zulip/docs/subsystems/typing-indicators.md

# Typing indicators

Zulip supports a feature called "typing indicators."

Typing indicators are status messages (or visual indicators) that
tell you when another user is writing a message to you. Zulip's
typing UI is similar to what you see in other chat/text systems.

This document describes how we have implemented the feature in
the Zulip web app, and our main audience is developers who want to
understand the system and possibly improve it. Any client should
be able follow the protocol documented here.

Typing indicators are implemented for both direct message conversations
and channel conversations in the web app.

There are two major roles for users in this system:

- The "writing user" is composing a message.
- The "receiving user" is waiting to receive a message (or possibly
  ready to shift their attention elsewhere).

Any Zulip user can play either one of these roles, and sometimes
they can be playing both roles at once. Having said that, you
can generally understand the system in terms of a single message
being composed by the "writing user."

On a high level the typing indicators system works like this:

- The client for the "writing user" sends requests to the server.
- The server broadcasts events to other users.
- The clients for "receiving users" receive events and conditionally
  show typing indicators, depending on where the clients are narrowed.

## Privacy settings

Note that there is a user-level privacy setting to disable sending
typing notifications that a client should check when implementing
the "writing user" protocol below. See `send_private_typing_notifications`
in the `UserBaseSettings` model in `zerver/models/users.py` and in the
`user_settings` object in the `POST /register` response.

## Writing user

When a "writing user" starts to compose a message, the client
sends a request to `POST /typing` with an `op` of `start` and
a list of potential message recipients. The web app function
that facilitates this is called `send_typing_notification_ajax`.

If the "writing user" is composing a long message, we want to send
repeated updates to the server so that downstream clients know the
user is still typing. Zulip messages tend to be longer than
messages in other chat/text clients, so this detail is important.

We have a small state machine in `web/shared/src/typing_status.ts`
that makes sure subsequent "start" requests get sent out. The
frequency of these requests is determined by
`server_typing_started_wait_period_milliseconds` in the
`POST /register` response.

If the "writing user" goes for a while without any text input,
then we send a request to `POST /typing` with an `op` of `stop`.
The time period a client should wait before sending the request
is determined by `server_typing_stopped_wait_period_milliseconds`
in the `POST /register` response. We also immediately send "stop"
notification when the user explicitly aborts composing a message
by closing the compose box (or other actions).

A common scenario is that a user is just pausing to think for a few
seconds, but they still intend to finish the message. Of course,
that's hard to distinguish from the scenario of the user got pulled
away from their desk. For the former case, where the "writing user"
completes the message with lots of pauses for thinking, a series of
"start" and "stop" messages may be sent over time. Timeout values
reflect tradeoffs, where we have to guess how quickly people type,
how long they pause to think, and how frequently they get interrupted.

## Server

The server piece of typing notifications is currently pretty
straightforward, since we take advantage of Zulip's
[events system](events-system.md).

We deliberately designed the server piece to be stateless,
which minimizes the possibility of backend bugs and gives clients
more control over the user experience.

As such, the server piece here is basically a single Django view
function with a small bit of library support to send out events
to clients.

Requests come in to `send_notification_backend`, which is in
`zerver/views/typing.py`. For direct message typing notifications,
the call to `check_send_typing_notification` does the heavy lifting.

One of the main things that the server does is to validate that
the user IDs in the `to` parameter are for valid, active users in
the realm.

Once the request has been validated, the server sends events to
potential recipients of the message. The event type for that
payload is `typing`. See the function `do_send_typing_notification`
in `zerver/actions/typing.py` for more details.

For channel typing notifications, the server also handles the logic
for determining which users should receive the typing events based
on channel subscribers.

## Receiving user

When a user plays the role of a "receiving user," the client handles
incoming "typing" events from the server, and the client will
display a typing indicator only when both of these conditions are
true:

- The "writing user" is still likely typing.
- The "receiving user" is in a view where they'd see the eventual
  message.

The client code starts by processing events, and it maintains data
structures, and then it eventually shows or hides status messages.

We'll describe the flow of data through the web app
as a concrete example.

The events will come in to `web/src/server_events_dispatch.js`.
The `stop` and `start` operations get further handled by
`web/src/typing_events.ts`.

The main goal is then to triage which events should lead to
display changes.

The web app client maintains a list of incoming "typists" using
code in `web/src/typing_data.ts`. The API here has functions
like the following:

- `add_typist`
- `remove_typist`
- `get_group_typists`
- `get_all_direct_message_typists`

One subtle thing that the client has to do here is to maintain
timers for typing notifications. The value of
`server_typing_started_expiry_period_milliseconds` in the
`POST /register` response is used to determine when the
"writing user" has abandoned the message. Of course, the
"writing user" will also explicitly send us `stop` notifications
at certain times.

When it finally comes to displaying the notification, the web
app eventually calls `render_notifications_for_narrow`.

## Ecosystem

Even though the server is stateless, any developer working on
a client needs to be mindful of timing/network considerations
that affect the overall system.

In general, client developers should agree on timeout parameters
for how frequently we "kickstart" typing notifications for users
sending long messages. This means standardizing the "writing
user" piece of the system. It's possible that certain clients
will have slightly different mechanisms for detecting that users
have abandoned messages, but the re-transmit frequency should be
similar.

When implementing the "receiving user" piece, it's important to
realize how clients behave on the other end of the protocol. It's
possible, for example, to never receive a "stop" notification
from a client that was shut down abruptly. You should allow
reasonable amounts of time for the other side to send notifications,
allowing for network delays and server delays, but you should
not let the notifications become too "sticky" either.

## Roadmap

An area for refinement is to tune the timing values a bit.
Right now, we are possibly too aggressive about sending `stop`
messages when users are just pausing to think. It's possible
to better account for typing speed or other heuristic things
like how much of the message has already been typed.