matrix-spec-proposals/proposals/3960-sliding-sync-receipts.md

MSC3960: Sliding Sync Extension: Receipts

This MSC is an extension to MSC3575 which includes support for room receipts in the /sync response.

Proposal

MSC3575 does not include support for receipts. This extension will add support for both public and private room receipts.

The proposal is to introduce a new extension called receipts. It processes the core extension arguments enabled, rooms and lists, but defines no arguments of its own.

{
    "enabled": true, // sticky
    "lists": ["rooms", "dms"], // sticky
    "rooms": ["!abcd:example.com"] // sticky
}

If enabled is true, then the sliding sync response MAY include the following response fields in the receipts extension response:

{
    "rooms": {
        "!foo:bar": {
            // m.receipt EDU
        },
        "!foo2:bar": {
            // m.receipt EDU
        }
    }
}

If a lists or rooms argument is given to the extension, the typing extension response SHOULD only include rooms belonging at least one of the sliding windows in lists, or rooms whose IDs are in rooms. See also MSC3575's "Extensions" section.

The m.receipt event in this response is the same event that would appear in the array rooms.join["!foo:bar"].ephemeral.events under /sync. This includes the m.read.private key in the receipt EDU for private read receipts.

Receipts MUST only be sent for rooms returned in the sliding sync response. Receipts MUST be returned for events sent in the timeline section for each room of the sliding sync response. In addition, public and private receipts sent by the client MUST be returned, even if the events in question are not in the timeline. Delta tokens MUST be ignored when calculating the events in the timeline, or else additional read receipts for the same event would never be returned to the client. The receipts themeselves MAY be subject to delta tokens such that only read receipt deltas are returned to the client.

Overall, these steps prevent the sliding sync response from scaling with the amount of rooms on the client. This contrasts with the existing /sync mechanism which sends all receipts for all users in all rooms.

Potential issues

By not sending receipts for events not in the returned timeline, the client will not have receipt information for old events (e.g retrieved via /messages). An additional MSC would be required to add receipt information to events returned from /messages. It is unacceptable to return receipts not in the returned timeline because this can be extremely large e.g for Matrix HQ this is 50,000+ read receipts just for that room, for events that the client will potentially never view!

Alternatives

This extension could be bundled into the core MSC3575, but this would force all clients to receive this data. In practice clients can function extremely well without the need for read receipts, so forcing all clients to receive this data feels like the wrong design.

Security considerations

No additional security considerations beyond what the current /sync implementation provides.

Unstable prefix

No unstable prefix as Sliding Sync is still in review. To enable this extension, just add this to your request JSON:

{
    "extensions": {
        "receipts": {
            "enabled": true
        }
    }
}

Dependencies

This MSC builds on MSC3575, which at the time of writing has not yet been accepted into the spec.