Pending Queue Size ­ The number of messages in the queue that have yet to bedelivered to any clientDispatched Counter ­ The number of message in the queue that have beendelivered to a client, but not yet acknowledgedEnqueue Counter ­ The number of messages that have been sent to this queueDequeue Counter ­ the number of messages that have been consumed from thisqueue

pending messages = number of messages CURRENTLY waiting for delivery in the destination (the current size of the queue)

enqueued messages = number of messages that where enqueued in the destination since the last statistic reset. This number can only rise.

dequeued messages = messages delivered from the destination to consumers. this number can be higher that the number of enqueued messages if a message was delivered to multiple consumers (topics).

properties是否是配置文件，重启系统即可见效

Message selectors

session.createConsumer(destination, selector), selector用来确定consumer需要什么样的message， selector是string，使用jms selector syntax来确定需要的mesage

ConnectionFactory:connection is open tcp socket between client and provider, overhead is large, connection used by client to create Session

Destination:created by Session, unique to Connection. temporary destinations are used for request/reply messaging

transport connectors : client to broker

network connectors : brokers cluster, channels configured between brokers

forwarding bridge : 

 Messages sent to queues and topics are stored differently

storage for queues in FIFO, one message is dispatched to a consumer at a time, only acknowledged can be deleted from broker message store

topics‘ consumer get a copy of message, message can‘t be deleted until it‘s delivered to every subscriber

kahaDB

ActiveMQ的failover没有想象中那么智能，还是另起一个线程用于连接connection以保证万一broker不存在的话不会让获得验证码的线程处于堵塞状态