HP Wei wrote at 05/13/2012 03:53 PM:
Suppose I am on a master machine A
and there are two other machines B and C.
On A, in racket, I would like to programatically initiate
one server on B and another on C.
The 'server' is presumably a 'repl' that can execute a block
of code, sent from A.
[ Let's say security is not an issue here. ]
i.e. this is the intention:
invoke-server-on B and C (via rsh or ssh)
send-code B (asynchronously)
send-code C (asynchronously)
wait-for-result-from B and C
...
kill-server-on B and C
How about the following?
* Master machine has a thread for each slave machine.
* Each slave machine handling thread in the master does the following:
first uses the "subprocess" procedure (or similar) to SSH (or "rsh") to
the slave machine and start the slave program, keeps that connection
open, and then runs in a loop, writing Racket expressions to stdin of
that remote process over SSH, and reading results from the stdout of
that process.
* The slave program on each machine can read Racket expressions from
stdin and write the results to stdout in a format readable by the "read"
procedure. You might be able to do this in a single thread.
* Regarding killing the slave process, you can send an expression that
tells a slave to die, and/or you can have the process die when you kill
the SSH connection.
That's a very simple setup that might do everything you need, and it
could be implemented from this outline by a second-year CS student.
If, however, you also want to do things like have the slave signal any
exceptions back to the master, permit the master to interrupt a slave,
take advantage of custodians for better resilience, have work queues,
etc., then there would be additional work you have to do. All the
details and subtleties of doing this well can't practically be covered
in an off-the-cuff email list post (considering that highly-skilled
people make their livelihoods doing such things well).
BTW, if you use SSH, you'll want to verify that small payloads get
padded so that they move promptly even if there is nothing else to
send. Offhand, I don't recall the SSH way to do that, but you should
look it up, so that you're not increasing latency or even intermittently
deadlocking a slave connection.
Neil V.
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