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/*
* Copyright 2012-2016 Amazon.com, Inc. or its affiliates. All Rights Reserved.
*
* Licensed under the Apache License, Version 2.0 (the "License").
* You may not use this file except in compliance with the License.
* A copy of the License is located at
*
* http://aws.amazon.com/apache2.0
*
* or in the "license" file accompanying this file. This file is distributed
* on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
* express or implied. See the License for the specific language governing
* permissions and limitations under the License.
*/
package com.amazonaws.services.sqs.buffered;
import java.util.ArrayList;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
import java.util.concurrent.Executor;
import java.util.concurrent.TimeUnit;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import com.amazonaws.AmazonClientException;
import com.amazonaws.services.sqs.AmazonSQS;
import com.amazonaws.services.sqs.model.ChangeMessageVisibilityBatchRequest;
import com.amazonaws.services.sqs.model.ChangeMessageVisibilityBatchRequestEntry;
import com.amazonaws.services.sqs.model.GetQueueAttributesRequest;
import com.amazonaws.services.sqs.model.Message;
import com.amazonaws.services.sqs.model.ReceiveMessageRequest;
import com.amazonaws.services.sqs.model.ReceiveMessageResult;
/**
* The ReceiveQueueBuffer class is responsible for dequeueing of messages from a single SQS queue.
* It uses the provided executor to pre-fetch messages from the server and keeps them in a buffer
* which it uses to satisfy incoming requests. The number of requests pre-fetched and kept in the
* buffer, as well as the maximum number of threads used to retrieve the messages are configurable.
*
* Synchronization strategy: - Threads must hold the TaskSpawnSyncPoint object monitor to spawn a
* new task or modify the number of inflight tasks - Threads must hold the monitor of the "futures"
* list to modify the list - Threads must hold the monitor of the "finishedTasks" list to modify the
* list - If you need to lock both futures and finishedTasks, lock futures first and finishedTasks
* second
*/
public class ReceiveQueueBuffer {
/**
* Simple interface to represent a condition
*
* @param
*/
private interface Predicate {
/**
* @param t
* Object being evaluated against the condition
* @return True if t meets the condition, false if not
*/
boolean test(T t);
}
private static Log log = LogFactory.getLog(ReceiveQueueBuffer.class);
private final QueueBufferConfig config;
private final String qUrl;
private final Executor executor;
private final AmazonSQS sqsClient;
private long bufferCounter = 0;
/**
* This buffer's queue visibility timeout. Used to detect expired message that should not be
* returned by the {@code receiveMessage} call. Synchronized by {@code receiveMessageLock}. -1
* indicates that the time is uninitialized.
*/
private volatile long visibilityTimeoutNanos = -1;
/**
* Used as permits controlling the number of in flight receive batches. Synchronized by
* {@code taskSpawnSyncPoint}.
*/
private volatile int inflightReceiveMessageBatches;
/**
* synchronize on this object to create new receive batches or modify inflight message count
*/
private final Object taskSpawnSyncPoint = new Object();
/** shutdown buffer does not retrieve any more messages from sqs */
volatile boolean shutDown = false;
/** message delivery futures we gave out */
private final LinkedList futures = new LinkedList();
/** finished batches are stored in this list. */
private LinkedList finishedTasks = new LinkedList();
ReceiveQueueBuffer(AmazonSQS paramSQS, Executor paramExecutor, QueueBufferConfig paramConfig, String url) {
config = paramConfig;
executor = paramExecutor;
sqsClient = paramSQS;
qUrl = url;
}
/**
* Prevents spawning of new retrieval batches and waits for all in-flight retrieval batches to
* finish
*/
public void shutdown() {
shutDown = true;
try {
while (inflightReceiveMessageBatches > 0)
Thread.sleep(100);
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
}
}
/**
* Submits the request for retrieval of messages from the queue and returns a future that will
* be signalled when the request is satisfied. The future may already be signalled by the time
* it is returned.
*
* @return never null
*/
public QueueBufferFuture receiveMessageAsync(ReceiveMessageRequest rq,
QueueBufferCallback callback) {
if (shutDown) {
throw new AmazonClientException("The client has been shut down.");
}
// issue the future...
int numMessages = 10;
if (rq.getMaxNumberOfMessages() != null) {
numMessages = rq.getMaxNumberOfMessages();
}
QueueBufferFuture toReturn = issueFuture(numMessages, callback);
// attempt to satisfy it right away...
satisfyFuturesFromBuffer();
// spawn more receive tasks if we need them...
spawnMoreReceiveTasks();
return toReturn;
}
/**
* Creates and returns a new future object. Sleeps if the list of already-issued but as yet
* unsatisfied futures is over a throttle limit.
*
* @return never null
*/
private ReceiveMessageFuture issueFuture(int size,
QueueBufferCallback callback) {
synchronized (futures) {
ReceiveMessageFuture theFuture = new ReceiveMessageFuture(callback, size);
futures.addLast(theFuture);
return theFuture;
}
}
/**
* Attempts to satisfy some or all of the already-issued futures from the local buffer. If the
* buffer is empty or there are no futures, this method won't do anything.
*/
private void satisfyFuturesFromBuffer() {
synchronized (futures) {
synchronized (finishedTasks) {
// attempt to satisfy futures until we run out of either futures or
// finished tasks
while ((!futures.isEmpty()) && (!finishedTasks.isEmpty())) {
// Remove any expired tasks before attempting to fufill the future
pruneExpiredTasks();
// Fufill the future from a non expired task if there is one. There is still a
// slight chance that the first task could have expired between the time we
// pruned and the time we fufill the future
if (!finishedTasks.isEmpty()) {
fufillFuture(futures.poll());
}
}
}
}
}
/**
* Fills the future with whatever results were received by the full batch currently at the head
* of the completed batch queue. Those results may be retrieved messages, or an exception. This
* method assumes that you are holding the finished tasks lock locks when invoking it. violate
* this assumption at your own peril
*/
private void fufillFuture(ReceiveMessageFuture future) {
ReceiveMessageBatchTask task = finishedTasks.getFirst();
ReceiveMessageResult result = new ReceiveMessageResult();
LinkedList messages = new LinkedList();
result.setMessages(messages);
Exception exception = task.getException();
int numRetrieved = 0;
boolean batchDone = false;
while (numRetrieved < future.getRequestedSize()) {
Message msg = task.removeMessage();
// a non-empty batch can still give back a null
// message if the message expired.
if (msg != null) {
messages.add(msg);
++numRetrieved;
} else {
batchDone = true;
break;
}
}
// we may have just drained the batch.
batchDone = batchDone || task.isEmpty() || (exception != null);
if (batchDone) {
finishedTasks.removeFirst();
}
result.setMessages(messages);
// if after the above runs the exception is not null,
// the finished batch has encountered an error, and we will
// report that in the Future. Otherwise, we will fill
// the future with the receive result
if (exception != null) {
future.setFailure(exception);
} else {
future.setSuccess(result);
}
}
/**
* Prune any expired tasks that do not have an exception associated with them. This method
* assumes that you are holding the finishedTasks lock when invoking it
*/
private void pruneExpiredTasks() {
int numberExpiredTasksPruned = pruneHeadTasks(new Predicate() {
@Override
public boolean test(ReceiveMessageBatchTask t) {
return t.isExpired() && t.getException() == null;
}
});
// If we pruned any tasks because they are expired we also want to prune any empty tasks
// afterwards so we have a chance to receive those expired messages again.
if (numberExpiredTasksPruned > 0) {
pruneHeadTasks(new Predicate() {
@Override
public boolean test(ReceiveMessageBatchTask t) {
return t.isEmpty() && t.getException() == null;
}
});
}
}
/**
* Prune all tasks at the beginning of the finishedTasks list that meet the given condition.
* Once a task is found that does not meet the given condition the pruning stops. This method
* assumes that you are holding the finishedTasks lock when invoking it.
*
* @param pruneCondition
* Condition on whether a task is eligible to be pruned
* @return Number of total tasks pruned from finishedTasks
*/
private int pruneHeadTasks(Predicate pruneCondition) {
int numberPruned = 0;
while (!finishedTasks.isEmpty()) {
if (pruneCondition.test(finishedTasks.getFirst())) {
finishedTasks.removeFirst();
numberPruned++;
} else {
break;
}
}
return numberPruned;
}
/**
* maybe create more receive tasks. extra receive tasks won't be created if we are already at
* the maximum number of receive tasks, or if we are at the maximum number of prefetched buffers
*/
private void spawnMoreReceiveTasks() {
if (shutDown) {
return;
}
int desiredBatches = config.getMaxDoneReceiveBatches();
desiredBatches = desiredBatches < 1 ? 1 : desiredBatches;
synchronized (finishedTasks) {
if (finishedTasks.size() >= desiredBatches)
return;
// if we have some finished batches already, and
// existing inflight batches will bring us to the limit,
// don't spawn more. if our finished tasks cache is empty, we will
// always spawn a thread.
if (finishedTasks.size() > 0 && (finishedTasks.size() + inflightReceiveMessageBatches) >= desiredBatches) {
return;
}
}
synchronized (taskSpawnSyncPoint) {
if (visibilityTimeoutNanos == -1) {
GetQueueAttributesRequest request = new GetQueueAttributesRequest().withQueueUrl(qUrl)
.withAttributeNames("VisibilityTimeout");
ResultConverter.appendUserAgent(request, AmazonSQSBufferedAsyncClient.USER_AGENT);
long visibilityTimeoutSeconds = Long.parseLong(sqsClient.getQueueAttributes(request).getAttributes()
.get("VisibilityTimeout"));
visibilityTimeoutNanos = TimeUnit.NANOSECONDS.convert(visibilityTimeoutSeconds, TimeUnit.SECONDS);
}
int max = config.getMaxInflightReceiveBatches();
// must allow at least one inflight receive task, or receive won't
// work at all.
max = max > 0 ? max : 1;
int toSpawn = max - inflightReceiveMessageBatches;
if (toSpawn > 0) {
ReceiveMessageBatchTask task = new ReceiveMessageBatchTask(this);
++inflightReceiveMessageBatches;
++bufferCounter;
if (log.isTraceEnabled()) {
log.trace("Spawned receive batch #" + bufferCounter + " (" + inflightReceiveMessageBatches + " of "
+ max + " inflight) for queue " + qUrl);
}
executor.execute(task);
}
}
}
/**
* This method is called by the batches after they have finished retrieving the messages.
*/
void reportBatchFinished(ReceiveMessageBatchTask batch) {
synchronized (finishedTasks) {
finishedTasks.addLast(batch);
if (log.isTraceEnabled()) {
log.info("Queue " + qUrl + " now has " + finishedTasks.size() + " receive results cached ");
}
}
synchronized (taskSpawnSyncPoint) {
--inflightReceiveMessageBatches;
}
satisfyFuturesFromBuffer();
spawnMoreReceiveTasks();
}
/**
* Clears and nacks any pre-fetched messages in this buffer.
*/
public void clear() {
boolean done = false;
while (!done) {
ReceiveMessageBatchTask currentBatch = null;
synchronized (finishedTasks) {
currentBatch = finishedTasks.poll();
}
if (currentBatch != null) {
currentBatch.clear();
} else {
// ran out of batches to clear
done = true;
}
}
}
private class ReceiveMessageFuture extends QueueBufferFuture {
/* how many messages did the request ask for */
private int requestedSize;
ReceiveMessageFuture(QueueBufferCallback cb, int paramSize) {
super(cb);
requestedSize = paramSize;
}
public int getRequestedSize() {
return requestedSize;
}
}
/**
* Task to receive messages from SQS.
*
* The batch task is constructed {@code !open} until the {@code ReceiveMessage} completes. At
* that point, the batch opens and its messages (if any) become available to read.
*/
private class ReceiveMessageBatchTask implements Runnable {
private Exception exception = null;
private List messages;
private long visibilityDeadlineNano;
private boolean open = false;
private ReceiveQueueBuffer parentBuffer;
/**
* Constructs a receive task waiting the specified time before calling SQS.
*
* @param waitTimeMs
* the time to wait before calling SQS
*/
ReceiveMessageBatchTask(ReceiveQueueBuffer paramParentBuffer) {
parentBuffer = paramParentBuffer;
messages = Collections.emptyList();
}
synchronized boolean isEmpty() {
if (!open) {
throw new IllegalStateException("batch is not open");
}
return messages.isEmpty();
}
/**
* @return the exception that was thrown during execution, or null if there was no exception
*/
synchronized Exception getException() {
if (!open) {
throw new IllegalStateException("batch is not open");
}
return exception;
}
/**
* Returns a message if one is available.
*
* The call adjusts the message count.
*
* @return a message or {@code null} if none is available
*/
synchronized Message removeMessage() {
if (!open) {
throw new IllegalStateException("batch is not open");
}
// our messages expired.
if (isExpired()) {
messages.clear();
return null;
}
if (messages.isEmpty())
return null;
else
return messages.remove(messages.size() - 1);
}
boolean isExpired() {
return System.nanoTime() > visibilityDeadlineNano;
}
/**
* Nacks and clears all messages remaining in the batch.
*/
synchronized void clear() {
if (!open) {
throw new IllegalStateException("batch is not open");
}
if (!isExpired()) {
ChangeMessageVisibilityBatchRequest batchRequest = new ChangeMessageVisibilityBatchRequest()
.withQueueUrl(qUrl);
ResultConverter.appendUserAgent(batchRequest, AmazonSQSBufferedAsyncClient.USER_AGENT);
List entries = new ArrayList(
messages.size());
int i = 0;
for (Message m : messages) {
entries.add(new ChangeMessageVisibilityBatchRequestEntry().withId(Integer.toString(i))
.withReceiptHandle(m.getReceiptHandle()).withVisibilityTimeout(0));
++i;
}
try {
batchRequest.setEntries(entries);
sqsClient.changeMessageVisibilityBatch(batchRequest);
} catch (AmazonClientException e) {
// Log and ignore.
log.warn("ReceiveMessageBatchTask: changeMessageVisibility failed " + e);
}
}
messages.clear();
}
/**
* Attempts to retrieve messages from SQS and upon completion (successful or unsuccessful)
* reports the batch as complete and open
*/
public void run() {
try {
visibilityDeadlineNano = System.nanoTime() + visibilityTimeoutNanos;
ReceiveMessageRequest request = new ReceiveMessageRequest(qUrl).withMaxNumberOfMessages(config
.getMaxBatchSize());
ResultConverter.appendUserAgent(request, AmazonSQSBufferedAsyncClient.USER_AGENT);
if (config.getVisibilityTimeoutSeconds() > 0) {
request.setVisibilityTimeout(config.getVisibilityTimeoutSeconds());
visibilityDeadlineNano = System.nanoTime()
+ TimeUnit.NANOSECONDS.convert(config.getVisibilityTimeoutSeconds(), TimeUnit.SECONDS);
}
if (config.isLongPoll()) {
request.withWaitTimeSeconds(config.getLongPollWaitTimeoutSeconds());
}
messages = sqsClient.receiveMessage(request).getMessages();
} catch (AmazonClientException e) {
exception = e;
} finally {
// whatever happened, we are done and can be considered open
open = true;
parentBuffer.reportBatchFinished(this);
}
}
}
}