xiangwangcheng commented on a change in pull request #793: [RIP-9] Add the
introduction of the basic concepts in RocketMQ
URL: https://github.com/apache/rocketmq/pull/793#discussion_r259195241
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File path: docs/en/Concept.md
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+# Basic Concept
+
+## 1 Message Model
+
+The RocketMQ message model is mainly composed of Producer, Broker and
Consumer.The Producer is responsible for producing messages, the Consumer is
responsible for consuming messages, and the Broker is responsible for storing
messages.
+The Broker corresponds to one server during actual deployment, and each Broker
can store messages from multiple topics, and messages from each Topic can be
stored in a different Broker by sharding strategy.
+The Message Queue is used to store physical offsets of messages, and the
Message addresses in multiple Topic are stored in multiple Message queues.The
Consumer Group consists of multiple Consumer instances.
+## 2 Producer
+The Producer is responsible for producing messages, typically by business
systems.It sends messages generated by the business application systems to
brokers. The RocketMQ provides multiple paradigms of sending: synchronous,
asynchronous,sequential and one-way.Both synchronous and asynchronous methods
require the Broker to return confirmation information, while one-way sending is
not required.
+## 3 Consumer
+ The Consumer is responsible for consuming messages, typically the background
system is responsible for asynchronous consumption.The Consumer pulls messages
from brokers and feeds them into application. In perspective of user
application, two types of consumers are provided:Pull Consumer and Push
Consumer.
+## 4 Topic
+The Topic refers to a collection of one kind message. Each topic contains
several messages and one message can only belong to one topic. The Topic is the
basic unit of RocketMQ for message subscription.
+## 5 Broker Server
+As the role of the transfer station, the Broker Server stores, and forwards
messages.In the RocketMQ system, the Broker Server is responsible for receiving
messages sent from producers, storing them and preparing to handle pull
requests.It also stores message related meta data, including consumer groups,
consuming progress, topics and queues info.
+## 6 Name Server
+The Name Server serves as the provider of routing service.The Producer or
Consumer can find the list of Broker IP addresses corresponding each topic
through name server.Multiple Name Servers can be deployed as a cluster, but are
independent of each other and do not exchange information.
+## 7 Pull Consumer
+ A type of Consumer.Applications are usually pulls messages from brokers by
actively calling the Consumer's pull message method,and the application has the
advantages of controlling the timing and frequency of pulling messages. Once
batches of messages are pulled, user application initiates consuming process.
+## 8 Push Consumer
+ A type of Consumer.Under this mode, after the Broker receives the data, it
will actively push it to the consumer, which is generally of high real-time
performance.
+## 9 Producer Group
+ A collection of the same type of Producer, which sends the same type of
messages with consistent logic.If a transaction message is sent and the
original producer crashes after sending, the Broker Server will contacts other
Producer in the same Producer group to commit or rollback the transactional
message.
+## 10 Consumer Group
+ A collection of the same type of Consumer, which sends the same type of
messages with consistent logic.The Consumer Group makes load-balance and
fault-tolerance super easy in terms of message consuming.
+Warning: consumer instances of one consumer group must have exactly the same
topic subscription(s).
+
+RocketMQ supports two type of consumption mode:Clustering and Broadcasting.
+## 11 Consumption Mode - Clustering
+Under the Clustering mode, each Consumer instance of the same Consumer Group
allocates messages equally.
+## 12 Consumption Mode - Broadcasting
+Under the Broadcasting mode, each Consumer instance of the same Consumer Group
receives a full number of messages.
+## 13 Normal Ordered Message
+Under the Normal Ordered Message mode, the messages received by consumers
through the same consumption queue are sequential, while the messages received
by different message queues may be non-sequential.
+## 14 Strictly Ordered Message
+Under the Strictly Ordered Message mode, all messages received by the consumer
are sequential.
+## 15 Message
+The physical carrier of information transmitted by a messaging system, the
smallest unit of production and consumption data, each message must belong to a
topic.
+Each Message in RocketMQ has a unique Message ID and can carry a Key with a
business identity.The system provides the function to query messages by Message
ID and Key.
+## 16 Tag
+ Flags set for messages to distinguish different types of messages under the
same topic.Messages from the same business unit can set different tag under the
same topic in terms of different business purposes.The Tag can effectively
maintain the clarity and consistency of the code and optimize the query system
provided by RocketMQ.The Consumer can realize different consumption logic for
different subtopics according to Tag, so as to achieve better expansibility.
Review comment:
1. Flags set for messages to distinguish different types of messages under
the same topic, **functioning as a "sub-topic"**.
2. Messages from the same business unit can set different **tags** under the
same topic.
3. The Consumer can realize different consumption logic for different
**"sub-topic"** **by using Tag**, so as to achieve better expansibility.
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