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From Doug Daniels <daniels.doug...@gmail.com>
Subject Re: Using thrift as part of a game network protocol
Date Sat, 04 Apr 2009 04:45:48 GMT
The bi-directional single socket RPC is exactly what I'm looking for! The
idea of using async messages is probably the most efficient way to just
deliver a message, and then wait on the client's RPC TProcessor receive side
for messages from the server. I also think it'd be beneficial to support
full synchronous RPC calls client to server and server to client not just
async ones.

At first I thought it would be great to send/receive any type of Thrift
struct over a socket and have application code sort out what type it's
handling, but in the end if you either package an "ALL" message with each
message as an optional field and a type indicator, or if you can
send/receive arbitrary message structs you still deal with the issue of
dispatching those messages once your appliction receives it. The dispatching
part is essentially what a well-defined RPC interface allows you to do
without having to write code to dispatch it you just write the code to
actually process it. If you wanted to support the send/receive "any" message
you could just declare a one-to-one message to async RPC service method.

For the client/server games I'm imagining I think being able to scale to 20k
concurrent would be fine. Low latency and fast response times is more
important than handling more concurrent connections in a "fair" manner for
most games, although a connect send/receive then disconnect might be
acceptable for some games and you could queue messages needing to be
delivered to "disconnected" clients until they poll again, very similiar to
the AJAX COMETD long-polling idea (http://cometdproject.dojotoolkit.org/),
COMETD can be used as AJAX style Sockets.

I began hacking some of the bi-directional RPC myself, my first thought was
to allow regular synchronous (and async) RPC client to server calls, but to
support server to client calls I'd do something like this:

I'd create a TProtocol that would wrap a TTransport, something like a
BidirectionalProtocolAdapter (it wouldn't directly implement TProtocol). The
BidirectionalProtocolAdapter would expose a TSendRPCProtocol and a
TReceiveRPCProtocol that would split the messages being read as follows:

 - TClientSendProtocol
    - would use the oproto to send RPC messages of type (TMessageType.CALL)
to the server
    - readMessage() would only read messages of type (TMessageType.REPLY,
and TMessageType.EXCEPTION)

- TServerReceiveProtocol
  - would use the oproto to send messages of type (REPLY and Exception)
  - readMessage() would only read messages of type (CALL) and dispatch it to
a TProcessor implementing the appropriate service


Some issues/difficulties I see with this are:
- Only one of the client or server protocols could receive Exception
messages (it makes sense that the RPC Client receives exceptions is it
possible for a Thrift client to send an RPC server an Exception message?)
- The BidirectionalProtocolAdapter will probably need to use wait() and
notify() to allow the Client and Server protocols to read the correct
message type.
- Client and Server are tightly coupled to each other instead of just the
client knowing the server's RPC services the server must know the client's
RPC services


On Fri, Apr 3, 2009 at 10:48 PM, Will Lowe <will.lowe@gmail.com> wrote:

> I'm actually considering using Thrift in a similar way: as a fast,
> cross-language serialization-and-transport mechanism between a bunch of
> different apps in a pub/sub architecture.
>
> There are a number of possible message types -- 100? -- and it won't always
> be possible for each consumer to know if the other supports a given message
> type,  so I'd like to avoid each app having an RPC service for each possible
> message type;  I'd rather hand them objects and let them just ignore the
> ones they don't care about.
>
> I see a few different ways to do this:
>
> 1. Define only one struct with all possible fields for all possible
> messages,  and a "type" field that lets you figure out what it is.  It seems
> kinda stupid to do this,  since one of the major reasons I'm interested in
> Thrift is type-awareness.
>
> 2. Modify the TService layer so that RPC arguments aren't statically typed:
>  make it possible to declare an RPC call that accepts any struct.  Feels
> like (void*),  and probably also irritates purists who like the simple
> no-object-inheritance, no-function-overloading model Thrift uses today.
>
> 3. Build something custom at the TProcessor layer and skip TService
> altogether.
>
> Both #2 and #3 require changing some guts.  I think that would go something
> like this:
>
> * Add a TMessageType (T_STRUCT?) that indicates "I'm sending you data, not
> calling a function!".  Or is that what T_ONEWAY is for?
> * Modify TBinaryProtocol so that writeStructBegin() and writeStructEnd()
> aren't noops -- otherwise the receiver doesn't know what he's receiving!
> * Implement a TProcessor that can read the struct type, instantiate it,
>  and do some sort of dispatch to the client app.
>
> Any thoughts on this?  Has someone else already solved this?
>
> Will
>
>
>
> On Apr 3, 2009, at 6:22 PM, Brian Hammond 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)!
>>
>> 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.
>>
>> 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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