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package com.hubspot.singularity.async;
import com.google.common.base.Suppliers;
import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
import java.util.Optional;
import java.util.Queue;
import java.util.concurrent.Callable;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
import java.util.concurrent.locks.StampedLock;
import java.util.function.Supplier;
/**
* AsyncSemaphore guarantees that at most N executions
* of an underlying completablefuture exeuction are occuring
* at the same time.
*
* The general strategy is to try acquiring a permit for execution.
* If it succeeds, the semaphore just proceeds normally. Otherwise,
* it pushes the execution onto a queue.
*
* At the completion of any execution, the queue is checked for
* any pending executions. If any executions are found, they are
* executed in order.
*
* @param
*/
public class AsyncSemaphore {
private final StampedLock stampedLock = new StampedLock();
private final AtomicInteger concurrentRequests = new AtomicInteger();
private final Queue> requestQueue;
private final com.google.common.base.Supplier queueRejectionThreshold;
private final Supplier timeoutExceptionSupplier;
private final PermitSource permitSource;
/**
* Create an AsyncSemaphore with the given limit.
*
* @param concurrentRequestLimit - A supplier saying how many concurrent requests are allowed
*/
public static AsyncSemaphoreBuilder newBuilder(
Supplier concurrentRequestLimit,
ScheduledExecutorService flushingExecutor
) {
return new AsyncSemaphoreBuilder(
new PermitSource(concurrentRequestLimit),
flushingExecutor
);
}
/**
* Create an AsyncSemaphore with the given permit source.
*
* @param permitSource - A source for the permits used by the semaphore
*/
public static AsyncSemaphoreBuilder newBuilder(
PermitSource permitSource,
ScheduledExecutorService flushingExecutor
) {
return new AsyncSemaphoreBuilder(permitSource, flushingExecutor);
}
AsyncSemaphore(
PermitSource permitSource,
Queue> requestQueue,
Supplier queueRejectionThreshold,
Supplier timeoutExceptionSupplier,
boolean flushQueuePeriodically,
ScheduledExecutorService flushingExecutor
) {
this.permitSource = permitSource;
this.requestQueue = requestQueue;
this.queueRejectionThreshold =
Suppliers.memoizeWithExpiration(queueRejectionThreshold::get, 1, TimeUnit.MINUTES);
this.timeoutExceptionSupplier = timeoutExceptionSupplier;
if (flushQueuePeriodically) {
flushingExecutor.scheduleAtFixedRate(() -> flushQueue(), 1, 1, TimeUnit.SECONDS);
}
}
public CompletableFuture call(Callable> execution) {
return callWithQueueTimeout(execution, Optional.empty());
}
/**
* Try to execute the supplier if there are enough permits available.
* If not, add the execution to a queue (if there is room).
* If the queue attempts to start the execution after the timeout
* has passed, short circuit the execution and complete the future
* exceptionally with TimeoutException
*
* @param execution - The execution of the item
* @param timeout - The time before which we'll short circuit the execution
* @param timeUnit
* @return
*/
public CompletableFuture callWithQueueTimeout(
Callable> execution,
long timeout,
TimeUnit timeUnit
) {
return callWithQueueTimeout(
execution,
Optional.of(TimeUnit.MILLISECONDS.convert(timeout, timeUnit))
);
}
private CompletableFuture callWithQueueTimeout(
Callable> execution,
Optional timeoutInMillis
) {
if (timeoutInMillis.isPresent() && timeoutInMillis.get() <= 0) {
return CompletableFutures.exceptionalFuture(timeoutExceptionSupplier.get());
} else if (tryAcquirePermit()) {
CompletableFuture responseFuture = executeCall(execution);
pollQueueOnCompletion(responseFuture);
return responseFuture;
} else {
DelayedExecution delayedExecution = new DelayedExecution<>(
execution,
timeoutExceptionSupplier,
timeoutInMillis
);
if (!tryEnqueueAttempt(delayedExecution)) {
return CompletableFutures.exceptionalFuture(
new RejectedExecutionException("Could not queue future for execution.")
);
}
return delayedExecution.getResponseFuture();
}
}
private void pollQueueOnCompletion(CompletableFuture future) {
future.whenComplete(
(ignored1, ignored2) -> {
// iterate through expired executions rather than using callbacks
// to avoid StackoverflowError if futures are completed or expired
while (true) {
DelayedExecution nextExecutionDue = requestQueue.poll();
if (nextExecutionDue == null) {
releasePermit();
return;
} else if (nextExecutionDue.isExpired()) {
nextExecutionDue
.getResponseFuture()
.completeExceptionally(timeoutExceptionSupplier.get());
} else {
// reuse the previous permit for the queued request
CompletableFuture nextExecution = nextExecutionDue.execute();
if (!nextExecution.isDone()) {
pollQueueOnCompletion(nextExecution);
return;
}
}
}
}
);
}
private boolean tryAcquirePermit() {
boolean acquired = permitSource.tryAcquire();
if (acquired) {
concurrentRequests.incrementAndGet();
}
return acquired;
}
private int releasePermit() {
permitSource.release();
return concurrentRequests.decrementAndGet();
}
private static CompletableFuture executeCall(
Callable> execution
) {
try {
return execution.call();
} catch (Throwable t) {
return CompletableFutures.exceptionalFuture(t);
}
}
/**
* enqueue the attempt into our underlying queue. since it's expensive to dynamically
* resize the queue, we have a separate rejection threshold which, if less than 0 is
* ignored, but otherwise is the practical cap on the size of the queue.
*/
private boolean tryEnqueueAttempt(DelayedExecution delayedExecution) {
int rejectionThreshold = queueRejectionThreshold.get();
if (rejectionThreshold < 0) {
return requestQueue.offer(delayedExecution);
}
long stamp = stampedLock.readLock();
try {
while (requestQueue.size() < rejectionThreshold) {
long writeStamp = stampedLock.tryConvertToWriteLock(stamp);
if (writeStamp != 0L) {
stamp = writeStamp;
return requestQueue.offer(delayedExecution);
} else {
stampedLock.unlock(stamp);
stamp = stampedLock.writeLock();
}
}
return false;
} finally {
stampedLock.unlock(stamp);
}
}
private void flushQueue() {
if (tryAcquirePermit()) {
// Pass in an already completed future so that we execute the callback on this thread
pollQueueOnCompletion(CompletableFuture.completedFuture(true));
}
}
static class DelayedExecution {
private static final AtomicIntegerFieldUpdater EXECUTED_UPDATER = AtomicIntegerFieldUpdater.newUpdater(
DelayedExecution.class,
"executed"
);
private final Callable> execution;
private final CompletableFuture responseFuture;
private final Supplier timeoutExceptionSupplier;
private final long deadlineEpochMillis;
@SuppressWarnings("unused") // use the EXECUTED_UPDATER
private volatile int executed = 0;
private DelayedExecution(
Callable> execution,
Supplier timeoutExceptionSupplier,
Optional timeoutMillis
) {
this.execution = execution;
this.responseFuture = new CompletableFuture<>();
this.timeoutExceptionSupplier = timeoutExceptionSupplier;
this.deadlineEpochMillis =
timeoutMillis.map(x -> System.currentTimeMillis() + x).orElse(0L);
}
private CompletableFuture getResponseFuture() {
return responseFuture;
}
@SuppressFBWarnings("NP_NONNULL_PARAM_VIOLATION") // https://github.com/findbugsproject/findbugs/issues/79
private CompletableFuture execute() {
if (!EXECUTED_UPDATER.compareAndSet(this, 0, 1)) {
return CompletableFuture.completedFuture(null);
}
return executeCall(execution)
.whenComplete(
(response, ex) -> {
if (ex == null) {
responseFuture.complete(response);
} else {
responseFuture.completeExceptionally(ex);
}
}
)
.thenApply(ignored -> null);
}
private boolean isExpired() {
return deadlineEpochMillis > 0 && System.currentTimeMillis() > deadlineEpochMillis;
}
}
public int getQueueSize() {
long stamp = stampedLock.tryOptimisticRead();
int queueSize = requestQueue.size();
if (!stampedLock.validate(stamp)) {
stamp = stampedLock.readLock();
try {
queueSize = requestQueue.size();
} finally {
stampedLock.unlockRead(stamp);
}
}
return queueSize;
}
public int getConcurrentRequests() {
return concurrentRequests.get();
}
}
