为什么 SQS 消息写入和删除会给出错误的计数？

Question

我编写了以下代码，将一些消息写入 SQS 队列并立即删除它们。但是我在输出中看到每条消息都被立即删除。尽管如此，我看到消息计数非常随机。我的意思是它最初显示 1 然后它再次显示 2 尽管删除了一个。我已经将可见性超时设置为 50 秒，就像这篇文章中提到的那样。我作为程序的一部分尝试了相同的方法。

所以我怀疑删除消息是否会立即发生。如果是这样，这将如何影响线程数。请参阅下面的代码和输出。

__author__ = 'Dhanapathi.Marepalli'

import boto.sqs

conn = boto.sqs.connect_to_region("us-east-1", aws_access_key_id="********",
                                  aws_secret_access_key="************")
my_queue = conn.get_queue('SMSQueue')


def write_messages():
    print("Writing messages.")
    for i in range(1, 5):
        new_message = Message()
        message_body = str(random.randint(1111111111, 9999999999))
        print("Message body is ", message_body)
        new_message.set_body(message_body)
        my_queue.write(new_message)
        print("Message with message body {0} is written.".format(message_body))
    print("Messages completed. Totally {0} messages written to Queue.".format(my_queue.count()))


def retrieve_messages():
    while True:
        counter = 0
        rs = my_queue.get_messages()
        for message in rs:
            counter = 1
            body = message.get_body()
            print(current_thread().getName(), " Message is ", body)
            print(current_thread().getName(), " Assume something happened in between.")
            is_message_deleted = my_queue.delete_message(message)
            if is_message_deleted:
                print("{0}: Message with body {1} is deleted.".format(current_thread().getName(), body))
            else:
                print("{0}: Message with body {1} failed to delete.".format(current_thread().getName(), body))

            print(current_thread().getName(), " thread count is ", my_queue.count())

        if counter == 0:
            print("Thread1:No messages inside the Queue.")
            break

if __name__ == "__main__":
   write_messages()
   retrieve_messages()

生成的输出如下：

Writing messages.
Message body is  8111076750
Message with message body 8111076750 is written.
Message body is  4927934264
Message with message body 4927934264 is written.
Message body is  8511692382
Message with message body 8511692382 is written.
Message body is  9718698315
Message with message body 9718698315 is written.
Messages completed. Totally 4 messages written to Queue.
MainThread  Message is  8111076750
MainThread  Assume something happened in between.
MainThread: Message with body 8111076750 is deleted.
MainThread  thread count is  3
MainThread  Message is  4927934264
MainThread  Assume something happened in between.
MainThread: Message with body 4927934264 is deleted.
MainThread  thread count is  2
MainThread  Message is  9718698315
MainThread  Assume something happened in between.
MainThread: Message with body 9718698315 is deleted.
MainThread  thread count is  1
MainThread  Message is  8511692382
MainThread  Assume something happened in between.
MainThread: Message with body 8511692382 is deleted.
MainThread  thread count is  2
Thread1:No messages inside the Queue.

提前致谢。

Answer 1

SQS 是一个分布式系统。你想要它的规模和可靠性。但是，由于系统的分布式特性，它不能保证 100% 的一致性。为了获取队列中的项目数，boto 使用了 queue 属性ApproximateNumberOfMessages，顾名思义，它只能提供队列中可用消息的近似计数。

如果您的应用程序需要队列中消息的精确计数，SQS 可能不是您问题的正确答案。然而，对于了解其一致性限制的应用程序来说，它是一种极其可靠且可扩展的排队服务。