Project: Apache RocketMQ (App Definition and Development / Streaming & Messaging)
File: FAQ.md

The following questions are frequently asked with regard to the RocketMQ project in general. Why did we create rocketmq project instead of selecting other products? Please refer to Do I have to install other softeware, such as zookeeper, to use RocketMQ? No. RocketMQ can run independently. &#8195;1) If the topic sends a message within three days, then the consumer start consuming messages from the first message saved in the server. &#8195;2) If the topic sends a message three days ago, the consumer starts to consume messages from the latest message in the server, in other words, starting from the tail of message queue. &#8195;3) If such consumer is rebooted, then it starts to consume messages from the last consumption location. &#8195;1) Cluster consumption pattern, The consumer business logic code returns Action.ReconsumerLater, NULL, or throws an exception, if a message failed to be consumed, it will retry for up to 16 times, after that, the message would be descarded. &#8195;2) Broadcast consumption patternThe broadcaset consumption still ensures that a message is consumered at least once, but no resend option is provided. &#8195;1) Using topic query by time, you can query messages within a period of time. &#8195;2) Using Topic and Message Id to accurately query the message. &#8195;3) Using Topic and Message Key accurately query a class of messages with the same Message Key. RocketMQ ensures that all messages are delivered at least once. In most cases, the messages are not repeated. &#8195;1) Start up a new broker and register it to the same list of name servers. &#8195;2) By default, only internal system topics and consumer groups are created automatically. If you would like to have your business topic and consumer groups on the new node, please replicate them from the existing broker. Admin tool and command lines are provided to handle this. The following answers are all default values and can be modified by configuration. Stored messages will be saved for up to 3 days, and messages that are not consumed for more than 3 days will be deleted. Generally 256KB. When you start Consumer, set a ConsumeThreadNums property, example is as follows: ``` consumer.setConsumeThreadMin(20);

``` Reason: Using the same Producer /Consumer Group to launch multiple instances of Producer/Consumer in the same JVM may cause the client fail to start. Solution: Make sure that a JVM corresponding to one Producer /Consumer Group starts only with one Producer/Consumer instance. Reason: Fastjson version is too low to allow the broadcast consumer to load local offsets.json, causing the consumer boot failure. Damaged fastjson file can also cause the same problem. Solution: Fastjson version has to be upgraded to rocketmq client dependent version to ensure that the local offsets.json can be loaded. By default offsets.json file is in /home/{user}/.rocketmq_offsets. Or check the integrity of fastjson. &#8195;1) Master crashes Messages can no longer be sent to this broker set, but if you have another broker set available, messages can still be sent given the topic is present. Messages can still be consumed from slaves. &#8195;2) Some slave crash As long as there is another working slave, there will be no impact on sending messages. There will also be no impact on consuming messages except when the consumer group is set to consume from this slave preferably. By default, consumer group consumes from master. &#8195;3) All slaves crash There will be no impact on sending messages to master, but, if the master is SYNCMASTER, producer will get a SLAVENOT_AVAILABLE indicating that the message is not sent to any slaves. There will also be no impact on consuming messages except that if the consumer group is set to consume from slave preferably. By default, consumer group consumes from master. This happens when you are trying to send messages to a topic whose routing info is not available to the producer. &#8195;1) Make sure that the producer can connect to a name server and is capable of fetching routing meta info from it. &#8195;2) Make sure that name servers do contain routing meta info of the topic. You may query the routing meta info from name server through topicRoute using admin tools or web console. &#8195;3) Make sure that your brokers are sending heartbeats to the same list of name servers your producer is connecting to. &#8195;4) Make sure that the topics permission is 6(rw-), or at least 2(-w-). If you cant find this topic, create it on a broker via admin tools command updateTopic or web console.