Backup Accounts feature

[gamemode-3]

Motivation

Because many homeservers are run by independent individuals or non-profits and funded by donations, it's not guaranteed that the owners will always be able to continue maintaining them. If a homeserver were to suddenly break down it would be beneficial to have a backup account on another server that you could just continue using.

Mechanism (Sketch)

The basic mechanism would be as follows:

Say account @A:a.org and @B:b.org are associated, A is the primary account and B is the secondary (backup) account.
A enters a new chat with @C:c.org, the client automatically adds B to the room. A and B both send new m.accountlink (or different name) events to make sure they are configured correctly and to notify C of the association. When A sends messages B and C receive them. If C's client supports Backup Accounts it would not show them as separate members but would show that A has an associated backup account. If B were to send a message, it would look identical to C as when A sends a message (or maybe just with a little hint).

(If A can't add B, the event would also be sent so that compatible admin clients or the server could add B. They would first query B to determine whether B agrees that it is a backup account of A)

If a.org now goes down, the owner will simply be able to log into B and everything will be backed up there. There would then be the option to unlink A which would remove A from all rooms where B has the permission and will send an m.accountunlink (again, name is just for illustration) event to notify other participants. Where this is implemented the admin clients or server could then make sure the unlinked account is removed based on the event (provided there was prior authentication that this link actually existed and was acknowledged by A).

This is what comes to my mind, it would have to be discussed how rooms hosted on a.org would be handled, and it might also be that none of this is feasible (i'm pretty new to matrix)

Curious to hear your thoughts