Andrew Bennetts wrote: > So it is impossible for the internet to route the messages such that B > will arrive before A. > > Neither PB nor TCP guarantee anything about when the recipient > “processes” those messages though, just that they will arrive in the > order they were sent. For example consider if A and B arrive very close > together (in the same millisecond, say) and the client processes them by > dispatching to a thread pool,
To expand on what Andrew said: Foolscap (which is descended from PB and has very similar semantics) makes the following guarantee: within any given client/server pair, if the client invokes callRemote(A) before invoking callRemote(B) then the server will invoke remote_A() before invoking remote_B() Your server's remote_A() method might return a Deferred, and not fire it for an arbitrary period of time, whereas it might return an immediate value from remote_B(). In this case, the client will see the responses come back in a different order. Your server might also choose to delay processing of one message for whatever reason (like by dispatching to a threadpool, or dispatching a request to a third server in order to service the request), but will be visible in the implementation of remote_A(). The only promise made by Foolscap and PB is that your remote_* methods will be invoked in the same order as the sender called callRemote(*). (in the future, Foolscap might offer some options to use multiple connections and send messages over multiple parallel paths, or to interleave message calls on a single connection, or to switch to UDP or something, which might weaken this guarantee, but the default will remain FIFO ordering) Also note that there are no ordering guarantees between messages sent to different machines (and therefore over different connections), of course. cheers, -Brian Foolscap: http://foolscap.lothar.com/trac _______________________________________________ Twisted-Python mailing list Twisted-Python@twistedmatrix.com http://twistedmatrix.com/cgi-bin/mailman/listinfo/twisted-python
