MSC for Search Filtering in Federation /publicRooms

Signed-off-by: Olivier Wilkinson (reivilibre) <olivier@librepush.net>

proposals/2197-search_filter_in_federation_publicrooms.md

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+# MSC2197 – Search Filtering in Public Room Directory over Federation
+
+This MSC proposes introducing the `POST` method to the `/publicRooms` Federation API endpoint,
+including a `filter` argument which allows server-side filtering of rooms.
+
+We are motivated by the opportunity to make searching the public Room Directory more efficient over
+Federation.
+
+## Motivation
+
+Although the Client-Server API includes the filtering capability in `/publicRooms`, the Federation API
+currently does not.
+
+This leads to a situation that is wasteful of effort and network traffic for both homeservers; searching
+a remote server involves first downloading its entire room list and then filtering afterwards.
+
+## Proposal
+
+Having a filtered `/publicRooms` API endpoint means that irrelevant or uninteresting rooms can be
+excluded from a room directory query response. In turn, this means that these room directory query
+responses can be generated more quickly and then, due to their smaller size, transmitted over the network more quickly, owing to their
+smaller size.
+
+These benefits have been exploited in the Client-Server API, which implements search filtering 
+using the `filter` JSON body parameter in the `POST` method on the `/publicRooms` endpoint.
+
+It should be noted that the Client-Server and Federation APIs both currently possess `/publicRooms`
+endpoints which, whilst similar, are not equivalent.
+
+Ignoring the `server` parameter in the Client-Server API, the following specific differences are
+noticed:
+
+* the Federation API endpoint only accepts the `GET` method whereas the Client-Server API accepts
+  the `POST` method as well.
+* the Federation API accepts `third_party_instance_id` and `include_all_networks` parameters through
+  the `GET` method, whereas the Client-Server API only features these in the `POST` method.
+
+This MSC proposes to introduce support for the `POST` method in the Federation API's `/publicRooms`
+endpoint, with all but one of the parameters from that of the Client-Server API. The copied parameters
+shall have the same semantics as they do in the Client-Server API.
+
+In the interest of clarity, the proposed parameter set is listed below, along with a repetition of the
+definitions of used substructures. The response format has been omitted as it is the same as that of
+the current Client-Server and Federation APIs, which do not differ in this respect.
+
+### `POST /_matrix/federation/v1/publicRooms`
+
+#### Query Parameters
+
+There are no query parameters. Notably, we intentionally do not inherit the `server` query parameter
+from the Client-Server API.
+
+#### JSON Body Parameters
+
+* `limit` (`integer`): Limit the number of search results returned.
+* `since` (`string`): A pagination token from a previous request, allowing clients to get the next (or previous) batch of rooms. The direction of pagination is specified solely by which token is supplied, rather than via an explicit flag.
+* `filter` (`Filter`): Filter to apply to the results.
+* `include_all_networks` (`boolean`): Whether or not to include all known networks/protocols from application services on the homeserver. Defaults to false.
+* `third_party_instance_id` (`boolean`): The specific third party network/protocol to request from the homeserver. Can only be used if `include_all_networks` is false.
+
+### `Filter` Parameters
+
+* `generic_search_term` (`string`): A string to search for in the room metadata, e.g. name, topic, canonical alias etc. (Optional).
+
+## Tradeoffs
+
+An alternative approach might be for implementations to carry on as they are but also
+cache (and potentially index) remote homeservers' room directories. This would not require
+a spec change.
+
+However, this would be unsatisfactory because it would lead to outdated room directory results and/or
+caches that provide no benefit (as room directory searches are generally infrequent enough that a cache
+would be outdated before being reused, on small – if not most – homeservers).
+
+## Potential issues
+
+### Backwards Compatibility
+
+After this proposal is implemented, outdated homeservers will still exist which do not support the room
+filtering functionality specified in this MSC. In this case, homeservers will have to fall-back to downloading
+the entire room directory and performing the filtering themselves, as currently happens. This is not considered
+a problem since it will not lead to a situation that is any worse than the current one, and it is expected that
+large homeservers – which cause the most work with the current search implementations – would be quick to upgrade
+to support this feature once it is available.
+
+In addition, as the `POST` method was not previously accepted on the `/publicRooms` endpoint over federation,
+then it is not a difficult task to use a `405 Method Not Allowed` HTTP response as a signal that fallback is required.
+
+## Security considerations
+
+There are no known security considerations.
+
+## Privacy considerations
+
+At current, remote homeservers do not learn about what a user has searched for.
+
+However, under this proposal, in the context of using the Federation API to forward on queries from the Client-Server
+API, a client's homeserver would end up sharing the client's search terms with a remote homeserver, which may not be
+operated by the same party or even trusted. For example, users' search terms could be logged.
+
+It is uncertain, to the author of this MSC, what implications this has with regards to legislation, such as GDPR.
+
+## Conclusion
+
+By allowing homeservers to pass on search filters, we enable remote homeservers' room directories to be
+efficiently searched, because, realistically speaking, only the remote homeserver is in a position to be
+able to perform search efficiently, by taking advantage of indexing and other such optimisations.
+