This topic describes related terms of Message Queue for Apache RocketMQ to help you better understand and use Message Queue for Apache RocketMQ.

Topic
A topic is used to classify messages. It is the first-level classification. For more information, see Best practices of topics and tags.
Message
In Message Queue for Apache RocketMQ, a message is a carrier for transferring information.
Message ID
A message ID is a globally unique identifier for a message, which is automatically generated by the Message Queue for Apache RocketMQ system.
Message key
A message key is a unique identifier of the message's service logic, which is set by the message producer.
Tag
A tag is used to further classify the messages under a topic. It is the second-level classification. For more information, see Best practices of topics and tags.
Producer
A producer, also known as a message publisher, produces and sends messages.
Producer instance
A producer instance is an object instance of a producer. Different producer instances can run in different processes or on different machines. Producer instances can be shared among producer instance threads in the same process because these threads are secure.
Consumer
A consumer, also known as a message subscriber, receives and consumes messages.
Consumer instance
A consumer instance is an object instance of a consumer. Different consumer instances can run in different processes or on different machines. A thread pool is configured for a consumer instance to consume messages.
Group

A group is a type of producer or consumer that produces or consumes messages of the same type and publish or subscribe to messages based on the same logic.

Group ID
The ID of the group.
Queue
Each topic has one or more queues to store messages. The number of queues for each topic varies with the region of the instance. For more information about the exact number of queues, submit a ticket.
Notice The Standard Edition does not support changing the queue number, while the Platinum Enterprise Edition does.
Exactly-once delivery semantics
The exactly-once delivery semantics is used to specify that messages sent to the message system can be processed by consumers only once. In other words, even if the message producer sends a message to the messaging system again, the message can still only be consumed by a consumer once. For more information, see Exactly-once delivery semantics.
Clustering consumption
All consumers identified by the same group ID consume messages in an even manner. For example, a topic contains nine messages and a group contains three consumer instances. In this case, each instance consumes three messages. For more information, see Clustering consumption and broadcasting consumption.
Broadcasting consumption
Each of the consumers identified by the same group ID consumes all messages once. For example, a topic contains nine messages and a group contains three consumer instances. In this case, each instance consumes nine messages. For more information, see Clustering consumption and broadcasting consumption.
Scheduled message
A producer sends a message to the Message Queue for Apache RocketMQ broker, expecting the message to be delivered to a consumer at a specified time in the future. The message is a scheduled message. For more information, see Scheduled messages and delayed messages.
Delayed message
A producer sends a message to the Message Queue for Apache RocketMQ broker, expecting the message to be delivered to a consumer after a specified period of time. The message is a delayed message. For more information, see Scheduled messages and delayed messages.
Transactional message
Message Queue for Apache RocketMQ provides a distributed transaction processing function similar to X/Open XA to ensure transaction consistency in Message Queue for Apache RocketMQ. For more information, see Transactional messages.
Ordered message
An ordered message is a message that is published and consumed in order. Ordered messages in Message Queue for Apache RocketMQ are classified into globally ordered messages and partitionally ordered messages. For more information, see Ordered messages.
Globally ordered message
All messages under a specified topic are published and consumed in strict first-in-first-out (FIFO) order. For more information, see Ordered messages.
Partitionally ordered message
All messages under a specified topic are partitioned by the shard key. Messages in one shard are published and consumed strictly in FIFO order. A shard key is a key field that is used in ordered messages to distinguish different shards. It is completely different from the key used in normal messages. For more information, see Ordered messages.
Message accumulation
A producer has sent messages to the Message Queue for Apache RocketMQ broker but a consumer cannot consume all the messages in a short period of time due to limited consumption capability. Therefore, unconsumed messages are stored in the Message Queue for Apache RocketMQ broker. This process is called message accumulation.
Message filtering
Consumers can filter messages by tag to receive only the message type they want. Message filtering is completed on the Message Queue for Apache RocketMQ broker. For more information, see Message filtering.
Message trace
A message trace is a complete route record of a message from its publishing by a producer, to consumption by a consumer. It consists of the time, location, and other information on each node. The message trace helps troubleshooting by clearly displaying a complete route of a message from a producer, to the Message Queue for Apache RocketMQ broker, and then to a consumer. For more information, see Query a message trace.
Consumer offset reset
You can reset the consumption progress of a topic subscribed by a message consumer over the time span specified for persistent message storage (three days by default). After the consumption offset is reset, the consumer can receive a message sent by the message producer to the Message Queue for Apache RocketMQ broker after the set time point. For more information, see Reset consumer offsets.
Dead-letter queue

Dead-letter queues are used to process messages that cannot be consumed. When a message fails to be consumed for the first time, Message Queue for Apache RocketMQ will automatically perform Message retry. If the message still cannot be consumed after the maximum number of retries is reached, the message cannot be properly consumed. Instead of immediately discarding the message, Message Queue for Apache RocketMQ sends it to a particular queue of the corresponding consumer.

In Message Queue for Apache RocketMQ, a message that cannot be properly consumed is called a dead-letter message, and the particular queue that stores dead-letter messages is called a dead-letter queue.

For more information, see Dead-letter queue.

Message router
Message routers are often used to synchronize messages between regions to ensure data consistency between regions. Relying on Express Connect, developed by Alibaba Cloud based on its superb infrastructure, Message Queue for Apache RocketMQ message routers allow you to efficiently synchronize messages from country to country and region to region. For more information, see Message router.