SynchronousQueue

[Arkadiusz Gasiński]

Hi,

I watched Alan's Loom presentation from last year's Devoxx and there was
this SynchronousQueue example <https://youtu.be/vbGbXUjlRyQ?t=689>. I
decided to try it out with the latest loom build and actually one thing
caught me by surprise.

Here are the sources
<https://github.com/jigga/loom-workshop/blob/master/synchronous-queue/src/main/java/loom/SyncQueueDemo.java>
of mine rendition of Alan's example.

If you want to quickly run it, you can basically do the following:

docker run -it gasinski/debian-loom-workshop

root at 65a43986c5b0:/loom-workshop# ./gradlew :synchronous-queue:bootRun

What surprised me was the output, which always looks like this:

> Task :synchronous-queue:bootRun
Thread[Fiber,5,ShadowThreads]|Offering a task: {"id" : 1, "class" : "Task"}
Thread[Fiber,5,ShadowThreads]|Processing task: {"id" : 1, "class" : "Task"}
Thread[Fiber,5,ShadowThreads]|Offering a task: {"id" : 2, "class" : "Task"}
Thread[Fiber,5,ShadowThreads]|Offering a task: {"id" : 3, "class" : "Task"}
Thread[Fiber,5,ShadowThreads]|Processing task: {"id" : 2, "class" : "Task"}
Thread[Fiber,5,ShadowThreads]|Processing task: {"id" : 3, "class" : "Task"}
Thread[Fiber,5,ShadowThreads]|Offering a task: {"id" : 4, "class" : "Task"}
Thread[Fiber,5,ShadowThreads]|Offering a task: {"id" : 5, "class" : "Task"}
Thread[Fiber,5,ShadowThreads]|Processing task: {"id" : 4, "class" : "Task"}
Thread[Fiber,5,ShadowThreads]|Processing task: {"id" : 5, "class" : "Task"}
Thread[Fiber,5,ShadowThreads]|Offering a task: {"id" : 6, "class" : "Task"}
Thread[Fiber,5,ShadowThreads]|Offering a task: {"id" : 7, "class" : "Task"}
Thread[Fiber,5,ShadowThreads]|Processing task: {"id" : 6, "class" : "Task"}
Thread[Fiber,5,ShadowThreads]|Processing task: {"id" : 7, "class" : "Task"}
Thread[Fiber,5,ShadowThreads]|Offering a task: {"id" : 8, "class" : "Task"}
Thread[Fiber,5,ShadowThreads]|Offering a task: {"id" : 9, "class" : "Task"}
Thread[Fiber,5,ShadowThreads]|Processing task: {"id" : 8, "class" : "Task"}
Thread[Fiber,5,ShadowThreads]|Processing task: {"id" : 9, "class" : "Task"}

I assumed (as SynchronousQueue has no capacity) that the output would be
more like:

Produder.offer
Consumer.take
Producer.offer
Consumer.tak

And so on. But in the actual output almost each time 2 tasks are actually
offered and then 2 tasks are taken. Do you have any idea why this happens?

Thanks,
Arek