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MSC4000: Forwards fill (`/backfill` forwards)

Currently, /messages only backfills events when you paginate backwards with ?dir=b. The limiting factor here is /backfill which only looks backwards to find preceding events.

The spec just says this:

Since users in that room are able to request the history by the /messages client API endpoint, it’s possible that they might step backwards far enough into history before the homeserver itself was a member of that room.

To cover this case, the federation API provides a server-to-server analog of the /messages client API, allowing one homeserver to fetch history from another. This is the /backfill API.

-- https://spec.matrix.org/v1.5/server-server-api/#backfilling-and-retrieving-missing-events

This proposal is after adding a way to /backfill forwards, also known as forwards fill.

This also opens the door to adding backfill support to /context which returns events around a given event in both directions. Please note, that this MSC doesn't dictate when a server should /backfill or not. We can address this in a future MSC with spec or guidelines which is tracked by https://github.com/matrix-org/matrix-spec/issues/1281

Navigating the DAG forwards is harder because by the nature of prev_events, which connect the DAG together, gives us no indication of what comes next. We could give a list of events that we have as forward extremeties and ask other homeservers for any successor events (events that reference the given events in their prev_events).

Proposal

Add a new server API endpoint GET /_matrix/federation/v1/forwards_fill/{roomId} which is basically a mirror image of /backfill.

Retrieves a sliding-window history of subsequent PDUs that occurred in the given room. Starting from the PDU ID(s) given in the from_event_id query parameter, the PDUs that succeed them are retrieved, up to the total number given by the limit.


Rate-limited	No
Requires authentication	Yes

Request

Request parameters

Name	Type	Description
`roomId`	`string`	Required: The room ID to backfill.

Query parameters

Name	Type	Description
`limit`	`integer`	Required: The maximum number of PDUs to retrieve, including the given events.
`from_event_id`	`[string]`	Required: The event IDs to find successors from. TODO: Should we spec a hard limit to the number of events you can specify?

Responses

Status	Description
`200`	A transaction containing the PDUs that succeeded the given event(s), up to the given limit.

200 response transaction

Name	Type	Description
`origin`	`string`	Required: The server_name of the homeserver sending this transaction.
`origin_server_ts`	`integer`	Required: POSIX timestamp in milliseconds on originating homeserver when this transaction started.
`pdus`	`[PDU]`	Required: List of persistent updates to rooms. Note that events have a different format depending on the room version - check the room version specification for precise event formats.

Potential issues

Forwards fill has the same normal problems that /backfill has where the server will probably only exhaust a single chain of events up to the limit from the many given events to navigate from. This makes it harder to fill in all the gaps in one pass. But servers can keep hitting the endpoint with the new extremities until they've filled in everything. Is there a smarter heuristic guideline that servers should follow? Or something we should spec for how much depth/breadth a server should return given a list of many events to navigate from?

For example with Synapse, when it does a /messages?dir=b and there are backward extremities, it just does a single /backfill with the closest 5 extremities and just moves on with the /messages response regardless of how much timeline is missing. Perhaps this is better solved by allowing the server to indicate where the gaps are so the client can choose if it's important to them to fill them in, see MSC3871: Gappy timelines.

Alternatives

We could add a direction query parameter to /backfill but the semantics for are slightly different as backfill normally includes the events you requested with v as well preceding events versus with going forward, we just want to find the successors.

Security considerations

We're only exposing messages according to the existing message history setting in the room (m.room.history_visibility). No extra data is exposed, just a new way to navigate and fetch it all.

Auxiliary references

Unstable prefix

While this feature is in development, the endpoint can be used with the unstable prefix at GET /_matrix/federation/org.matrix.msc4000/forwards_fill/{roomId} instead of it's stable /v1 location as described above.

While the MSC is unstable

During this period, servers should optimistically try the unstable endpoint and look for the 404 M_UNRECOGNIZED error code to determine support (see MSC3743: Standardized error response for unknown endpoints).

Once the MSC is merged but not in a spec version

Once this MSC is merged, but is not yet part of the spec, servers should optimistically use the stable endpoint and fallback to the unstable endpoint to have maximum compatibility if desired.

Once the MSC is in a spec version

Once this MSC becomes a part of a spec version, servers should optimistically use the stable endpoint. If a given server doesn't support the endpoint, another server in the room can be tried until all are exhausted.

Servers can keep falling back to the unstable endpoint and are encouraged (not obligated) to serve the unstable fallback endpoint for a reasonable amount of time to help smooth over the transition for other servers. "Reasonable" is intentionally left as an implementation detail, however the MSC process currently recommends at most 2 months from the date of spec release.

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MSC4000: Forwards fill (/backfill forwards)