What is a widget?

Widgets are special kinds of messages. These include:

  • polls

  • TODO lists

  • /me messages

  • Trivia bot

Some widgets are used with a leading / (like /poll Tea or coffee?), similar to slash commands, but these two concepts are very different. Slash commands have nothing to do with message sending.

The trivia_quiz_bot does not use /’s. Instead, it sends “extra_data” in messages to invoke zforms (which enable button-based UIs in the messages).

/me messages

These are the least complex. We use our markdown processors to detect if a message is a /me message, plumb the flag through the message object (as is_me_message) and have the clients format it correctly. Related code (for the web app) lies in message_list_view.js in _maybe_format_me_message.

Polls, todo lists, and games

The most interactive widgets that we built during 2018 are for polls, todo lists, and games. You launch widgets by sending one of the following messages:

  • /poll

  • /todo

The web app client provides the “widget experience” by default. Other clients just show raw messages like “/poll”, and should be adding support for widgets soon.

Our customers have long requested a poll/survey widget. See this issue. There are workaround ways to do polls using things like emoji reactions, but our poll widget provides a more interactive experience.

Data flow

Some important code entities for the widget implementation are:

  • SubMessage database table

  • /json/submessage API endpoint

  • web/src/submessage.js

  • web/src/poll_widget.js

  • web/src/widgetize.js

  • web/src/zform.js

  • web/templates/widgets/

  • zerver/lib/widget.py

  • zerver/views/submessage.py

Both poll and todo widgets use the “submessage” architecture. We’ll use the poll widget as an example.

The SubMessage table, as the name indicates, allows you to associate multiple submessages to any given Message row. When a message gets sent, there’s a hook inside of widget.py that will detect widgets like “/poll”. If a message needs to be widgetized, an initial SubMessage row will be created with an appropriate msg_type (and persisted to the database). This data will also be included in the normal Zulip message event payload. Clients can choose to ignore the submessage-related data, in which case they’ll gracefully degrade to seeing “/poll”. Of course, the web app client actually recognizes the appropriate widgets.

The web app client will next collect poll options and votes from users. The web app client has code in submessage.js that dispatches events to widgetize.js, which in turn sends events to individual widgets. The widgets know how to render themselves and set up click/input handlers to collect data. They can then post back to /json/submessage to attach more data to the message (and the details are encapsulated with a callback). The server will continue to persist SubMessage rows in the database. These rows are encoded as JSON, and the schema of the messages is driven by the individual widgets. Most of the logic is in the client; things are fairly opaque to the server at this point.

If a client joins Zulip after a message has accumulated several submessage events, it will see all of those events the first time it sees the parent message. Clients need to know how to build/rebuild their state as each submessage comes in. They also need to tolerate misformatted data, ideally just dropping data on the floor. If a widget throws an exception, it’s caught before the rest of the message feed is affected.

As far as rendering is concerned, each widget module is given a parent elem when its activate function is called. This is just a <div> inside of the parent message in the message pane. The widget has access to jQuery and template.render, and the developer can create new templates in web/templates/widgets/.

A good way to learn the system is to read the code in web/src/poll_widget.js. It is worth noting that writing a new widget requires only minor backend changes in the current architecture. This could change in the future, but for now, a frontend developer mostly needs to know JS, CSS, and HTML.

It may be useful to think of widgets in terms of a bunch of clients exchanging peer-to-peer messages. The server’s only real role is to decide who gets delivered which submessages. It’s a lot like a “subchat” system.

Backward compatibility

Our “submessage” widgets are still evolving, and we want to have a plan for allowing future progress without breaking old messages.

Widget developers can revise code to improve a widget’s visual polish without too much concern for breaking how old messages get widgetized. They will need to be more cautious if they change the actual data structures passed around in the submessage payloads.

For significant schema changes, it would be worthwhile to add some kind of versioning scheme inside of SubMessages, either at the DB level or more at the JSON level within fields. This has yet to be designed. One thing to consider is that most widgets are somewhat ephemeral in nature, so it’s not the end of the world if upgrades cause some older messages to be obsolete, as long as the code degrades gracefully.

Mission-critical widgets should have a deprecation strategy. For example, you could add optional features for one version bump and then only make them mandatory for the next version, as long as you don’t radically change the data model. And if you’re truly making radical changes, you can always write a Django migration for the SubMessage data.

Adding widgets

Right now we don’t have a plugin model for the above widgets; they are served up by the core Zulip server implementation. Of course, anybody who wishes to build their own widget has the option of forking the server code and self-hosting, but we want to encourage folks to submit widget code to our codebase in PRs. If we get to a critical mass of contributed widgets, we will want to explore a more dynamic mechanism for “plugging in” code from outside sources, but that is not in our immediate roadmap.

This is sort of a segue to the next section of this document. Suppose you want to write your own custom bot, and you want to allow users to click buttons to respond to options, but you don’t want to have to modify the Zulip server codebase to turn on those features. This is where our “zform” architecture comes to the rescue.

zform (trivia quiz bot)

This section will describe our “zform” architecture.

For context, imagine a naive trivia bot. The trivia bot sends a question with the answers labeled as A, B, C, and D. Folks who want to answer the bot send back an answer have to send an actual Zulip message with something like @trivia_bot answer A to Q01, which is kind of tedious to type. Wouldn’t it be nice if the bot could serve up some kind of buttons with canned replies, so that the user just hits a button?

That is where zforms come in. Zulip’s trivia bot sends the Zulip server a JSON representation of a form it wants rendered, and then the client renders a generic “zform” with buttons corresponding to short_name fields inside a choices list inside of the JSON payload.

Here is what an example payload looks like:

    "extra_data": {
        "type": "choices",
        "heading": "05: What color is a blueberry?",
        "choices": [
                "type": "multiple_choice",
                "reply": "answer 05 A",
                "long_name": "red",
                "short_name": "A"
                "type": "multiple_choice",
                "reply": "answer 05 B",
                "long_name": "blue",
                "short_name": "B"
                "type": "multiple_choice",
                "reply": "answer 05 C",
                "long_name": "yellow",
                "short_name": "C"
                "type": "multiple_choice",
                "reply": "answer 05 D",
                "long_name": "orange",
                "short_name": "D"
    "widget_type": "zform"

When users click on the buttons, generic click handlers automatically simulate a client reply using a field called reply (in choices) as the content of the message reply. Then the bot sees the reply and grades the answer using ordinary chat-bot coding.

The beautiful thing is that any third party developer can enhance bots that are similar to the trivia_quiz bot without touching any Zulip code, because zforms are completely generic.

Data flow

We can walk through the steps from the bot generating the zform to the client rendering it.

First, here is the code that produces the JSON.

def format_quiz_for_widget(quiz_id: str, quiz: Dict[str, Any]) -> str:
    widget_type = 'zform'
    question = quiz['question']
    answers = quiz['answers']

    heading = quiz_id + ': ' + question

    def get_choice(letter: str) -> Dict[str, str]:
        answer = answers[letter]
        reply = 'answer ' + quiz_id + ' ' + letter

        return dict(

    choices = [get_choice(letter) for letter in 'ABCD']

    extra_data = dict(

    widget_content = dict(
    payload = json.dumps(widget_content)
    return payload

The above code processes data that is specific to a trivia quiz, but it follows a generic schema.

The bot sends the JSON payload to the server using the send_reply callback.

The bot framework looks for the optional widget_content parameter in send_reply and includes that in the message payload it sends to the server.

The server validates the schema of widget_content using check_widget_content.

Then code inside of zerver/lib/widget.py builds a single SubMessage row to contain the zform payload, and the server also sends this payload to all clients who are recipients of the parent message.

When the message gets to the client, the codepath for zform is actually quite similar to what happens with a more customized widget like poll. (In fact, zform is a sibling of poll and zform just has a somewhat more generic job to do.) In web/src/widgetize.js you will see where this code converges, with snippets like this:

widgets.poll = poll_widget;
widgets.todo = todo_widget;
widgets.zform = zform;

The code in web/src/zform.js renders the form (not shown here) and then sets up a click handler like below:

    $elem.find('button').on('click', function (e) {

        // Grab our index from the markup.
        var idx = $(e.target).attr('data-idx');

        // Use the index from the markup to dereference our
        // data structure.
        var reply_content = data.choices[idx].reply;

            message: opts.message,
            content: reply_content,

And then we are basically done!