Scaling Simply

try (var executor = Executors
		.newVirtualThreadPerTaskExecutor()) {
	for (int i = 0; i < 1_000_000; i++) {
		var number = i;
		executor.submit(() -> {
			Thread.sleep(Duration.ofSeconds(1));
			return number;
		});
	}
} // executor.close() is called implicitly, and waits

spring.threads.virtual.enabled=true

@GET
@Path("api")
@RunOnVirtualThread
public String handle() {
	// ...
}

// limits concurrent queries but pools 👎🏾
private static final ExecutorService DB_POOL =
	Executors.newFixedThreadPool(16);

public <T> Future<T> queryDatabase(Callable<T> query) {
	return DB_POOL.submit(query);
}

// limits concurrent queries without pool 👍🏾
private static final Semaphore DB_SEMAPHORE =
	new Semaphore(16);

public <T> T queryDatabase(Callable<T> query)
		throws Exception {
	DB_SEMAPHORE.acquire();
	try {
		return query.call();
	} finally {
		DB_SEMAPHORE.release();
	}
}

// guarantees sequential access, but pins (for now) 👎🏾
public synchronized String accessResource() {
	return access();
}

// guarantees sequential access without pinning 👍🏾
private static final ReentrantLock LOCK =
	new ReentrantLock();

public String accessResource() {
	// lock guarantees sequential access
	LOCK.lock();
	try {
		return access();
	} finally {
		LOCK.unlock();
	}
}

// copies value for each inheriting thread 👎🏾
static final ThreadLocal<Principal> PRINCIPAL =
	new ThreadLocal<>();

public void serve(Request request, Response response) {
	var level = request.isAdmin() ? ADMIN : GUEST;
	var principal = new Principal(level);
	PRINCIPAL.set(principal);
	Application.handle(request, response);
}

// immutable, so no copies needed 👍🏾
static final ScopedValue<Principal> PRINCIPAL =
	new ScopedValue<>();

public void serve(Request request, Response response) {
	var level = request.isAdmin() ? ADMIN : GUEST;
	var principal = new Principal(level);
	ScopedValue
		.where(PRINCIPAL, principal)
		.run(() -> Application
			.handle(request, response));
}

private static final ExecutorService VIRTUAL =
	Executors.newVirtualThreadPerTaskExecutor()

void handle(Request request, Response response)
		throws InterruptedException {
	var taskA = VIRTUAL.submit(this::doA);
	var taskB = VIRTUAL.submit(this::doB);
	response.send(taskA.get() + taskB.get());
}

private static final ExecutorService VIRTUAL =
	Executors.newVirtualThreadPerTaskExecutor()

void handle(Request request, Response response)
		throws InterruptedException {
	// what's the relationship between
	// this and the two spawned threads?
	// what happens when one of them fails?
	var taskA = VIRTUAL.submit(this::doA);
	var taskB = VIRTUAL.submit(this::doB);
	// what if we only need the faster one?
	response.send(taskA.get() + taskB.get());
}

void handle(Request request, Response response)
		throws InterruptedException {
	// define explicit success/error handling
	try (var scope = new StructuredTaskScope
			.ShutdownOnFailure()) {
		var taskA = scope.fork(this::doA);
		var taskB = scope.fork(this::doB);
		// wait explicitly until success criteria met
		scope.join();
		scope.throwIfFailed();
		response.send(taskA.get() + taskB.get());
	} catch (ExecutionException ex) {
		response.fail(ex);
	}
}