I just finished a second attempt at the asynchronous RPC implementation
incorporating Philip's feedback and some other ideas that I had. I think
it's easiest to explain how it works with an example. So here's a simple
IDL and schema:
IDL:
protocol Calculator {
int add(int arg1, int arg2);
}
Schema:
{"protocol":"Calculator","messages":{
"add":{
"request":[{"name":"arg1","type":"int"},{"name":"arg2","type":"int"}],
"response":"int"}}}
No changes are required to the IDL or schema to enable async RPCs. The Avro
Java compiler will generate two interfaces instead of one. The first
interface, Calculator, contains the standard synchronous methods. The
second interface, CalculatorClient, extends Calculator and adds asynchronous
methods for all two-way messages. The reason why the async methods are
separated out into a separate interface is that the responder/server side
doesn't need to know (and shouldn't know) about the client-side async
methods. So the Responder/server implements Calculator, and the
Requestor/client can either use Calculator or CalculatorClient to invoke the
RPCs. For reference, here is what the two generated interfaces look like
(without the PROTOCOL field and package names):
public interface Calculator {
int add(int arg1, int arg2) throws AvroRemoteException;
}
public interface CalculatorClient extends Calculator {
CallFuture<Integer> addAsync(int arg1, int arg2) throws IOException;
CallFuture<Integer> addAsync(int arg1, int arg2, Callback<Integer>
callback) throws IOException;
}
The CalculatorClient interface is the only new component. It has two
methods for each message, one that takes a Callback and one that does not.
Both methods return a CallFuture so that the client has the option of using
either the Future or the Callback to obtain the result of the RPC.
Future.get() blocks until the RPC is complete, and either returns the result
or throws an exception if one occurred during the RPC. The Callback
interface has two methods, handleResult(T result) and handleError(Exception
error). One or the other is always called depending on whether the RPC was
successful or an Exception was thrown.
In addition to the compiler changes, I had to make some changes in the
avro-ipc project to get the async plumbing to work correctly. Most of these
changes are in Requestor and NettyTransceiver. As part of the changes I had
to make to Requestor I ended up replacing a couple of large synchronized
blocks with finer-grained critical sections protected by reentrant locks. I
think this change improved performance overall, at least in the case where
multiple threads are using the same client. I implemented a rudimentary
performance test that spins up a bunch of threads, executes the same RPC
(Simple.hello(String)) repeatedly for a fixed amount of time, and then
calculates the average number of RPCs completed per second. With Avro 1.5.1
I got 7,450 RPCs/sec, and with my modified version of trunk I got 19,050
RPCs/sec. That was a very simple test, but if there is a standard benchmark
that the Avro team uses I'd be happy to rerun my tests using that.
So that's basically it. All existing unit tests pass, and I wrote
additional tests for all the new async functionality. I've documented all
public interfaces, and I think the changes are ready to be reviewed if any
of the committers have time to take a look. Should I post a patch
somewhere? AVRO-539? ReviewBoard?
-James
On Tue, May 31, 2011 at 9:09 PM, James Baldassari <[email protected]>wrote:
> Thanks for the helpful feedback!
>
> After thinking about this more, I agree that it would be cleaner and
> simpler to remove the "async" keyword/property from the IDL and schema.
> Instead I'll just generate the asynchronous companion methods for all
> two-way messages.
>
> Regarding the passing of RPC results and exceptions/errors back to the
> client asynchronously, I'm not sure what the best approach is. I had
> considered the use of both the future pattern and the callback pattern, but
> I think the callback pattern is more useful in general. One option I had
> considered was having the async methods accept a Callback parameter and also
> return a Future so that the client could choose which pattern to use on a
> case-by-case basis. I think I may go with the combined callback/future
> approach as it provides clients with the most flexibility.
>
> For Futures, error handling is straightforward: the get() method either
> returns the result of the callback or throws the exception if one occurred.
> The correct solution for the callback approach is less obvious, but I'll
> tell you the approach that I took, and I'm open to comments. The Callback
> interface has two methods, one for handling a successful RPC result, and one
> for handling exceptions that occur during RPC execution. For each RPC,
> either one or the other will be called, but never both. The methods look
> like this:
>
> void handleCallback(T result);
> void handleError(Exception error);
>
> I'll work on making the changes you suggested, and hopefully I'll have a
> patch that's in decent shape by the end of the week.
>
> -James
>
>
>
> On Tue, May 31, 2011 at 4:40 PM, Philip Zeyliger <[email protected]>wrote:
>
>> Hi James,
>>
>> I think this is a fine approach. You're correct that the place to do it
>> is in the code generator. It's not obvious to me that it belongs in the
>> schema, however, since you might have two different pieces of software that
>> want to use the same RPCs differently. Is there any harm in
>> always generating both types?
>>
>> As for your API, you'll want to specify very explicitly what happens to
>> the exceptions that an Avro RPC call may declare. Future<V> is one
>> mechanism. As is your Callback<Foo> mechanism, if there's a way to get at
>> exceptional states.
>>
>>
>> On Tue, May 31, 2011 at 12:08 AM, James Baldassari <[email protected]
>> > wrote:
>>
>>> Hi,
>>>
>>> I recently started playing with Avro RPCs, and while it's great that Avro
>>> can use Netty for asynchronous I/O, all client-facing Java RPC interfaces
>>> are currently synchronous (as far as I can tell). It would be nice to have
>>> asynchronous RPCs that use callbacks to take full advantage of Netty's
>>> asynchronous features. I found at least one other request for this feature
>>> on the Avro list (http://bit.ly/iCD0ae), and I believe this enhancement
>>> is already documented as AVRO-539.
>>>
>>> I took it on as a weekend project to add async Java RPCs to Avro, and I
>>> think I have it all working now including unit tests. I'd love to
>>> contribute this patch once I've gotten some feedback, cleaned up the
>>> documentation, and written a few more tests. I'll give a quick example
>>> demonstrating this new functionality. Consider the following IDL and its
>>> associated schema which use the asynchronous feature:
>>>
>>> IDL:
>>> protocol Calculator {
>>> int add(int arg1, int arg2) async;
>>> }
>>>
>>> Schema:
>>> {"protocol":"Calculator","messages":{
>>> "add":{
>>>
>>> "request":[{"name":"arg1","type":"int"},{"name":"arg2","type":"int"}],
>>> "response":"int",
>>> "async":true}}}
>>>
>>> When the "async" keyword/property is present in a message, the Avro Java
>>> compiler generates two methods instead of one: the standard synchronous
>>> method and an asynchronous companion method. For the example I gave above,
>>> the following interface would be generated (the static PROTOCOL field and
>>> package names are omitted for brevity):
>>>
>>> public interface Calculator {
>>> int add(int arg1, int arg2) throws AvroRemoteException;
>>> void addAsync(int arg1, int arg2, Callback<Integer> callback) throws
>>> IOException;
>>> }
>>>
>>> The addAsync method returns immediately without blocking (except one
>>> special case which I'll describe shortly). The callback provided as the
>>> last argument to addAsync will be invoked with the result of the RPC as soon
>>> as the server returns it. There are a couple of caveats. The first RPC,
>>> whether synchronous or asynchronous, must block until the client-server
>>> handshake has been completed. Subsequent async RPCs will never block.
>>> Also, only the NettyTransceiver currently works with async RPCs; all other
>>> transceivers throw UnsupportedOperationException.
>>>
>>> So that's the basic overview of my changes. Please let me know if there
>>> are any comments or questions. Is this something people are interested in?
>>> If so, should I post the patch in AVRO-539 or somewhere else?
>>>
>>> Thanks,
>>> James
>>>
>>>
>>
>
