[undertow-dev] scala, activate, nio, async jdbc, future, threads

[Jason Greene]

Hello Laurent,

Your hunch is correct. You never want to block the I/O thread as each thread is shared between many connections, and a pause prevents them from being processed. It’s totally reasonable to call dispatch(), which transfers the exchange to the worker pool where you can wait on a database reply, and in fact this is a normal pattern. The worker pool is sized to be pretty large as its expected to host lots of waiters. You can of course tweak this, or dispatch to your own special pool if you like. The big advantage to an async database call, even with requiring a worker thread, is that you can do other work while its running if your pattern fits (e.g. parallel db calls, iterative computation, etc). 

Alternatively, if the database provides a callback on complete option, you can save yourself a worker thread and reuse the thread they generate the callback from. At that callback point you can write your response out to the exchange.  Just note, that the thread policy of an exchange is that only one thread at a time can interact with it. If you have a use case where the a request bounces back and forth (e.g. a very long stream containing multiple records that must be written to the database as they com in), then you will need to coordinate locking around that. Such use-cases though should be weighed against the simplicity of just blocking in a worker thread.

To answer a second question you have in a comment from your example, startBlocking() just enables the use of a plain inputstream/outputstream, which can only be done once you have dispatched and blocking i/o is now ok.

Hope this helps,

-Jason

> On Dec 12, 2014, at 3:44 PM, Laurent Bedubourg <laurent at labe.me> wrote:
> 
> Ok, I think my mail could be reduced to :
> 
> Is it possible possible to transfer the HttpServerExchange responsibility to another thread ?
> 
> Regards
> Laurent
> 
> 
> On Fri Dec 12 2014 at 7:11:01 PM Laurent Bedubourg <laurent at labe.me> wrote:
> Hello,
> 
> I am evaluating Undertow and trying to fit an async database (with scala, Activate and https://github.com/mauricio/postgresql-async which use netty and nio).
> 
> The good news is that it more or less works.
> 
> My main concern is that the database system create threads using scala futures and that I am forced to "Await" for the result of my Futures to send the result to the HttpServerExchange.
> 
> I am forced to dispath() the request because Await locks the current thread and I shouldn't lock the IO thread from my Handler, should I?
> 
> It made me wondering : is is really a good idea to use futures + Await with undertow and more generaly nio? 
> 
> If the thread pool is limited and I lock threads, even if the database driver is async and uses nio, am I bitting my own leg?
> 
> Thanks for any input you can give me and sorry if it's too scala related :)
> 
> Regards
> Laurent
> 
> PS: Here's the handleRequest I am using to test this, any comment or help welcome too since I am discovering the API.
> 
> def handleRequest(x:io.undertow.server.HttpServerExchange){
> if (x.isInIoThread()){
>   x.dispatch(this)
>   return
> }
> // useful or not? no change
> // x.startBlocking()
> 
> // Ok, we are in worker thread we can work a little with database
> // val f = asyncTransactionalChain { implicit context =>
> //   for (
> //     a <- asyncTransactional { new AMember("aaa") };
> //     b <- asyncById[AMember](a.id)
> //   ) yield b
> // }
> 
> // Simulate database work with just a future
> val f = future {
>   Some(1)
> }(scala.concurrent.ExecutionContext.Implicits.global)
> 
> // send string to xchange
> def sendReply(str:String){
>   x.getResponseHeaders().put(io.undertow.util.Headers.CONTENT_TYPE, "text/plain")
>   x.getResponseSender().send(str)
>   // x.endExchange()
> }
> 
> // // this fails
> // f.onSuccess {
> //   case Some(result) => sendReply(result.toString)
> // }(scala.concurrent.ExecutionContext.Implicits.global)
> 
> // // this works
> val result = Await.result(f, Duration(1000, MILLISECONDS))
> sendReply(result.toString)
> }
> 
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--
Jason T. Greene
WildFly Lead / JBoss EAP Platform Architect
JBoss, a division of Red Hat