Persistence for what an OAuth client handle learns: the client it registered (or was given) and the tokens it holds.

Without a store both live in the handle and die with the client process. A host that wants them to survive a restart gives the {oauth, Config} a store => {Module, Arg} implementing this behaviour. Keys are client and tokens; values are maps the handle owns the shape of.

Deleting matters as much as writing: when the resource's authorization server changes, the handle drops both and registers again, and a store that ignored the delete would hand the old credential back on the next start.