[Hawkular-dev] [metrics] Looking for advice on ListenableFuture usage

[John Sanda]

In the metrics code, at least where a response from the C* driver is involved, the versions that take an executor is used almost everywhere. We do not want to block I/O threads doing work so in general it is best to use the overloaded versions of the methods.

> On Jan 16, 2015, at 4:47 AM, Thomas Segismont <tsegismo at redhat.com> wrote:
> 
> Hi everyone,
> 
> I'm looking for advice on how to use Futures#transform[1] and friends 
> (#addCallback, #withFallback)
> 
> These methods all have an overloaded version which takes a JDK Executor 
> instance as additional argument.
> 
> When the base method is called, Guava will use the so-called 
> "sameThreadExecutor". Consequently, the code will be executed:
> * by the caller thread, if the underlying Future has completed already
> * otherwise, by the thread completing the underlying Future
> 
> When the overloaded method is called, Guava will submit tasks to the 
> provided executor.
> 
> In the metrics code right now, the base method is used almost 
> everywhere. Considering that in most cases it will take more time to get 
> the response from C* than to complete the REST Handler code, it's the 
> Datastax driver threads which will execute the chain of callbacks, down 
> to the call to javax.ws.rs.container.AsyncResponse#resume
> 
> Could that be a problem? I guess it's safe when callbacks only 
> transforms the data loaded. I'm not sure about the call to 
> AsyncResponse#resume.
> 
> Thoughts?
> 
> Thanks,
> Thomas
> 
> 
> [1] 
> http://docs.guava-libraries.googlecode.com/git-history/v16.0.1/javadoc/com/google/common/util/concurrent/Futures.html 
> 
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