docs: Update `docs/subsystems/` files to use channel.

Updates descriptive text that refer to Zulip channels in the
`docs/subsystems` files to use channel instead of stream.

Part of the stream to channel rename project.

docs/subsystems/analytics.md

@@ -2,12 +2,12 @@
 
 Zulip has a cool analytics system for tracking various useful statistics
 that currently power the `/stats` page, and over time will power other
-features, like showing usage statistics for the various streams. It is
+features, like showing usage statistics for the various channels. It is
 designed around the following goals:
 
 - Minimal impact on scalability and service complexity.
 - Well-tested so that we can count on the results being correct.
-- Efficient to query so that we can display data in-app (e.g. on the streams
+- Efficient to query so that we can display data in-app (e.g. on the channels
   page) with minimum impact on the overall performance of those pages.
 - Storage size smaller than the size of the main Message/UserMessage
   database tables, so that we can store the data in the main PostgreSQL
@@ -164,7 +164,7 @@ a given realm). The only piece that you can't test here is the "Me"
 buttons, which won't have any data. For those, you can instead log in
 as the `shylock@analytics.ds` in the `analytics` realm and visit
 `/stats` there (which is only a bit more work). Note that the
-`analytics` realm is a shell with no streams, so you'll only want to
+`analytics` realm is a shell with no channels, so you'll only want to
 use it for testing the graphs.
 
 If you're adding a new stat/table, you'll want to edit

docs/subsystems/caching.md

@@ -47,7 +47,7 @@ work.
 As a side note, the policy of using these accessor functions wherever
 possible is a good idea, regardless of caching, because the functions
 also generally take care of details you might not think about
-(e.g. case-insensitive matching of stream names or email addresses).
+(e.g. case-insensitive matching of channel names or email addresses).
 It's amazing how slightly tricky logic that's duplicated in several
 places invariably ends up buggy in some of those places, and in
 aggregate we call these accessor functions hundreds of times in
@@ -252,7 +252,7 @@ multiple servers. We do have a few, however:
 
 Zulip makes extensive use of caching of data in the browser and mobile
 apps; details like which users exist, with metadata like names and
-avatars, similar details for streams, recent message history, etc.
+avatars, similar details for channels, recent message history, etc.
 
 This data is fetched in the `/register` endpoint (or `page_params`
 for the web app), and kept correct over time. The key to keeping these

docs/subsystems/events-system.md

@@ -15,7 +15,7 @@ to receive updates to the Zulip data. The simplest example is a new
 message being sent by one client; other clients must be notified in
 order to display the message. But a complete application like Zulip
 has dozens of different types of data that need to be synced to other
-clients, whether it be new streams, changes in a user's name or
+clients, whether it be new channels, changes in a user's name or
 avatar, settings changes, etc. In Zulip, we call these updates that
 need to be sent to other clients **events**.
 
@@ -68,7 +68,7 @@ Usually, this list of users is one of 3 things:
 - Everyone in the realm (e.g. for organization-level settings changes,
   like new realm emoji).
 - Everyone who would receive a given message (for messages, emoji
-  reactions, message editing, etc.); i.e. the subscribers to a stream
+  reactions, message editing, etc.); i.e. the subscribers to a channel
   or the people on a direct message thread.
 
 It is the responsibility of the caller of `send_event` to choose the
@@ -176,7 +176,7 @@ When a client starts up, it usually wants to get 2 things from the
 server:
 
 - The "current state" of various pieces of data, e.g. the current
-  settings, set of users in the organization (for typeahead), stream,
+  settings, set of users in the organization (for typeahead), channel,
   messages, etc. (aka the "initial state").
 - A subscription to receive updates to those data when they are
   changed by a client (aka an event queue).

docs/subsystems/hashchange-system.md

@@ -8,13 +8,13 @@ be used to deep-link into the application and allow the browser's
 Some examples are:
 
 - `/#settings/your-bots`: Bots section of the settings overlay.
-- `/#channels`: Streams overlay, where the user manages streams
+- `/#channels`: Channels overlay, where the user manages channels
   (subscription etc.)
-- `/#channels/11/announce`: Streams overlay with stream ID 11 (called
+- `/#channels/11/announce`: Channels overlay with channel ID 11 (called
   "announce") selected.
-- `/#narrow/stream/42-android/topic/fun`: Message feed showing stream
+- `/#narrow/stream/42-android/topic/fun`: Message feed showing channel
   "android" and topic "fun". (The `42` represents the id of the
-  stream.)
+  channel.)
 
 The main module in the frontend that manages this all is
 `web/src/hashchange.js` (plus `hash_util.js` for all the parsing
@@ -23,19 +23,19 @@ the reason that it's thorny is that it needs to support a lot of
 different flows:
 
 - The user clicking on an in-app link, which in turn opens an overlay.
-  For example the streams overlay opens when the user clicks the small
+  For example the channels overlay opens when the user clicks the small
   cog symbol on the left sidebar, which is in fact a link to
   `/#channels`. This makes it easy to have simple links around the app
   without custom click handlers for each one.
 - The user uses the "back" button in their browser (basically
   equivalent to the previous one, as a _link_ out of the browser history
   will be visited).
-- The user clicking some in-app click handler (e.g. "Stream settings"
-  for an individual stream), that potentially does
-  several UI-manipulating things including e.g. loading the streams
+- The user clicking some in-app click handler (e.g. "Channel settings"
+  for an individual channel), that potentially does
+  several UI-manipulating things including e.g. loading the channels
   overlay, and needs to update the hash without re-triggering the open
   animation (etc.).
-- Within an overlay like the streams overlay, the user clicks to
+- Within an overlay like the channels overlay, the user clicks to
   another part of the overlay, which should update the hash but not
   re-trigger loading the overlay (which would result in a confusing
   animation experience).
@@ -69,7 +69,7 @@ Internally you have these functions:
   a hash or using the back button) or triggered internally.
 - `hashchange.do_hashchange_normal` handles most cases, like loading the main
   page (but maybe with a specific URL if you are narrowed to a
-  stream or topic or direct messages, etc.).
+  channel or topic or direct messages, etc.).
 - `hashchange.do_hashchange_overlay` handles overlay cases. Overlays have
   some minor complexity related to remembering the page from
   which the overlay was launched, as well as optimizing in-page

docs/subsystems/html-css.md

@@ -46,7 +46,7 @@ styling, clean up now-unused CSS, etc., to keep things maintainable.
 
 Opt to write CSS in CSS files. Avoid using the `style=` attribute in
 HTML except for styles that are set dynamically. For example, we set
-the colors for specific streams (`{{stream_color}}`) on different
+the colors for specific channels (`{{stream_color}}`) on different
 elements dynamically, in files like `user_stream_list_item.hbs`:
 
 ```html

docs/subsystems/markdown.md

@@ -139,7 +139,7 @@ that depend on realm-specific or user-specific data. For example, the
 realm could have
 [linkifiers](https://zulip.com/help/add-a-custom-linkifier)
 or [custom emoji](https://zulip.com/help/custom-emoji)
-configured, and Zulip supports mentions for streams, users, and user
+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`
@@ -151,7 +151,7 @@ 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), stream descriptions,
+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
@@ -253,7 +253,7 @@ accurate.
 - Disable link-by-reference syntax,
   `[foo][bar]` ... `[bar]: https://google.com`.
 
-- Enable linking to other streams using `#**streamName**`.
+- Enable linking to other channels using `#**channelName**`.
 
 ### Code
 

docs/subsystems/notifications.md

@@ -42,7 +42,7 @@ as follows:
     `enable_online_push_notifications` flag is enabled). This data
     structure ignores users for whom the message is not notifiable,
     which is important to avoid this being thousands of `user_ids` for
-    messages to large streams with few currently active users.
+    messages to large channels with few currently active users.
 - The Tornado [event queue system](events-system.md)
   processes that data, as well as data about each user's active event
   queues, to (1) push an event to each queue needing that message and
@@ -153,7 +153,7 @@ structure of the system, when thinking about changes to it:
 - **Future configuration**. Notification settings are an area that we
   expect to only expand with time, with upcoming features like
   following a topic (to get notifications for messages only within
-  that topic in a stream). There are a lot of different workflows
+  that topic in a channel). There are a lot of different workflows
   possible with Zulip's threading, and it's important to make it easy
   for users to set up Zulip's notification to fit as many of those
   workflows as possible.

docs/subsystems/performance.md

@@ -179,7 +179,7 @@ Zulip is somewhat unusual among web apps in sending essentially all of the
 data required for the entire Zulip web app in this single request,
 which is part of why the Zulip web app loads very quickly -- one only
 needs a single round trip aside from cacheable assets (avatars, images, JS,
-CSS). Data on other users in the organization, streams, supported
+CSS). Data on other users in the organization, channels, supported
 emoji, custom profile fields, etc., is all included. The nice thing
 about this model is that essentially every UI element in the Zulip
 client can be rendered immediately without paying latency to the
@@ -211,21 +211,21 @@ typical organization but potentially multiple seconds for large open
 organizations with 10,000s of users. There is also smaller
 variability based on a individual user's personal data state,
 primarily in that having 10,000s of unread messages results in a
-somewhat expensive query to find which streams/topics those are in.
+somewhat expensive query to find which channels/topics those are in.
 
 We consider any organization having normal `page_params` fetch times
 greater than a second to be a bug, and there is ongoing work to fix that.
 
 It can help when thinking about this to imagine `page_params` as what
 in another web app would have been 25 or so HTTP GET requests, each
-fetching data of a given type (users, streams, custom emoji, etc.); in
+fetching data of a given type (users, channels, custom emoji, etc.); in
 Zulip, we just do all of those in a single API request. In the
 future, we will likely move to a design that does much of the database
 fetching work for different features in parallel to improve latency.
 
-For organizations with 10K+ users and many default streams, the
+For organizations with 10K+ users and many default channels, the
 majority of time spent constructing `page_params` is spent marshalling
-data on which users are subscribed to which streams, which is an area
+data on which users are subscribed to which channels, which is an area
 of active optimization work.
 
 ### Fetching message history
@@ -238,7 +238,7 @@ it does a large number of these requests:
 - Most of these requests are from users clicking into different views
   -- to avoid certain subtle bugs, Zulip's web app currently fetches
   content from the server even when it has the history for the
-  relevant stream/topic cached locally.
+  relevant channel/topic cached locally.
 - When a browser opens the Zulip web app, it will eventually fetch and
   cache in the browser all messages newer than the oldest unread
   message in a non-muted context. This can be in total extremely
@@ -314,7 +314,7 @@ but are also extremely cheap (~3ms).
 
 ### Other endpoints
 
-Other API actions, like subscribing to a stream, editing settings,
+Other API actions, like subscribing to a channel, editing settings,
 registering an account, etc., are vanishingly rare compared to the
 requests detailed above, fundamentally because almost nobody changes
 these things more than a few dozen times over the lifetime of their

docs/subsystems/pointer.md

@@ -1,6 +1,6 @@
 # Unread counts and the pointer
 
-When you're using Zulip and you reload, or narrow to a stream, how
+When you're using Zulip and you reload, or narrow to a channel, how
 does Zulip decide where to place you?
 
 Conceptually, Zulip takes you to the place where you left off
@@ -29,7 +29,7 @@ First a bit of terminology:
 ### Recipient bar: message you clicked
 
 If you enter a narrow by clicking on a message group's _recipient bar_
-(stream/topic or direct message recipient list at the top of a group
+(channel/topic or direct message recipient list at the top of a group
 of messages), Zulip will select the message you clicked on. This
 provides a nice user experience where you get to see the stuff near
 what you clicked on, and in fact the message you clicked on stays at
@@ -48,8 +48,8 @@ This provides the nice user experience of taking you to the start of
 the new stuff (with enough messages you've seen before still in view
 at the top to provide you with context), which is usually what you
 want. (When finding the "first unread message", Zulip ignores unread
-messages in muted streams or in muted topics within non-muted
-streams.)
+messages in muted channels or in muted topics within non-muted
+channels.)
 
 ### Unnarrow: previous sequence
 

docs/subsystems/sending-messages.md

@@ -37,10 +37,10 @@ and narrowing).
 The compose box does a lot of fancy things that are out of scope for
 this article. But it also does a decent amount of client-side
 validation before sending a message off to the server, especially
-around mentions (E.g. checking the stream name is a valid stream,
+around mentions (E.g. checking the channel name is a valid channel,
 displaying a warning about the number of recipients before a user can
 use `@**all**` or mention a user who is not subscribed to the current
-stream, etc.).
+channel, etc.).
 
 ## Backend implementation
 
@@ -257,7 +257,7 @@ For background, Zulip’s threading model requires tracking which
 individual messages each user has received and read (in other chat
 products, the system either doesn’t track what the user has read at
 all, or just needs to store a pointer for “how far the user has read”
-in each room, channel, or stream).
+in each room or channel).
 
 We track these data in the backend in the `UserMessage` table, storing
 rows `(message_id, user_id, flags)`, where `flags` is 32 bits of space
@@ -268,7 +268,7 @@ the database indexes on this table (with joins to the `Message` table
 containing the actual message content where required).
 
 The downside of this design is that when a new message is sent to a
-stream with `N` recipients, we need to write `N` rows to the
+channel with `N` recipients, we need to write `N` rows to the
 `UserMessage` table to record those users receiving those messages.
 Each row is just 3 integers in size, but even with modern databases
 and SSDs, writing thousands of rows to a database starts to take a few
@@ -278,10 +278,10 @@ This isn’t a problem for most Zulip servers, but is a major problem
 for communities like chat.zulip.org, where there might be 10,000s of
 inactive users who only stopped by briefly to check out the product or
 ask a single question, but are subscribed to whatever the default
-streams in the organization are.
+channels in the organization are.
 
 The total amount of work being done here was acceptable (a few seconds
-of total CPU work per message to large public streams), but the
+of total CPU work per message to large public channels), but the
 latency was unacceptable: The server backend was introducing a latency
 of about 1 second per 2000 users subscribed to receive the message.
 While these delays may not be immediately obvious to users (Zulip,
@@ -294,18 +294,18 @@ even simple questions).
 
 A key insight for addressing this problem is that there isn’t much of
 a use case for long chat discussions among 1000s of users who are all
-continuously online and actively participating. Streams with a very
+continuously online and actively participating. Channels with a very
 large number of active users are likely to only be used for occasional
 announcements, where some latency before everyone sees the message is
 fine. Even in giant organizations, almost all messages are sent to
-smaller streams with dozens or hundreds of active users, representing
+smaller channels with dozens or hundreds of active users, representing
 some organizational unit within the community or company.
 
-However, large, active streams are common in open source projects,
+However, large, active channels are common in open source projects,
 standards bodies, professional development groups, and other large
 communities with the rough structure of the Zulip development
 community. These communities usually have thousands of user accounts
-subscribed to all the default streams, even if they only have dozens
+subscribed to all the default channels, even if they only have dozens
 or hundreds of those users active in any given month. Many of the
 other accounts may be from people who signed up just to check the
 community out, or who signed up to ask a few questions and may never
@@ -330,14 +330,14 @@ organization for a few weeks, they are tagged as soft-deactivated.
 The way this works internally is:
 
 - We (usually) skip creating UserMessage rows for soft-deactivated
-  users when a message is sent to a stream where they are subscribed.
+  users when a message is sent to a channel where they are subscribed.
 
 - If/when the user ever returns to Zulip, we can at that time
   reconstruct the UserMessage rows that they missed, and create the rows
   at that time (or, to avoid a latency spike if/when the user returns to
   Zulip, this work can be done in a nightly cron job). We can construct
   those rows later because we already have the data for when the user
-  might have been subscribed or unsubscribed from streams by other
+  might have been subscribed or unsubscribed from channels by other
   users, and, importantly, we also know that the user didn’t interact
   with the UI since the message was sent (and thus we can safely assume
   that the messages have not been marked as read by the user). This is
@@ -369,9 +369,9 @@ The end result is the best of both worlds:
   with Zulip.
 
 Empirically, we've found this technique completely resolved the "send
-latency" scaling problem. The latency of sending a message to a stream
+latency" scaling problem. The latency of sending a message to a channel
 now scales only with the number of active subscribers, so one can send
-a message to a stream with 5K subscribers of which 500 are active, and
+a message to a channel with 5K subscribers of which 500 are active, and
 it’ll arrive in the couple hundred milliseconds one would expect if
 the extra 4500 inactive subscribers didn’t exist.
 
@@ -384,7 +384,7 @@ There are a few details that require special care with this system:
   assumed a `UserMessage` row would always exist for a message that
   triggers a notification to a given user.
 - Digest emails, which use the `UserMessage` table extensively to
-  determine what has happened in streams the user can see. We can use
+  determine what has happened in channels the user can see. We can use
   the user's subscriptions to construct what messages they should have
   access to for this feature.
 - Soft-deactivated users experience high loading latency when

docs/subsystems/typing-indicators.md

@@ -170,11 +170,7 @@ not let the notifications become too "sticky" either.
 
 ## Roadmap
 
-The most likely big change to typing indicators is that we will
-add them for stream conversations. This will require some thought
-for large streams, in terms of both usability and performance.
-
-Another area for refinement is to tune the timing values a bit.
+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

docs/subsystems/unread_messages.md

@@ -1,8 +1,8 @@
 # Unread message synchronization
 
-In general displaying unread counts for all streams and topics may require
+In general displaying unread counts for all channels and topics may require
 downloading an unbounded number of messages. Consider a user who has a muted
-stream or topic and has not read the backlog in a month; to have an accurate
+channel or topic and has not read the backlog in a month; to have an accurate
 unread count we would need to load all messages this user has received in the
 past month. This is inefficient for web clients and even more for mobile
 devices.