mochi-mqtt / server / 9038356648

Pull #402

github

Allow a Hook to reload an individual client session

In a clustered environment, client connections are distributed among
multiple Server instances on different machines. After a client
disconnects, leaving behind a persistent session state, its next
login is likely to be on a different node. Because of this, in such a
setup an individual Server instance should only keep Client instances
corresponding to online client connections, and it should be able to
reload an individual client's state (presumably from persistent
storage) when that client connects.

This commit adds support for such an environment by adding a new hook
`StoredClientByID`. An implementation finds and returns any
persistent client data for a given session ID. In practice the only
necessary information turned out to be the saved subscriptions and
in-flight ack messages. The hook also returns the prior 'Remote'
property since the server logs that.

The Server method `inheritClientSession` is extended to call this hook
if there is no matching in-memory Client session. If the hook returns
session data, it installs it into the Client object in the same way
as the existing code.

At the end of the Server method `attachClient`, after disconnection,
the existence of a `StoredClientByID` hook is checked; if present,
the method expires the Client instance so it won't hang around in
memory and so the next connection will go to the hook to reload
state.

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