[riku_iki]

> Either of those severely limits the scalability

you can avoid both issues by using 20yo executorservice.

[pron]

If the code is simple, blocking code, then the number of threads required in the pool is the average total duration of a request times the fanout times the request rate. That number can easily reach many thousands and more.

[riku_iki]

yes, you shouldn't add blocking code into executorservice..

[kaba0]

Wtf, where on Earth do you put blocking code then? Firing off some long-running task in a background thread through executors is bog-standard usecase.

[riku_iki]

discussion was about specific context: avoiding overhead from spawning millions of threads, in this case you shouldn't have any blocking code at all, all API should utilize epoll underneath or something similar.

[kaba0]

And where do you handle callbacks?

[riku_iki]

there are tons of variations, depending on your logic and API. The closest to virtual threads is ForkJoinPool and RecursiveTask, where you can have code like regular blocking code:

var f = async_api_returns_future();

...

var res = f.join();

but join() won't block OS/JVM thread, but make it to perform other tasks in the queue.

Or you can design API which will receive executorService as params, and run callback there, e.g.:

async_call(Callable callback, ExecutorService threadPool);

[pron]

Then you either don't get the same scalability that virtual threads give you or you get it but with asynchronous code that requires not just more work but can't enjoy the same observability/debuggability on the Java platform.

[riku_iki]

could you give example what requires more work exactly and where virtual threads give more "observability"?..

[pron]

Sure. Because handling server requests typically requires IO, if you wish not to block you need some way to sequence operations that is different from the ordinary sequential composition of the language (beforeIO(); blockingIO(); afterIO()). Similarly, other language constructs that build on top of basic sequential composition -- loops, exceptions, try/finally -- no longer work across the IO boundary. Instead you must reach for an asynchronous composition DSL (such as the one offered by CompletableFuture) which is not as composable as the basic language primitives.

Moreover, the platform now has no insight about your composition. Exceptions, which are designed to give context in the form of a thread stack trace, simply don't know about the context as it's not composed through the normal composition (in plain terms, stack traces in asynchronous code don't give you the operation's context). Debuggers cannot step through the asynchronous flow because the platform's built in debugging support works only by stepping through threads, and profilers are no longer able to assign IO to operations: a server that's under heavy load may show up as idle thread pools in a profiler because the platform cannot assign an asynchronous operation to some asynchronous pipeline such as CompletableFutures because these are not observable constructs of the Java platform.

Virtual threads give you the same scalability as asynchronous code but in a way that fits with the design of the Java platform. All language constructs work and compose well, debuggers step through code, and profilers can understand what's going on.

That's not to say that some other platform could not be designed around a different construct, but the Java platform -- from language, through libraries and bytecode, and all the way to the VM and its tooling interfaces -- was designed around the idea that sequential composition occurs by sequencing operations on a single thread. And virtual threads are just Java threads.

[riku_iki]

I am not certain many of your assessments are correct. If you give specific simple example of code, we can iterate there.