[zeromq-dev] Limitations of patterns?

Kelly Brock Kerby at inocode.com
Tue Aug 31 14:14:39 CEST 2010

Hi Matt,

> I'm trying to catch up on this, but you might think about using a
> "reverse server" pattern:
> Build a device which connects (XREP--XREP).
> Servers _and_ clients connect via REQ sockets.
> Your device contains a freelist of available servers.
> Each server sends an initial hello to put its identity on the freelist.
> You only recv new work from clients when a server is available, and
> assign the work to a server on the freelist.
> When you assign work keep a map (hash) from server_identity to
> client_identity.
> When a server sends a response put its identity back on the freelist,
> and send to client_identity.
> All unassigned work is now in the inbound client XREP socket, meaning
> if a server fails you only have one stuck request (at that server).
> Multiple recovery options are available (keeping state around about
> the requests at each server).
> Multiple server scheduling policies are available with a more
> sophisticated freelist.
> Does this help?

	I can work around the problem in my usage; that was not really an
issue.  The intention was more of a discussion to see if there is a method
of allowing push connections to support other distribution methods than
round robin.  Push is the most appropriate connection type but the round
robin distribution breaks down if your tasks are not fairly reliable at
retiring work in a consistently timely manner.

	More importantly is the idea that you shouldn't let tasks pile up on
the queues indiscriminately.  It's generally a bad idea to have tasks
stacked up beyond a couple, there is likely a reason they got backed up in
the first place.  (Someone runs a virus scan, decides to use your server as
a quake server, someone kicked the direct router out and you are routing to
Egypt and back instead of directly, etc etc.)

	So, the point of this was to reduce the case of major task pileups.
Additionally I "hope" we could solve for the broader usage and allow a first
come, first serve distribution on push without the queued messages.  I just
don't see how to get that without a separated ack ability.  :/


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