Amazon SQS是AWS上主流的消息队列服务，按理说它是有SDK的，那么为什么还要自己编写客户端呢？因为它提供的SDK太简单，就几个Web API，没有办法直接用。我们具体来说一说。
SQS SDK中的API，我们主要用到的也就是getQueueUrl, sendMessage, receiveMessage等。getQueueUrl能根据传入的queueName查找到queueUrl，后续用这个queueUrl来访问相应的queue（即：调用sendMessage发消息，或调用receiveMessage收消息）。主要复杂度在于收消息：这个API是要主动调用的，可是你怎么知道有没有新消息需要你去收呢？事实上，这个receiveMessage API是基于拉模式(pull mode)的，你需要轮询来不停地拉取新消息，这个比较像Kafka。随之而来的，就需要线程管理，需要一个对SDK做了进一步包装的客户端库。
Spring Cloud Messaging提供了SQS的客户端库。但是当我们在2023年3月构建基于SQS的应用程序时，我们用的是AWS SDK V2，而Spring Cloud Messaging尚未正式支持AWS SDK V2。因此，我们决定自己编写SQS的客户端库。而且我们的设计也与Spring Cloud Messaging的有所不同：我们同时使用多个AWS账号，为此，我们直接在配置中引用queueUrl（它其实是静态值，可直接引用）；而Spring Cloud Messaging只能在配置中引用queueName，然后再运行时获取当前AWS账号中相应的queueUrl。
现在就来讲一讲设计与实现。消息队列客户端遵循生产者-消费者模型，分为Producer和Consumer。SQS的消息体必须是不大于256KB的文本，因此可以把消息体当成一个String。
Producer
Producer很简单，把消息发出去就行了，顺便对超时和异常做适当的处理。库的用户可以自行决定消息体的序列化和反序列化方式，我们不干涉这件事。
Producer的使用方式很简单：
new SqsMessageProducer(queueUrl)
.produce(yourMessagePayload);Producer的完整实现代码大致如下：
/** How to use: Call produce() with your serialized message string. */
public class SqsMessageProducer {
private final String queueUrl;
private final int timeoutSeconds;
private final SqsAsyncClient client;
public SqsMessageProducer(String queueUrl, int timeoutSeconds) {
this.queueUrl = queueUrl;
this.timeoutSeconds = timeoutSeconds;
client = new SqsClientFactory().createSqsAsyncClient();
}
public void produce(String payload) {
var sendMessageFuture =
client.sendMessage(
SendMessageRequest.builder().queueUrl(queueUrl).messageBody(payload).build());
// 不能无限等待future，要有超时机制
try {
sendMessageFuture.get(timeoutSeconds, TimeUnit.SECONDS);
} catch (InterruptedException | ExecutionException | TimeoutException e) {
throw new ProducerException(e);
}
}
public static class ProducerException extends RuntimeException {
public ProducerException(Throwable cause) {
super(cause);
}
}
}如果想进一步提高Producer的性能，可以让它异步获取sendMessageFuture的结果，不用同步等待。但是这么做会降低可靠性，不能保证调用了Producer就一定成功发送了消息，因此需要权衡。
Consumer
Consumer的使用方式很简单，有效利用了函数式编程风格，不需要编写派生类，只需要创建Consumer的实例，传入一个消息处理函数，然后启动就可以。示例代码如下：
new SqsMessageConsumer(queueUrl, yourCustomizedThreadNamePrefix, yourMessageHandler)
.runAsync();Consumer的实现要复杂一些，需要实现消息驱动的异步计算风格。处理消息一般会比收取消息更花时间，因此它创建一个主循环线程用来轮询消息队列，创建一个工作线程池用来处理消息。主循环线程每次可能收到0~n个消息，把收到的消息分发给工作线程池来处理。因为工作线程池自带任务队列用于缓冲，所以这两种线程之间是互不阻塞的：如果工作线程慢了，主循环线程可以照常收取和分发新消息；如果主循环线程慢了，工作线程可以照常处理已有的消息。
注意一个要点：SQS不会自动清理已被收取的消息，因为它不知道你是否成功处理了消息。当一个消息被收取后，它会暂时被隐藏，以免其他消费者收到它，如果此消息一直没有被清理，它会在一段时间后(默认30秒，可配置)重新出现，被某个消费者再度收取。你需要一个机制来主动告知SQS某条消息已被处理，这个机制就是deleteMessage API：成功处理一个消息后，主动调deleteMessage来从队列中删除此消息；如果处理失败，什么都不用做，SQS会在一段时间后再次让消费者收取到此消息。
核心代码这么写：
private volatile boolean shouldShutdown = false;
// 只要没有关闭，主循环就一直收取消息
while (!shouldShutdown) {
List messages;
try {
messages = receiveMessages();
} catch (Throwable e) {
logger.error("failed to receive", e);
continue;
}
try {
dispatchMessages(queueUrl, messages);
} catch (Throwable e) {
logger.error("failed to dispatch", e);
}
}
// 收消息的具体实现
private List receiveMessages() throws ExecutionException, InterruptedException {
// visibilityTimeout = message handling timeout
// It is usually set at infrastructure level
var receiveMessageFuture =
client.receiveMessage(
ReceiveMessageRequest.builder()
.queueUrl(queueUrl)
.waitTimeSeconds(10)
.maxNumberOfMessages(maxParallelism)
.build());
// 上面已在请求中设置waitTimeSeconds=10，所以这里可以不设置超时
return receiveMessageFuture.get().messages();
}
// 把收到消息分发给工作线程池做处理
// 要显式地把处理好的消息从队列中删除
// 如果不删除，会在未来再次被主循环收取到
private void dispatchMessages(String queueUrl, List messages) {
for (Message message : messages) {
workerThreadPool.execute(
() -> {
String messageId = message.messageId();
try {
logger.info("Started handling message with id={}", messageId);
messageHandler.accept(message);
logger.info("Completed handling message with id={}", messageId);
// Should delete the succeeded message
client.deleteMessage(
DeleteMessageRequest.builder()
.queueUrl(queueUrl)
.receiptHandle(message.receiptHandle())
.build());
logger.info("Deleted handled message with id={}", messageId);
} catch (Throwable e) {
// Logging is enough. Failed message is not deleted, and will be retried on a future polling.
logger.error("Failed to handle message with id=$messageId", e);
}
});
}
} 在以上代码中，每次receiveMessage时设置waitTimeSeconds=10，即最多等待10秒，若没有新消息就返回0条消息；若有新消息，就提前返回所收到的1或多条消息。之所以不无限等待，是怕网关自动关闭长时间静默的网络连接。
还需要一个优雅关闭机制，让服务器能顺利关闭和清理资源：
Thread mainLoopThread = Thread.currentThread();
// JVM awaits all shutdown hooks to complete
// https://stackoverflow.com/questions/8663107/how-does-the-jvm-terminate-daemon-threads-or-how-to-write-daemon-threads-that-t
Runtime.getRuntime()
.addShutdownHook(
new Thread(
() -> {
shouldShutdown = true;
mainLoopThread.interrupt();
try {
workerThreadPool.shutdown();
boolean terminated = workerThreadPool.awaitTermination(1, TimeUnit.MINUTES);
if (!terminated) {
List runnables = workerThreadPool.shutdownNow();
logger.info("shutdownNow with {} runnables undone", runnables.size());
}
} catch (RuntimeException e) {
logger.error("shutdown failed", e);
throw e;
} catch (InterruptedException e) {
logger.error("shutdown interrupted", e);
throw new IllegalStateException(e);
}
})); 有时网络连接不稳定，主循环频繁报错比较noisy，改成指数退避的重试：
while (!shouldShutdown) {
List messages;
try {
messages = receiveMessages();
// after success, restore backoff to the initial value
receiveBackoffSeconds = 1;
} catch (Throwable e) {
logger.error("failed to receive", e);
logger.info("Gonna sleep {} seconds for backoff", receiveBackoffSeconds);
try {
//noinspection BusyWait
Thread.sleep(receiveBackoffSeconds * 1000L);
} catch (InterruptedException ex) {
logger.error("backoff sleep interrupted", ex);
}
// after failure, increment next backoff (≤ limit)
receiveBackoffSeconds = exponentialBackoff(receiveBackoffSeconds, 60);
continue;
}
try {
dispatchMessages(queueUrl, messages);
} catch (Throwable e) {
logger.error("failed to dispatch", e);
}
}
private int exponentialBackoff(int current, int limit) {
int next = current * 2;
return Math.min(next, limit);
} 工作线程池是一个ThreadPoolExecutor，使用一个有界的BlockingQueue来实现回压(back-pressure)，当这个queue一满，主循环线程就会被迫暂停，以防止本地的消息积压过多：如果积压过多，既会浪费内存，又会导致很多消息被收取却得不到及时处理，这时还不如让给其他消费者实例去收取。创建工作线程池的相关代码如下：
workerThreadPool =
new ThreadPoolExecutor(
maxParallelism,
maxParallelism,
0,
TimeUnit.SECONDS,
// bounded queue for back pressure
new LinkedBlockingQueue<>(100),
new CustomizableThreadFactory(threadPoolPrefix + "-pool-"),
new TimeoutBlockingPolicy(30));
// Used by workerThreadPool
private static class TimeoutBlockingPolicy implements RejectedExecutionHandler {
private final long timeoutSeconds;
public TimeoutBlockingPolicy(long timeoutSeconds) {
this.timeoutSeconds = timeoutSeconds;
}
@Override
public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
try {
BlockingQueue queue = executor.getQueue();
if (!queue.offer(r, this.timeoutSeconds, TimeUnit.SECONDS)) {
throw new RejectedExecutionException("Timeout after " + timeoutSeconds + " seconds");
}
} catch (InterruptedException e) {
throw new IllegalStateException(e);
}
}
}
Consumer的完整实现代码大致如下：
/**
* How to use:
* 1. create a consumer instance with a queue name and a stateless messageHandler function.
* 2. call runAsync() method to start listening to the queue.
*/
public class SqsMessageConsumer implements Runnable {
private static final Logger logger = LoggerFactory.getLogger(SqsMessageConsumer.class);
private final String queueUrl;
private final Consumer messageHandler;
private final int maxParallelism;
private final SqsAsyncClient client;
private final ExecutorService workerThreadPool;
private volatile boolean shouldShutdown = false;
public SqsMessageConsumer(
String queueUrl,
String threadPoolPrefix,
Consumer messageHandler) {
this(queueUrl, threadPoolPrefix, messageHandler, 8);
}
public SqsMessageConsumer(
String queueUrl,
String threadPoolPrefix,
Consumer messageHandler,
int maxParallelism) {
this.queueUrl = queueUrl;
this.messageHandler = messageHandler;
this.maxParallelism = maxParallelism;
client = new SqsClientFactory().createSqsAsyncClient();
workerThreadPool =
new ThreadPoolExecutor(
maxParallelism,
maxParallelism,
0,
TimeUnit.SECONDS,
// bounded queue for back pressure
new LinkedBlockingQueue<>(100),
new CustomizableThreadFactory(threadPoolPrefix + "-pool-"),
new TimeoutBlockingPolicy(30));
}
/** Use this method by default, it is asynchronous and handles threading for you. */
public void runAsync() {
Thread mainLoopThread = new Thread(this);
mainLoopThread.start();
}
/**
* Use this method only if you run it in your own thread pool, it runs synchronously in the
* contextual thread.
*/
@Override
public void run() {
Thread mainLoopThread = Thread.currentThread();
// JVM awaits all shutdown hooks to complete
// https://stackoverflow.com/questions/8663107/how-does-the-jvm-terminate-daemon-threads-or-how-to-write-daemon-threads-that-t
Runtime.getRuntime()
.addShutdownHook(
new Thread(
() -> {
shouldShutdown = true;
mainLoopThread.interrupt();
try {
workerThreadPool.shutdown();
boolean terminated = workerThreadPool.awaitTermination(1, TimeUnit.MINUTES);
if (!terminated) {
List runnables = workerThreadPool.shutdownNow();
logger.info("shutdownNow with {} runnables undone", runnables.size());
}
} catch (RuntimeException e) {
logger.error("shutdown failed", e);
throw e;
} catch (InterruptedException e) {
logger.error("shutdown interrupted", e);
throw new IllegalStateException(e);
}
}));
logger.info("polling loop started");
int receiveBackoffSeconds = 1;
// "shouldShutdown" state is more reliable than Thread interrupted state
while (!shouldShutdown) {
List messages;
try {
messages = receiveMessages();
// after success, restore backoff to the initial value
receiveBackoffSeconds = 1;
} catch (Throwable e) {
logger.error("failed to receive", e);
logger.info("Gonna sleep {} seconds for backoff", receiveBackoffSeconds);
try {
//noinspection BusyWait
Thread.sleep(receiveBackoffSeconds * 1000L);
} catch (InterruptedException ex) {
logger.error("backoff sleep interrupted", ex);
}
// after failure, increment next backoff (≤ limit)
receiveBackoffSeconds = exponentialBackoff(receiveBackoffSeconds, 60);
continue;
}
try {
dispatchMessages(queueUrl, messages);
} catch (Throwable e) {
logger.error("failed to dispatch", e);
}
}
}
private int exponentialBackoff(int current, int limit) {
int next = current * 2;
return Math.min(next, limit);
}
private List receiveMessages() throws ExecutionException, InterruptedException {
// visibilityTimeout = message handling timeout
// It has usually been set at infrastructure level
var receiveMessageFuture =
client.receiveMessage(
ReceiveMessageRequest.builder()
.queueUrl(queueUrl)
.waitTimeSeconds(10)
.maxNumberOfMessages(maxParallelism)
.build());
// Consumer can wait infinitely for the next message, rely on library default timeout.
return receiveMessageFuture.get().messages();
}
private void dispatchMessages(String queueUrl, List messages) {
for (Message message : messages) {
workerThreadPool.execute(
() -> {
String messageId = message.messageId();
try {
logger.info("Started handling message with id={}", messageId);
messageHandler.accept(message);
logger.info("Completed handling message with id={}", messageId);
// Should delete the succeeded message
client.deleteMessage(
DeleteMessageRequest.builder()
.queueUrl(queueUrl)
.receiptHandle(message.receiptHandle())
.build());
logger.info("Deleted handled message with id={}", messageId);
} catch (Throwable e) {
// Logging is enough. Failed message is not deleted, will be retried at next polling.
logger.error("Failed to handle message with id=$messageId", e);
}
});
}
}
// Used by workerThreadPool
private static class TimeoutBlockingPolicy implements RejectedExecutionHandler {
private final long timeoutSeconds;
public TimeoutBlockingPolicy(long timeoutSeconds) {
this.timeoutSeconds = timeoutSeconds;
}
@Override
public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
try {
BlockingQueue queue = executor.getQueue();
if (!queue.offer(r, this.timeoutSeconds, TimeUnit.SECONDS)) {
throw new RejectedExecutionException("Timeout after " + timeoutSeconds + " seconds");
}
} catch (InterruptedException e) {
throw new IllegalStateException(e);
}
}
}
}