Re: [Twisted-Python] order of remote requests
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
Re: [Twisted-Python] order of remote requests
Wolfgang Rohdewald wrote: Hi, when I send two remote calls A and B over the network, in that order, using the twisted perspective broker callRemote method, is it granted that A is processed first on the other side even if B arrives first? Or should I only send B after I got an answer to A? It depends on the server, and what exactly you want to happen. Perspective broker can have many concurrent remote calls on the same connection, and depending on how the server is implemented it might process those sequentially in the order they arrive, or concurrently, or even in an arbitrary order. So if call A really must be completed before starting B, then the client must not make call B until it knows A has finished. -Andrew. ___ Twisted-Python mailing list Twisted-Python@twistedmatrix.com http://twistedmatrix.com/cgi-bin/mailman/listinfo/twisted-python
Re: [Twisted-Python] order of remote requests
On Tuesday 29 December 2009, Andrew Bennetts wrote: Wolfgang Rohdewald wrote: when I send two remote calls A and B over the network, in that order, using the twisted perspective broker callRemote method, is it granted that A is processed first on the other side even if B arrives first? Or should I only send B after I got an answer to A? It depends on the server, and what exactly you want to happen. Perspective broker can have many concurrent remote calls on the same connection, and depending on how the server is implemented it might process those sequentially in the order they arrive, or concurrently, or even in an arbitrary order. So if call A really must be completed before starting B, then the client must not make call B until it knows A has finished. I should have given more details, sorry. I am writing both the server and the client with twisted pb. The server (a game server) sends messages to four player clients. Say the game server sends messages A and B to a player client. Is it granted that the player client always processes A first even if the internet routes the messages differently and B arrives first? -- Wolfgang ___ Twisted-Python mailing list Twisted-Python@twistedmatrix.com http://twistedmatrix.com/cgi-bin/mailman/listinfo/twisted-python
Re: [Twisted-Python] order of remote requests
On 2009.12.29 12:53:07 +0100, Wolfgang Rohdewald wrote: On Tuesday 29 December 2009, Andrew Bennetts wrote: Wolfgang Rohdewald wrote: when I send two remote calls A and B over the network, in that order, using the twisted perspective broker callRemote method, is it granted that A is processed first on the other side even if B arrives first? Or should I only send B after I got an answer to A? It depends on the server, and what exactly you want to happen. Perspective broker can have many concurrent remote calls on the same connection, and depending on how the server is implemented it might process those sequentially in the order they arrive, or concurrently, or even in an arbitrary order. So if call A really must be completed before starting B, then the client must not make call B until it knows A has finished. I should have given more details, sorry. I am writing both the server and the client with twisted pb. The server (a game server) sends messages to four player clients. Say the game server sends messages A and B to a player client. Is it granted that the player client always processes A first even if the internet routes the messages differently and B arrives first? TCP guarantees in-order message delivery. If you send A before B over the same TCP connection, and A and B reach the client, then the client will receive A before B. Each Twisted process is single-threaded by default. So if you use TCP, and keep the message processing code simple, then I think you'll get messages processed in the order you expect. I've never noticed a problem with out-of-order messages in my PB game, anyway. -- David Riptondrip...@ripton.net ___ Twisted-Python mailing list Twisted-Python@twistedmatrix.com http://twistedmatrix.com/cgi-bin/mailman/listinfo/twisted-python
Re: [Twisted-Python] order of remote requests
On Tuesday 29 December 2009, David Ripton wrote: TCP guarantees in-order message delivery. If you send A before B over the same TCP connection, and A and B reach the client, then the client will receive A before B. Of course - now I wonder what made me think otherwise. Thank you for your explanation! -- Wolfgang ___ Twisted-Python mailing list Twisted-Python@twistedmatrix.com http://twistedmatrix.com/cgi-bin/mailman/listinfo/twisted-python
Re: [Twisted-Python] order of remote requests
Wolfgang Rohdewald wrote: [...] I should have given more details, sorry. I am writing both the server and the client with twisted pb. The server (a game server) sends messages to four player clients. Say the game server sends messages A and B to a player client. Is it granted that the player client always processes A first even if the internet routes the messages differently and B arrives first? A PB connection uses a single TCP connection. TCP guarantees that the bytes will arrive in order (or not at all), so the messages will arrive at the client in the order the server sent them, assuming there is one persistent PB connection between each client and the server (which seems likely from your description). 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, which might take 10s or even 100s of milliseconds to fully process the message (and determine a reply, if applicable)... then depending on the exact work required and exactly how the OS schedules the threads, then B might be fully processed before A is, even though A arrived first. Apologies for such a pedantically precise and abstract answer, but what exactly you mean when you say “processes” is a bit ambiguous. I hope this reply helps. -Andrew. ___ Twisted-Python mailing list Twisted-Python@twistedmatrix.com http://twistedmatrix.com/cgi-bin/mailman/listinfo/twisted-python
