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From Joel Meyer <joel.me...@gmail.com>
Subject Re: Using thrift as part of a game network protocol
Date Sat, 04 Apr 2009 17:17:42 GMT
Hi Brian, responses inline below:

On Fri, Apr 3, 2009 at 6:22 PM, Brian Hammond <brian@brianhammond.com>wrote:

> That's neat Joel.  However, does this scale?  I mean, the underlying
> assumption here is that clients are using persistent connections to the
> service, and you [not so simply] are sending messages back to the client
> over that same connection.  Thus, your service now has to handle a
> potentially large number of client connections.  Unless you're using
> something libev[ent] I don't see this scaling beyond say 20K connections.
>  Two things, I could be missing something here, and this level of
> scalability is probably just fine for *many* types of services (perhaps not
> for a chat server though)!
>

The code I've shared won't scale nearly as well as a kqueue/epoll/select
based implementation because it requires a thread on the server for every
client. Another problem with my example is that every client gets every
message, and when a message is sent the sending is done using the thread
handling requests from the client that sent the original message. Some of
this could be avoided by using the THaHsServer (I think that's the class
name, not positive) which is select/NIO based (in Java) and wouldn't require
a new thread for every client. Calling it a chat server was done more to
give a quick idea of what the code does - I'd definitely NOT recommend
designing your chat server this way :)

The concept that I was trying to demonstrate (sending messages both ways
using a single socket), however, should scale just fine - at least as well
as having socket connections in both directions. The number of connections a
single server can handle will be determined more by the workload required of
the server for each received message and how active each client is. If the
clients are dormant a majority of the time a libev[ent] based server should
scale quite well.


>
> I'm curious to hear other people's thoughts on this and how it could be
> made scalable since, well, I'm planning on using polling in my project since
> I am expecting potentially a very large number of simultaneous users of the
> service and my servers can only handle so many connections.


Me too, it'd be great to hear thoughts/experiences from other people who are
scaling Thrift based RPC servers.

Cheers,
Joel


>
>
> Thanks for sharing this.
>
> Brian
>
>
> On Apr 3, 2009, at 7:50 PM, Joel Meyer wrote:
>
>  On Thu, Apr 2, 2009 at 4:17 PM, Joel Meyer <joel.meyer@gmail.com> wrote:
>>
>>  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
>>>
>>>
>> Thanks for the feedback. I've created a simple example in Java
>> demonstrating
>> this in action:
>> http://www.joelpm.com/wp-content/uploads/2009/04/bidimessages.tgz
>>
>> Post with a few details on the implementation:
>> http://www.joelpm.com/2009/04/03/thrift-bidirectional-async-rpc/
>>
>> Please add me to the list of people who think there's value in a full
>> async
>> transport that provides (optional?) synchronization at the api level using
>> futures/deferreds/etc.
>>
>> 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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