mirror of https://github.com/zulip/zulip.git
58 lines
3.0 KiB
Markdown
58 lines
3.0 KiB
Markdown
# Presence
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This document explains the model for Zulip's presence.
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In a chat tool like Zulip, users expect to see the “presence” status
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of other users: is the person I want to talk to currently online? If
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not, were they last online 5 minutes ago, or more like an hour ago, or
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a week? Presence helps set expectations for whether someone is likely
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to respond soon. To a user, this feature can seem like a simple thing
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that should be easy. But presence is actually one of the hardest
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scalability problems for a team chat tool like Zulip.
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There's a lot of performance-related details in the backend and
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network protocol design that we won't get into here. The focus of
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this is what one needs to know to correctly implement a Zulip client's
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presence implementation (e.g. web app, mobile app, terminal client, or
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other tool that's intended to represent whether a user is online and
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using Zulip).
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A client should report to the server every minute a `POST` request to
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`/users/me/presence`, containing the current user's status. The
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requests contains a few parameters. The most important is "status",
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which had 2 valid values:
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- "active" -- this means the user has interacted with the client
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recently. We use this for the "green" state in the web app.
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- "idle" -- the user has not interacted with the client recently.
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This is important for the case where a user left a Zulip tab open on
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their desktop at work and went home for the weekend. We use this
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for the "orange" state in the web app.
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The client receives in the response to that request a data set that,
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for each user, contains their status and timestamp that we last heard
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from that client. There are a few important details to understand
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about that data structure:
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- It's really important that the timestamp is the last time we heard
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from the client. A client can only interpret the status to display
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about another user by doing a simple computation using the (status,
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timestamp) pair. E.g. a user who last used Zulip 1 week ago will
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have a timestamp of 1 week ago and a status of "active". Why?
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Because this correctly handles the race conditions. For example, if
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the threshold for displaying a user as "offline" was 5 minutes
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since the user was last online, the client can at any time
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accurately compute whether that user is offline (even if the last
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data from the server was 45 seconds ago, and the user was last
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online 4:30 before the client received that server data).
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- Users can disable their own presence updates in user settings
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(`UserProfile.presence_enabled` is the flag storing [this user
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preference](https://zulip.com/help/status-and-availability#disable-updating-availability)).
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- The `status_from_timestamp` function in `static/js/presence.js` is
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useful sample code; the `OFFLINE_THRESHOLD_SECS` check is critical
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to correct output.
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- We provide the data for e.g. whether the user was online on their
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desktop or the mobile app, but for a basic client, you will likely
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only want to parse the "aggregated" key, which shows the summary
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answer for "is this user online".
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