MSC2753: Peeking via sync (take 2)

proposals/2753-peeking-via-sync-v2.md

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+# MSC2753: Peeking via Sync (Take 2)
+
+## Problem
+
+Peeking into rooms without joining them currently relies on the deprecated v1 /initialSync and /events APIs.
+
+This poses the following issues:
+
+ * Servers and clients must implement two separate sets of event-syncing logic, doubling complexity.
+ * Peeking a room involves setting a stream of long-lived /events requests
+   going. Having multiple streams is racey, competes for resources with the
+   /sync stream, and doesn't scale given each room requires a new /events
+   stream.
+ * v1 APIs are deprecated and not implemented on new servers.
+
+This MSC likely obsoletes [MSC1776](https://github.com/matrix-org/matrix-doc/pull/1776).
+
+## Proposal
+
+We add an CS API called `/peek/{roomIdOrAlias}`, very similar to `/join/{roomIdOrAlias}`.
+
+Calling `/peek`:
+ * Resolves the given room alias to a room ID, if needed.
+ * Adds the room (if permissions allow) to a new section of the /sync response called `peeking` - but only for the device which called `/peek`.
+ * The `peeking` section is identical to `joined`, but shows the live activity of rooms for which that device is peeking.
+
+The API returns 404 on an unrecognised room ID or alias, or 403 if the room does not allow peeking.
+
+Similar to `/join`, `/peek` lets you specify `server_name` querystring parameters to specify which server(s) to try to peek into the room via (when coupled with [MSC2444](https://github.com/matrix-org/matrix-doc/pull/2444)).
+
+If a user subsequently `/join`s the room they're peeking, we atomically move the room to the `joined` block of the `/sync` response, allowing the client to build on the state and history it has already received without re-sending it down /sync.
+
+To stop peeking, the user calls `/unpeek` on the room, similar to `/leave` or `/forget`.  This returns 200 on success, 404 on unrecognised ID, or 400 if the room was not being peeked in the first place.
+
+## Potential issues
+
+It could be seen as controversial to add another new block to the `/sync` response.  We could use the existing `joined` block, but:
+
+ * it's a misnomer (given the user hasn't joined the rooms)
+ * `joined` refers to rooms which the *user* is in, rather than that they are peeking into using a given device
+ * we risk breaking clients who aren't aware of the new style of peeking.
+ * there's already a precedent for per-device blocks in the sync response (for to-device messages)
+
+It could be seen as controversial to make peeking a per-device rather than per-user feature.  When thinking through use cases for peeking, however:
+
+ 1. Peeking a chatroom before joining it is the common case, and is definitely per-device - you would not expect peeked rooms to randomly pop up on other devices, or consume their bandwidth.
+ 2. [MSC1769](https://github.com/matrix-org/matrix-doc/pull/1769) (Profiles as rooms) is also per device: if a given client wants to look at the Member Info for a given user in a room, it shouldn't pollute the others with that data.
+ 3. [MSC1772](https://github.com/matrix-org/matrix-doc/pull/1772) (Groups as rooms) uses room joins to indicate your own membership, and peeks to query the group membership of other users.  Similarly to profiles, it's not clear that this should be per-user rather than per-device (and worse case, it's a matter of effectively opting in rather than trying to filter out peeks you don't care about).
+
+## Alternatives
+
+[MSC1776](https://github.com/matrix-org/matrix-doc/pull/1776) defined an alternative approach, where you could use filters to add peeked rooms into a given `/sync` response as needed.  This however had some issues:
+
+ * You can't specify what servers to peek remote rooms via.
+ * You can't identify rooms via alias, only ID
+ * It feels hacky to bodge peeked rooms into the `joined` block of a given `/sync` response
+ * Fiddling around with custom filters feels clunky relative to just calling a `/peek` endpoint similar to `/join`.
+
+While experimenting with implementing MSC1776, I came up with this as an alternative that empirically feels much simpler and tidier.
+
+## Security considerations
+
+Servers should ratelimit calls to `/peek` to stop someone DoSing the server.
+
+Servers may stop maintaining a `/peek` if its device has not `/sync`ed recently, and thus reclaim resources.  At the next `/sync` the server would need to restore the `peek` and provide a gappy update.  This is most relevant in the context of peeking into a remote room via [MSC2444](https://github.com/matrix-org/matrix-doc/pull/2444) however.
+
+## Unstable prefix
+
+TBD