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From Joel Meyer <joel.me...@gmail.com>
Subject Re: Using thrift as part of a game network protocol
Date Thu, 02 Apr 2009 23:17:18 GMT
On Tue, Mar 24, 2009 at 5:01 PM, Doug Daniels <daniels.douglas@gmail.com>wrote:

> Ok I definitely plan on giving the Async RPC methods a try tonight, but I
> figured I'd just throw out some questions before I get home to start
> hacking
> on this stuff.
>
> The one-to-one message to RPC call Async solution will let a client send
> messages of any given type in my defined protocol, but how would a server
> respond to a client with a message that the client didn't request? For
> example say I was trying to write a FPS like Quake and I want to server to
> send position updates for all clients to all clients, how would i model
> that
> as a client RPC request for that. With the Async RPC solutions I could make
> a RPC call for Map<Integer, Position> getPositionUpdates(), Now say that
> the
> client needs to request 50 other messages to be notified of. I guess the
> solution would be to make an Async RPC call requesting those updates and
> respond to it when I receive it asynchronously and then reissue another
> Async RPC call for the next set of updates. It just seems inefficient to
> actively make the client request for data when the server could implicitly
> know that when connected on this game protocol I can just send these
> messages to the clients without them asking for it. Not to mention you'd
> have make sure you don't "miss" sending a client a message if they finished
> their Async call but haven't reestablished a new one.
>

I think I've done something similar to what you're trying to do, and as long
as you can commit to using only async messages it's possible to pull it off
without having to start a server on the client to accept RPCs from the
server.

When your RPC is marked as async the server doesn't send a response and the
client doesn't try to read one. So, if all your RPC calls from the client to
the server are async you have effectively freed up the inbound half of the
socket connection. That means that you can use it for receiving async
messages from the server - the only catch is that you have to start a new
thread to read and dispatch the incoming async RPC calls.

In a typical Thrift RPC system you'd create a MyService.Processor on your
server and a MyService.Client on your client. To do bidirectional async
message sending you'll need to go a step further and create a
MyService.Client on your server for each client that connects (this can be
accomplished by providing your own TProcessorFactory) and then on each
client you create a MyService.Processor. (This assumes that you've gone with
a generic MyService definition like you described above that has a bunch of
optional messages, another option would be to define separate service
definitions for the client and server.) With two clients connected the
objects in existence would look something like this:

Server:
  MyService.Processor mainProcessor - handles incoming async RPCs
  MyService.Client clientA - used to send outgoing async RPCs to ClientA
  MyService.Client clientB - used to send outgoing async RPCs to ClientB

ClientA:
  MyService.Client - used to send messages to Server
  MyService.Processor clientProcessor - used (by a separate thread) to
process incoming async RPCs

ClientB:
  MyService.Client - used to send messages to Server
  MyService.Processor clientProcessor - used (by a separate thread) to
process incoming async RPCs

Hopefully that explains the concept. If you need example code I can try and
pull something together (it will be in Java). The nice thing about this
method is that you don't have to establish two connections, so you can get
around the firewall issues others have mentioned. I've been using this
method on a service in production and haven't had any problems. When you
have a separate thread in your client running a Processor you're basically
blocking on a read, waiting for a message from the server. The benefit of
this is that you're notified immediately when the server shuts down instead
of having to wait until you send a message and then finding out that the TCP
connection was reset.

Cheers,
Joel


>
> The biggest issue is that not all client request will result in a single
> response (like shooting a bullet, may blowup an entity, and damage all
> players in the area those events are seperate messages sent from the
> respective entities).
>
> At a game development studio I used to work at we developed a cross
> language
> IDL network protocol definition (C++, Java)  very similiar to Protocol
> Buffers and Thrift (without some of the more mature features like being
> transport agnostic we explicitly built it for binary TCP socket transport,
> or protocol versioning), the stream of packets would contain as the first
> 32
> bits a message ID that would be a key to a map a Message class that would
> have methods to read in that message type from a byte[] stream.
>
> Looking through Thrift code in the TBinaryProtocol writeMessage it looks
> like it's including the name of the message being sent and it's type (is
> the
> concept of Message in thrift the same as RPC?), if so what's the
> corresponding code pathway for the client waiting for an RPC response
> because if I could just use this message name or type to key into what I
> need to serialize off the network from both client and server end then that
> would be perfect.
>
>
>
> On Tue, Mar 24, 2009 at 1:51 PM, Ted Dunning <ted.dunning@gmail.com>
> wrote:
>
> > I really think that using async service methods which are matched one to
> > one
> > with the message types that you want to send gives you exactly the
> > semantics
> > that are being requested with very simple implementation cost.
> >
> > It is important to not get toooo hung up on what RPC stands for.  I use
> > async methods all the time to stream data structures for logging and it
> > works great.  Moreover, it provides a really simple way of building
> > extractors and processors for this data since I have an interface sitting
> > there that will tell me about all of the methods (data types) that I need
> > to
> > handle or explicitly ignore.
> >
> > So the trick works and works really well.  Give it a try!
> >
> > On Tue, Mar 24, 2009 at 8:23 AM, Bryan Duxbury <bryan@rapleaf.com>
> wrote:
> >
> > > Optional fields are not serialized onto the wire. There is a slight
> > > performance penalty at serialization time if you have a ton of unset
> > fields,
> > > but that's it.
> > >
> > >  Am I over complicating things
> > >>
> > >
> > > Personally, sounds like it to me. Why do you need this streaming
> behavior
> > > or whatnot? Hotwiring the rpc stack to let you send any message you
> want
> > is
> > > going to be a ton of work and not really that much of a functionality
> > > improvement.
> > >
> > > -Bryan
> > >
> >
> >
> >
> > --
> > Ted Dunning, CTO
> > DeepDyve
> >
>

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