MSC2444: peeking over federation via /peek

proposals/2444-peeking-over-federation-peek-api.md

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+# Proposal for implementing peeking over federation (peek API)
+
+## Problem
+
+Currently you can't peek over federation, as it was never designed or
+implemented due to time constraints when peeking was originally added to Matrix
+in 2016.
+
+As well as stopping users from previewing rooms before joining, the fact that
+servers can't participate in remote rooms without joining them first is
+inconvenient in many other ways:
+
+ * You can't reliably participate in E2E encryption in rooms you're invited to
+   unless the server is actually participating in the room
+   (https://github.com/vector-im/riot-web/issues/2713)
+ * You can't use rooms as generic pubsub mechanisms for synchronising data like
+   profiles, groups, device-lists etc if you can't peek into them remotely.
+ * Search engines can't work if they can't peek remote rooms.
+
+## Solution
+
+We let servers participate in peekable rooms (i.e. those with `world_readable`
+`m.room.history_visibility`) without having actually joined them.
+
+We do this by subscribing to a room on one or more servers via a new `/peek`
+S2S API, which lets users on a given server declare their interest in viewing
+events in that room.  Having started peeking into the room, the server(s)
+being peeked will relay *all* events it sees in that room to the peeking
+server (including ones from other servers).  Backfill
+and event-retrieval APIs should be changed to be queryable from servers not
+in the room if the room is peekable.
+
+This continues until the peeking server calls DELETE on the peek it initiated.
+
+To start peeking, firstly the peeking server must pick server(s) to peek via.
+This is typically the same server you would use to try to join the room via
+(i.e. one taken from the alias, or the via param on a matrix.to URL). The
+server could also call S2S /state on m.room.members to find other servers
+participating in the room and try to peek them from too.
+
+The peeking server starts to peek by PUTting to `/peek` on the peeked server.
+The request takes an empty object as a body as a placeholder for future (where
+we might put filters). The peeking server selects an ID for the peeking
+subscription for the purposes of idempotency. The ID must be 8 or less bytes
+of ASCII and should be unique for a given peeking & peeked server.
+
+PUT and DELETE return 200 OK with an empty `{}` on success.
+
+```
+PUT /_matrix/federation/v2/peek/{roomId}/{peekId}
+{}
+
+200 OK
+{}
+```
+
+```
+DELETE /_matrix/federation/v1/peek/{roomId}/{peekId}
+{}
+
+200 OK
+{}
+```
+
+If the peeking server hasn't heard any events from the peeked server for a
+while, it should attempt to re-PUT the /peek. If the peeked server is
+unavailable, it should retry via other servers from the room's members until
+it can reestablish.
+
+## Security considerations
+
+The peeked server becomes a centralisation point which could conspire against
+the peeking server to withhold events.  This is not that dissimilar to trying
+to join a room via a malicious server, however, and can be mitigated somewhat
+if the peeking server tries to query missing events from other servers.
+The peeking server could also peek to multiple servers for resilience against
+this sort of attack.
+
+The peeked server will be able to track the metadata surrounding which servers
+are peeking into which of its rooms, and when.  This could be particularly
+sensitive for single-person servers peeking at profile rooms.
+
+## Design considerations
+
+This doesn't solve the problem that rooms wink out of existence when all
+participants leave (https://github.com/matrix-org/matrix-doc/issues/534),
+unlike other approaches to peeking (e.g. MSC1777)
+
+Do we allow filtering the peek? (e.g. if you only care about particular
+events, or particular servers - e.g. if load-balancing peeking via multiple
+servers). Similarly, is it concerning that this significantly overlaps with
+the /sync CS API?
+
+How do we handle backpressure or rate limiting on the event stream (if at
+all?)
+
+## Dependencies
+
+This unblocks MSC1769 (profiles as rooms) and MSC1772 (groups as rooms)
+and is required for MSC1776 (peeking via /sync) to be of any use.
+
+## History
+
+This would close https://github.com/matrix-org/matrix-doc/issues/913
+
+An earlier rejected solution is MSC1777, which proposed joining a pseudouser
+(`@:server`) to a room in order to peek into it.  However, being forced to write
+to a room DAG (by joining such a user) in order to perform a read-only operation
+(peeking) was deemed inefficient and rejected.