Device IDs about automerge-repo HOT 4 CLOSED

Some initial thoughts on how this might work:

• ID is set within repo, and stored under a common ID in whichever storage adapter is attached
• Each time the Repo is initialised, it checks storage for an existing ID and creates a new one if absent
• Can be overridden by passing in a DeviceId/RepoId to the Repo constructor; useful if you have a local Repo instance which is not connected to storage, for example.

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This seems very sensible; we can also keep track of sync states with the tuple repoId/docId/syncState.

Note that some peers may not be persistent. Browser tabs have their own local sync state but don't store anything in localstorage. We should be careful not to clutter up storage with the state of ephemeral peers.

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Perhaps relatedly, @orionz in automerge/automerge#617 added a kind of "branch ID" concept, which is an actorId extended with an integer to allow several branches to exist side-by-side. Seems like having multiple peers for a single device is a similar idea?

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Interesting! I think we can't do quite the same thing without some kind of leader mechanism (and solving the attendant problems that follow)... My gut reaction is that a "peer" in our current terminology should converge to become a "repo", which is to say a stored (or ephemeral) instance of some documents. Peers should include a hint about whether they plan to return during handshaking.

This is a bit of a puzzle... in the case of blutack, where we have messages run from tab -> shared_worker -> sync_server -> shared_worker -> tab, I suppose the "sync state" from an application perspective is whether the shared_worker is in sync with the server, and the "peer state" is the sync state from the other party's shared_worker. Tabs should panic any time they fall out of sync with the shared_worker and otherwise be treated as disposable.

In this case, I believe only the shared_worker's repo, stored in localStorage is really important.

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