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- Release Notes and Announcements
- Product Introduction
- Purchase Guide
- RocketMQ 5.x
- RocketMQ 4.x
- Product Introduction
- Purchase Guide
- Getting Started
- Operation Guide
- Development Guide
- SDK Documentation
- Migration to Cloud
- API Documentation
- FAQs
- TDMQ FOR ROCKETMQ SERVICE LEVEL AGREEMENT
- Contact Us
This document lists the common concepts and their definitions in TDMQ for RocketMQ.
Message (Message)
A message is the physical carrier of information transmitted by the messaging system. It is the smallest unit of the produced or consumed data. A producer encapsulates the load and extended attributes of business data into messages and sends the messages to a TDMQ for RocketMQ broker. Then, the broker delivers the messages to the consumer based on the relevant semantics.
Topic (Topic)
A topic is the collection of a type of messages. It is the basic unit for message subscription in TDMQ for RocketMQ. Each topic contains several messages.
Message Tag (MessageTag)
Tags are used to categorize different types of messages in the same topic. Topic and tag are basically the first-level and second-level classifications of messages, respectively.
Message Queue (MessageQueue)
A message queue (also known as a message partition) is a physical entity for message storage, and a topic can contain multiple queues. Messages in a queue can only be consumed by one consumer rather than multiple consumers in one consumer group.
Message Offset (MessageQueueOffset)
Messages are stored in multiple queues of a specified topic based on the order in which they arrive at the TDMQ for RocketMQ broker. Each message has a unique coordinate of type
Long in the queue, which is defined as the message offset.Consumption Offset (ConsumerOffset)
A message is not removed from the queue immediately after it has been consumed by a consumer. TDMQ for RocketMQ will record the offset of the last consumed message based on each consumer group. Such an offset is defined as the consumption offset.
Message Index (MessageKey)
A message index is a message-oriented index property in TDMQ for RocketMQ. By setting the message index, you can quickly find the corresponding message content.
Producer (Producer)
A producer in TDMQ for RocketMQ is a functional messaging entity that creates messages and sends them to the broker. It is typically integrated into the business system to encapsulate data as messages and send them to the broker.
Consumer (Consumer)
A consumer is an entity that receives and processes messages in TDMQ for RocketMQ. It is usually integrated into the business system to obtain messages from TDMQ for RocketMQ brokers and convert the messages into information that can be perceived and processed by business logic.
Group (Group)
Groups include producer groups and consumer groups.
Producer group: It is the collection of the same type of producers that send the same type of messages with the same sending logic. If a producer sends transactional messages and crashes afterward, the broker will contact other producer instances in the producer group to commit or cancel the transaction.
Consumer group: It is the collection of the same type of consumers that consume the same type of messages with the same consumption logic. It can ensure load balancing and fault tolerance in the message consumption process. Consumer instances in a consumer group must subscribe to the same topics.
Message Type (MessageType)
Messages are classified by message transmission characteristic for message type management and security verification. TDMQ for RocketMQ has four message types: general message, sequential message, transactional message, and scheduled/delayed message.
General Message
The general message is a basic message type. After the produced general messages are delivered to a specified topic, they will be consumed by consumers that subscribe to this topic. A topic with general messages is sequence-insensitive. Therefore, you can use multiple topic partitions to improve message production and consumption efficiency. This approach performs best when dealing with high throughput.
Sequential Message
In TDMQ for RocketMQ, the sequential message is an advanced message type. Sequential messages in a specified topic are published and consumed in a First In First Out (FIFO) manner, that is, the first produced messages are first consumed.
Retry Letter Queue
A retry letter queue is designed to ensure that messages are consumed normally. When a message is consumed for the first time by a consumer but is not acknowledged, it will be placed in the retry letter queue and will be retried there until the maximum number of retries is reached. It will then be delivered to the dead letter queue.
In actual scenarios, messages may not be processed promptly due to temporary issues such as network jitter and service restart. The retry mechanism of the retry letter queue can be a good solution in this case.
Dead Letter Queue
A dead letter queue is a special type of message queue used to centrally process messages that cannot be consumed normally. If a message cannot be consumed after a specified number of retries in the retry letter queue, TDMQ will determine that the message cannot be consumed under the current situation and deliver it to the dead letter queue.
In actual scenarios, messages may not be consumed due to service downtime or network disconnection. In this case, they will not be discarded immediately; instead, they will be persistently stored in the dead letter queue. After fixing the problem, you can create a consumer to subscribe to the dead letter queue to process such messages.
Clustering Consumption
Clustering consumption: If the clustering consumption mode is used, each message only needs to be processed by any of the consumers in the cluster. This mode is suitable for scenarios where each message only needs to be processed once.
Broadcasting Consumption
Broadcasting consumption: If the broadcasting consumption mode is used, each message will be pushed to all registered consumers in the cluster to ensure that the message is consumed by each consumer at least once. This mode is suitable for scenarios where each message needs to be processed by each consumer in the cluster.
Message Filtering
A consumer can filter messages by subscribing to specified message tags to ensure that it only receives the filtered messages. The whole filtering process is completed in the TDMQ for RocketMQ broker.
Consumption Offset Reset
Resetting the consumption offset means resetting a consumer group's consumption offset for subscribed topics within the persistent message storage period based on the time axis. After the offset is reset, the consumers will receive the messages that the producer sends to the TDMQ for RocketMQ broker after the set time point.
Message Trace
The message trace records the entire lifecycle of a message from the time it is sent by the producer to the time it is received and processed by the consumer. With this feature, you can track the entire trace of a message, starting from its production by a producer, its storage and distribution within the TDMQ for RocketMQ broker, and finally its consumption by one or more consumers. This helps you troubleshoot any problems that may occur during message processing.
Message Heap
Message heap occurs in scenarios where the producer has sent messages to the TDMQ for RocketMQ broker but the consumer fails to normally consume all these messages promptly due to its consumption capability limit. In this case, the unconsumed messages will be heaped in the broker. The message heap data is collected once every minute. After the message retention period (3 days by default) elapses, the unconsumed messages will no longer be heaped in the broker because they have been deleted by the broker.
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