What is the point of /keys/changes ?

[richvdh]

Link to problem area:

https://spec.matrix.org/v1.12/client-server-api/#tracking-the-device-list-for-a-user

Issue

The spec says:

Periodically, Alice’s client stores the next_batch field of the result from /sync in persistent storage. If Alice later restarts her client, it can obtain a list of the users who have updated their device list while it was offline by calling /keys/changes, passing the recorded next_batch field as the from parameter. If the client is tracking the device list of any of the users listed in the response, it marks them as outdated. It combines this list with those already flagged as outdated, and initiates a /keys/query request for all of them.

... but won't that return just the same information as calling /sync with the next_batch token (which we will presumably do anyway)?

I don't get it. Which is a particular shame, given I wrote it.

[richvdh]

oh! I remember!

It's intended for clients which do not persist the other results of /sync (and will therefore be calling /sync without a since token, rather than passing the next_batch token). The client can just cache the device list, and bring it up to date with /keys/changes, rather than doing a complete incremental sync since next_batch.

This could do with clarifying in the spec.