Load balancing in RocketMQ is accomplished on Client side. Specifically, it can be divided into load balancing at Producer side when sending messages and load balancing at Constumer side when subscribing messages.
### 4.1 Producer Load Balancing
### 1 Producer Load Balancing
When the Producer sends a message, it will first find the specified TopicPublishInfo according to Topic. After getting the routing information of TopicPublishInfo, the RocketMQ client will select a queue (MessageQueue) from the messageQueue List in TopicPublishInfo to send the message by default.Specific fault-tolerant strategies are defined in the MQFaultStrategy class.
Here is a sendLatencyFaultEnable switch variable, which, if turned on, filters out the Broker agent of not available on the basis of randomly gradually increasing modular arithmetic selection. The so-called "latencyFault Tolerance" refers to a certain period of time to avoid previous failures. For example, if the latency of the last request exceeds 550 Lms, it will evade 3000 Lms; if it exceeds 1000L, it will evade 60000 L; if it is closed, it will choose a queue (MessageQueue) to send messages by randomly gradually increasing modular arithmetic, and the latencyFault Tolerance mechanism is the key to achieve high availability of message sending.
### 4.2 Consumer Load Balancing
### 2 Consumer Load Balancing
In RocketMQ, the two consumption modes (Push/Pull) on the Consumer side are both based on the pull mode to get the message, while in the Push mode it is only a kind of encapsulation of the pull mode, which is essentially implemented as the message pulling thread after pulling a batch of messages from the server. After submitting to the message consuming thread pool, it continues to try again to pull the message to the server. If the message is not pulled, the pull is delayed and continues. In both pull mode based consumption patterns (Push/Pull), the Consumer needs to know which message queue - queue from the Broker side to get the message. Therefore, it is necessary to do load balancing on the Consumer side, that is, which Consumer consumption is allocated to the same ConsumerGroup by more than one MessageQueue on the Broker side.
