matrix-doc/proposals/2197-search_filter_in_federation_publicrooms.md

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.

These benefits have been utilised in the Client-Server API, which implements search filtering using the filter JSON body parameter in the POST method on the /publicRooms endpoint.

Ignoring the server parameter in the Client-Server API, the following specific differences are noticed between the Client-Server and Federation API's /publicRooms endpoints:

• 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 parameter that is intentionally omitted is the server query parameter, as it does not make sense to include it – the requesting homeserver could make a direct request instead of requesting that a request be relayed.

The parameters which are copied, however, 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.

As the POST method was not previously accepted on the /publicRooms endpoint over federation, then requesting servers should fall back to the old behaviour, if one of the following errors is encountered:

• an HTTP 400 response with an M_UNRECOGNIZED standard error response errcode (this is what would be typically expected in this situation)
• a 404 (Not Found) HTTP error response
• a 405 (Method Not Allowed) HTTP error response

Security considerations

There are no known security considerations.

Privacy considerations

At present, 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.

The privacy implications of this proposal are not overly major, as the data that's being shared is [1]:

• only covered by GDPR if:
  • the search terms contain personal data, or
  • the user's homeserver IP address or domain name is uniquely identifying (because it's a single-person homeserver, perhaps)
• likely to be expected to be shared with the remote homeserver

For the sake of clarity, clients are strongly encouraged to display a warning that a remote search will take the user's data outside the jurisdiction of their own homeserver, before using the server parameter of the Client-Server API /publicRooms, as it can be assumed that this will lead to the server invoking the Federation API's /publicRooms – on the specified remote server – with the user's search terms.

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.