several places:

The backend Markdown processor (zerver/lib/markdown/__init__.py).
The frontend Markdown processor (web/src/markdown.ts and sometimes web/third/marked/lib/marked.js), or markdown.contains_backend_only_syntax if your changes won't be supported in the frontend processor.
If desired, the typeahead logic in web/src/composebox_typeahead.ts.
The test suite, probably via adding entries to zerver/tests/fixtures/markdown_test_cases.json.
The in-app Markdown documentation (markdown_help_rows in web/src/info_overlay.ts).
The list of changes to Markdown at the end of this document.

Important considerations for any changes are:

Security: A bug in the Markdown processor can lead to XSS issues. For example, we should not insert unsanitized HTML from a third-party web application into a Zulip message.
Uniqueness: We want to avoid users having a bad experience due to accidentally triggering Markdown syntax or typeahead that isn't related to what they are trying to express.
Performance: Zulip can render a lot of messages very quickly, and we'd like to keep it that way. New regular expressions similar to the ones already present are unlikely to be a problem, but we need to be thoughtful about expensive computations or third-party API requests.
Database: The backend Markdown processor runs inside a Python thread (as part of how we implement timeouts for third-party API queries), and for that reason we currently should avoid making database queries inside the Markdown processor. This is a technical implementation detail that could be changed with a few days of work, but is an important detail to know about until we do that work.
Testing: Every new feature should have both positive and negative tests; they're easy to write and give us the flexibility to refactor frequently.

Per-realm features

Zulip's Markdown processor's rendering supports a number of features that depend on realm-specific or user-specific data. For example, the realm could have linkifiers or custom emoji configured, and Zulip supports mentions for channels, users, and user groups (which depend on data like users' names, IDs, etc.).

At a backend code level, these are controlled by the message_realm object and other arguments passed into do_convert (sent_by_bot, translate_emoticons, mention_data, etc.). Because Python-Markdown doesn't support directly passing arguments into the Markdown processor, our logic attaches these data to the Markdown processor object via e.g. _md_engine.zulip_db_data, and then individual Markdown rules can access the data from there.

For non-message contexts (e.g. an organization's profile (aka the thing on the right-hand side of the login page), channel descriptions, or rendering custom profile fields), one needs to just pass in a message_realm (see, for example, zulip_default_context for the organization profile code for this). But for messages, we need to pass in attributes like sent_by_bot and translate_emoticons that indicate details about how the user sending the message is configured.

Zulip's Markdown philosophy

Note that this discussion is based on a comparison with the original Markdown, not newer Markdown variants like CommonMark.

Markdown is great for group chat for the same reason it's been successful in products ranging from blogs to wikis to bug trackers: it's close enough to how people try to express themselves when writing plain text (e.g. emails) that it helps more than getting in the way.

The main issue for using Markdown in instant messaging is that the Markdown standard syntax used in a lot of wikis/blogs has nontrivial error rates, where the author needs to go back and edit the post to fix the formatting after typing it the first time. While that's basically fine when writing a blog, it gets annoying very fast in a chat product; even though you can edit messages to fix formatting mistakes, you don't want to be doing that often. There are basically 2 types of error rates that are important for a product like Zulip:

What fraction of the time, if you pasted a short technical email that you wrote to your team and passed it through your Markdown implementation, would you need to change the text of your email for it to render in a reasonable way? This is the "accidental Markdown syntax" problem, common with Markdown syntax like the italics syntax interacting with talking about char *s.
What fraction of the time do users attempting to use a particular Markdown syntax actually succeed at doing so correctly? Syntax like required a blank line between text and the start of a bulleted list raise this figure substantially.

Both of these are minor issues for most products using Markdown, but they are major problems in the instant messaging context, because one can't edit a message that has already been sent before others read it and users are generally writing quickly. Zulip's Markdown strategy is based on the principles of giving users the power they need to express complicated ideas in a chat context while minimizing those two error rates.

Zulip's changes to Markdown

Below, we document the changes that Zulip has against stock Python-Markdown; some of the features we modify / disable may already be non-standard.

Note This section has not been updated in a few years and is not accurate.

Basic syntax

Enable nl2br extension: this means one newline creates a line break (not paragraph break).
Allow only * syntax for italics, not _. This resolves an issue where people were using _ and hitting it by mistake too often. Asterisks surrounded by spaces won't trigger italics, either (e.g. with stock Markdown You should use char * instead of void * there would produce undesired results).
Allow only ** syntax for bold, not __ (easy to hit by mistake if discussing Python __init__ or something).
Add ~~ syntax for strikethrough.
Disable special use of \ to escape other syntax. Rendering \\ as \ was hugely controversial, but having no escape syntax is also controversial. We may revisit this. For now you can always put things in code blocks.

Lists

Allow tacking a bulleted list or block quote onto the end of a paragraph, i.e. without a blank line before it.
Allow only * for bulleted lists, not + or - (previously created confusion with diff-style text sloppily not included in a code block).
Disable ordered list syntax: stock Markdown automatically renumbers, which can be really confusing when sending a numbered list across multiple messages.

Code

Enable fenced code block extension, with syntax highlighting.
Disable line-numbering within fenced code blocks -- the <table> output confused our web client code.

Other

Disable headings, both # foo and == foo == syntax: they don't make much sense for chat messages.
Disabled images with ![]() (images from links are shown as an inline preview).
Allow embedding any avatar as a tiny (list bullet size) image. This is used primarily by version control integrations.
We added the ~~~ quote block quote syntax.

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