消息队列（二）AMQ
AMQ是个轻量级的消息队列，下载下来几十兆，马上就可以用，还自带web管理端，而且功能比较多。一般的小项目需求都能满足。在消息队列（一）中说明了消息队列设计的几个问题，下面针对AMQ的处理方式关键点进行下分析。
设计问题
Slow Consumers
慢消费问题会导致topics会有大量缓存，对于需要持久化的topic，可以直接进行持久化操作。对于Non-Durable Topics处理方式有如下：
- block/slow the producer
- drop the slow consumer
- spool messages to disk
- discard messages for the slow consumer
AMQ中的默认配置是>block producers until the slow consumer catches up (for non-durable topics here).
对于此问题，可以专门设立慢消费的侦测程序Slow Consumer Detector
Messages redeliver
消息重发出现的场景如下
- A transacted session is used and rollback() is called.
- A transacted session is closed before commit is called.
- A session is using CLIENT_ACKNOWLEDGE and Session.recover() is called.
- A client connection times out
此外对于一直从发失败的消息，可以看作Poison ack（message was considered a poison pill），处理方式如下：
Broker then takes the message and sends it to a Dead Letter Queue so that it can be analyzed later on. By default, ActiveMQ will not place undeliverable non-persistent messages on the dead-letter queue. By default, ActiveMQ will never expire messages sent to the DLQ but from v5.12, the deadLetterStrategy supports an expiration attribute where the value is in milliseconds.
Supporting IO Streams
support for streaming files over ActiveMQ of any arbitrary size
Message Store
可通过日志方式记录消息记录，与log4xx雷系，可以配置单文件大小（typically 32mb in size），压缩，定时清理，还支持异步IO。
Dispatch Policies
prefetch value预先读取的方式，SEDA,as much work as possible asynchronously. Async Sends，The cases that we are forced to send in sync mode are when persistent messages are being sent outside of a transaction.
基础配置
AcknowledgeMode
- AUTO_ACKNOWLEDGE，回调给上层后自动ack，上层需要保证消息处理的。
- CLIENT_ACKNOWLEDGE，需要客户端主动调用ack。
- SESSION_TRANSACTED，在一个session上，也可以调用transcate方法提交。
- INDIVIDUAL_ACKNOWLEDGE
- DUPS_OK_ACKNOWLEDGE
消息优先级
0到9的优先级路线级别，0是最低
消息正文的类型
- StreamMessage – Java原始值的数据流
- MapMessage–一套名称-值对
- TextMessage–一个字符串对象
- ObjectMessage–一个序列化的 Java对象
- BytesMessage–一个未解释字节的数据流
高级功能
Exclusive Consumer
consumer可以配置优先级。可以实现类似负载均衡，主备的功能。
JMS Selectors
实现高级订阅过滤
Composite Destinations
复制转发消息
Pending Message Limit Strategy
预读策略，提速
Mirrored Queues
指定一个queue为外一个queue的镜像，这样能够监控queue中的消息，而不必在queue端在打日志了。
Wildcards
支持正则表达式方式订阅
Strict Order Dispatch Policy
对于特殊的场景，多个Producer保证多个consumer接收到是顺序一致的，AMQ中默认是不开启的
MYQA
Q：transacted session 调用 commit() 与CLIENT_ACKNOWLEDGE Session 调用 acknowledge()对消息ack 区别？
A: 作用对象不一样，acknowledge是确认单条消息，commit可以是对一个session的一批消息，session也有acknowledge方法是对session没有ack的消息全部ack.
Q: 如果一个queue有多个订阅者，消息需要ack，默认配置是其中一个确认后即可？
A:> A JMS Queue implements load balancer semantics. A single message will be received by exactly one consumer. If there are no consumers available at the time the message is sent it will be kept until a consumer is available that can process the message.
多个订阅者情况下是负载均衡机制
Q: producer与consumer都建立session都可以设置类型，不一致会怎样？
A: producer的session与consumer的session没有关系