docs: Delete legacy presence subsystem page.

Everything on this page is now better explained in the API
documentation for presence.

docs/subsystems/index.md

@@ -34,7 +34,6 @@ django-upgrades
 release-checklist
 api-release-checklist
 input-pills
-presence
 unread_messages
 billing
 widgets

docs/subsystems/notifications.md

@@ -64,7 +64,7 @@ as follows:
     if a user was present 1 minute before a message was sent, and then
     closed their laptop, the user will not be in
     `presence_idle_user_ids` (because it takes a
-    [few minutes](presence.md) of being idle for Zulip
+    [few minutes](https://zulip.com/api/update-presence) of being idle for Zulip
     clients to declare to the server that the user is actually idle),
     and so without an additional mechanism, messages sent shortly after
     a user leaves would never trigger a notification (!).

docs/subsystems/performance.md

@@ -149,12 +149,12 @@ thousands of concurrent users.
 `POST /users/me/presence` requests, which submit the current user's
 presence information and return the information for all other active
 users in the organization, account for about 36% of all HTTP requests
-on production Zulip servers. See
-[presence](presence.md) for details on this system and
-how it's optimized. For this article, it's important to know that
-presence is one of the most important scalability concerns for any
-chat system, because it cannot be cached long, and is structurally a
-quadratic problem.
+on production Zulip servers. See the [presence API
+documentation](https://zulip.com/api/update-presence) for details on
+this system and how it's optimized. For this article, it's important
+to know that presence is one of the most important scalability
+concerns for any chat system, because it cannot be cached long, and is
+structurally a quadratic problem.
 
 Because typical presence requests consume 10-50ms of server-side
 processing time (to fetch and send back live data on all other active

docs/subsystems/presence.md

@@ -1,52 +0,0 @@
-# Presence
-
-This document explains the model for Zulip's presence.
-
-In a chat tool like Zulip, users expect to see the “presence” status
-of other users: is the person I want to talk to currently online? If
-not, were they last online 5 minutes ago, or more like an hour ago, or
-a week? Presence helps set expectations for whether someone is likely
-to respond soon. To a user, this feature can seem like a simple thing
-that should be easy. But presence is actually one of the hardest
-scalability problems for a team chat tool like Zulip.
-
-There's a lot of performance-related details in the backend and
-network protocol design that we won't get into here. The focus of
-this is what one needs to know to correctly implement a Zulip client's
-presence implementation (e.g., web app, mobile app, terminal client, or
-other tool that's intended to represent whether a user is online and
-using Zulip).
-
-A client should report to the server every minute a `POST` request to
-`/users/me/presence`, containing the current user's status. The
-requests contains a few parameters. The most important is "status",
-which had 2 valid values:
-
-- "active" -- this means the user has interacted with the client
-  recently.
-- "idle" -- the user has not interacted with the client recently.
-  This is important for the case where a user left a Zulip tab open on
-  their desktop at work and went home for the weekend.
-
-The client receives in the response to that request a data set that,
-for each user, contains their status and timestamp that we last heard
-from that client. There are a few important details to understand
-about that data structure:
-
-- It's really important that the timestamp is the last time we heard
-  from the client. A client can only interpret the status to display
-  about another user by doing a simple computation using the (status,
-  timestamp) pair. E.g., a user who last used Zulip 1 week ago will
-  have a timestamp of 1 week ago and a status of "active". Why?
-  Because this correctly handles the race conditions. For example, if
-  the threshold for displaying a user as "offline" was 5 minutes
-  since the user was last online, the client can at any time
-  accurately compute whether that user is offline (even if the last
-  data from the server was 45 seconds ago, and the user was last
-  online 4:30 before the client received that server data).
-- Users can disable their own presence updates in user settings
-  (`UserProfile.presence_enabled` is the flag storing [this user
-  preference](https://zulip.com/help/status-and-availability#disable-updating-availability)).
-- The `status_from_timestamp` function in `web/src/presence.js` is
-  useful sample code; the `OFFLINE_THRESHOLD_SECS` check is critical
-  to correct output.

zerver/actions/presence.py

@@ -30,8 +30,7 @@ def send_presence_changed(
     # sends a message, recipients may still see that user as offline!
     # We solve that by sending an immediate presence update clients.
     #
-    # See https://zulip.readthedocs.io/en/latest/subsystems/presence.html for
-    # internals documentation on presence.
+    # The API documentation explains this interaction in more detail.
     if settings.CAN_ACCESS_ALL_USERS_GROUP_LIMITS_PRESENCE:
         user_ids = get_user_ids_who_can_access_user(user_profile)
     else:

zerver/models/presence.py

@@ -14,9 +14,6 @@ class UserPresence(models.Model):
     NOTE: Users can disable updates to this table (see UserProfile.presence_enabled),
     so this cannot be used to determine if a user was recently active on Zulip.
     The UserActivity table is recommended for that purpose.
-
-    This is a tricky subsystem, because it is highly optimized.  See the docs:
-      https://zulip.readthedocs.io/en/latest/subsystems/presence.html
     """
 
     user_profile = models.OneToOneField(UserProfile, on_delete=CASCADE, unique=True)

zerver/openapi/zulip.yaml

@@ -10289,11 +10289,6 @@ paths:
         Zulip clients like mobile/desktop apps will want to use the [main
         presence endpoint](/api/get-presence), which returns data for all
         active users in the organization, instead.
-
-        See [Zulip's developer documentation][subsystems-presence]
-        for details on the data model for presence in Zulip.
-
-        [subsystems-presence]: https://zulip.readthedocs.io/en/latest/subsystems/presence.html
       parameters:
         - name: user_id_or_email
           in: path
@@ -10797,9 +10792,6 @@ paths:
         use the modern [`last_update_id`](#parameter-last_update_id) protocol to
         minimize fetching duplicate user presence data.
 
-        See [Zulip's developer documentation][subsystems-presence] for details
-        on the data model for user presence in Zulip.
-
         The Zulip server is responsible for implementing [invisible mode][invisible],
         which disables sharing a user's presence data. Nonetheless, clients
         should check the `presence_enabled` field in user objects in order to
@@ -10811,7 +10803,6 @@ paths:
         `true`, then user presence data in the response is [limited to users
         the current user can see/access][limit-visibility].
 
-        [subsystems-presence]: https://zulip.readthedocs.io/en/latest/subsystems/presence.html
         [limit-visibility]: /help/guest-users#configure-whether-guests-can-see-all-other-users
         [invisible]: /help/status-and-availability#invisible-mode
         [availability]: /help/status-and-availability#availability
@@ -12647,10 +12638,9 @@ paths:
         setting is set to `true`, presence information of only accessible
         users are returned.
 
-        See [Zulip's developer documentation][subsystems-presence]
-        for details on the data model for presence in Zulip.
-
-        [subsystems-presence]: https://zulip.readthedocs.io/en/latest/subsystems/presence.html
+        Complete Zulip apps are recommended to fetch presence
+        information when they post their own state using the [`POST
+        /presence`](/api/update-presence) API endpoint.
       responses:
         "200":
           description: Success.

zproject/default_settings.py

@@ -574,9 +574,7 @@ STAGING = False
 #
 # The default for OFFLINE_THRESHOLD_SECS is chosen as
 # `PRESENCE_PING_INTERVAL_SECS * 3 + 20`, which is designed to allow 2
-# round trips, plus an extra in case an update fails. See
-# https://zulip.readthedocs.io/en/latest/subsystems/presence.html for
-# details on the presence architecture.
+# round trips, plus an extra in case an update fails.
 #
 # How long to wait before clients should treat a user as offline.
 OFFLINE_THRESHOLD_SECS = 200