[zeromq-dev] Load distribution vs fail over

[gonzalo diethelm]

The butterfly tutorial is a good example of using 0MQ's capabilities for
load balancing using UPSTREAM / DOWNSTREAM sockets: you have one process
S that binds on a socket and N identical processes Ri that connect to
that socket; then the S process starts sending messages to the Ri
processes, distributing them in a round-robin fashion among all Ri. You
might call this "active-active load distribution" (because all the Ri
processes are actively processing messages simultaneously).

I am wondering how one would build a variation of this design, where the
S process sends ALL messages to a single Ri (R0, let's say). All the
other Ri processes are there just in case R0 fails; when S detects that
R0 is not receiving any messages, it switches to sending them to R1; and
so on. You might call this "active-passive load distribution" (because
only one Ri process is actively processing messages, while all the other
processes are passively waiting to become the active one). One reason it
might be advisable to process all messages with just one process is
concurrency problems; if all Ri processes access the same database,
running them simultaneosly might lead to contention, locking and even
deadlocking.

How would you approach this second scenario? One way I thought of was
using a PUB socket in S and SUB sockets in each Ri, so that all of them
receive all messages. The Ri processes also send a heart-beat message
between themselves, and have a way of deciding which one is the current
active process (such as voting). However, this is complex and forces
each Ri to know about all other instances.

Would it be possible to have a socket option (or type) that tells 0MQ
not to round-robin among all Ri processes, but stick to one process
only, and make the switch to the next one on a failure condition? Of
course, you would have to define a policy for declaring such a failure.

The failure condition also made me wonder: is there any way to control
the queueing policy in 0MQ? Can I know how many messages are waiting in
a queue to be delivered? Or how much "space" the queue has left? Can I
control these values somehow?

Your comments are most welcome. Thanks and best regards.

-- 
Gonzalo Diethelm


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