socket.io: emit a message from the server to the server (!)
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I can happily send messages to the clients from the server and also to the server from the clients.  I, however, seem to need to send (broadcast) **messages to the (server) sockets from the server itself** -- and I don't know how to go about it.

The case is that the server learns (via a Redis subscription) that a subset of the possibly very numerous clients need to be sent a message.  Each server side socket can determine on its own if it needs to fire or not.  I just don't know how to let them know they have to do the check (possibly in much the same way as 'normally' by a message from the client (?)).

FWIW the check can be made by using the handshakeData only (and other non-socket-instance-specific server side structures).  If there's a way to get to the handshakeData 'from the outside', it's also a possibility; but by no means really pretty.

Any thoughts/pointers anyone?