matrix-spec-proposals/proposals/3959-sliding-sync-account-data.md

MSC3959: Sliding Sync Extension: Account Data

This MSC is an extension to MSC3575 which includes support for global and room account data in the /sync response.

Proposal

MSC3575 does not include support for account data. This extension will add support for both global and room account data.

The prosposal is to introduce a new extension called account_data. 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 account_data extension response:

{
    "global": [
        {
           // account data JSON
        }
    ],
    "rooms": {
        "!foo:bar": [
           {
              // account data JSON
           },
           {
              // account data JSON
           }
        ],
        "!foo2:bar": [
            {
               // account data JSON
            }
        ]
    }
}

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.

All keys are optional and clients MUST check if they exist prior to use. The semantics of these fields is exactly the same as the current /sync implementation whereby:

• This extension's global key maps to the top-level account_data.events field in /sync.
• This extension's rooms key maps to a joined room's account_data field in /sync, e.g the path rooms.join["!foo:bar"].account_data.events.

For sliding sync, an initial response must include all global account data. This data is subject to delta tokens and SHOULD be omitted when delta tokens are used and the client already has these events. Only rooms returned in the sliding sync response will be included in the rooms key. The server MUST NOT send room account data for rooms the client is not aware of. This prevents the sliding sync response from scaling with the amount of room account data on the client. These events are also subject to delta tokens and SHOULD be omitted when the client already has these events.

Potential issues

If clients do not use delta tokens, every initial sliding sync response will include all global account data. In practice, this can be very large: specifically the m.push_rules and m.direct account data events scale sub-linearly with the number of rooms on the clients account. For very large accounts, these two events can take >100KB, which impacts time-to-first-use on clients. Delta tokens can be used to prevent these events from being sent all the time. Similarly, room account data is sent every time sliding sync sends an initial: true room response. If rooms frequently reappear in the sliding window this can result in needless bandwidth consumption. This can also be mitigated via delta tokens.

There is no filtering capabilities in this extension. Clients may not care about some kinds of events but are unable to filter them out of the response.

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 account data, so forcing all clients to receive this data feels like the wrong design.

This extension could also not exist, but this would mean that Sliding Sync has no way to push live-updated account data to clients, relying on clients to manually GET the HTTP endpoints /user/<user_id>/account_data/<type> and /user/{userId}/rooms/{roomId}/account_data/{type}.

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": {
        "account_data": {
            "enabled": true
        }
    }
}

Dependencies

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