This topic describes the terms used in Message Queue for Apache Kafka . This helps you better understand and use Message Queue for Apache Kafka.

Apache Kafka
A distributed, open source platform for data stream processing. This platform allows you to publish, subscribe to, store, and process data streams in real time. For more information, see Apache Kafka.
Message Queue for Apache Kafka
A fully managed Apache Kafka service provided by Alibaba Cloud. This service frees you from deployments and O&M, and offers you a cost-effective service that features high scalability, reliability, and throughput. For more information, see What is Message Queue for Apache Kafka?.
Zookeeper
A distributed, open source coordination service for applications. In Message Queue for Apache Kafka , ZooKeeper is used to manage clusters and configurations, and elect leaders. ZooKeeper is part of the Message Queue for Apache Kafka architecture. You do not need to learn about ZooKeeper.
Broker
A node server in a Message Queue for Apache Kafka cluster. Message Queue for Apache Kafka provides a fully managed service that automatically changes the broker quantity and configuration based on the traffic specifications of your instances. You do not need to learn about the specific broker information.
Cluster
A collection of multiple brokers.
Instance
An independent Message Queue for Apache Kafka resource entity that corresponds to a cluster.
VPC-connected instance
An instance that provides only a VPC endpoint, and can be accessed only from the VPC.
Internet- and VPC-connected instance
An Internet- and VPC-connected instance that provides public and VPC endpoints, and can be accessed from the Internet and the VPC.
Major version upgrade
An upgrade across versions. For example, you can upgrade a Message Queue for Apache Kafka instance from version 0.10.x to version 2.x. For more information, see Upgrade the instance version.
Minor version upgrade
An upgrade that does not cross versions. For example, you can upgrade a Message Queue for Apache Kafka instance from version 0.10 to version 0.10.2, or from version 0.10.2 to the 0.10.2 kernel-optimized version. For more information, see Upgrade the instance version.
Endpoint
An address that is used for producers or consumers to connect to Message Queue for Apache Kafka .This address consists of the broker URL and port number. For more information, see Comparison among endpoints.
Message
A carrier for transferring information in Message Queue for Apache Kafka . Messages can be page views, server logs, and the information about system resources, such as CPU and memory resources. In Message Queue for Apache Kafka , a message is a byte array.
Message retention period
The maximum message retention period when the disk capacity is sufficient.
Maximum message size
The maximum size of a message that you can send and receive in Message Queue for Apache Kafka .
Publish/subscribe pattern
An asynchronous communication model between services. A publisher sends a message directly to a specified topic, and does not need to know the subscriber that receives the message. A subscriber receives a message directly from a specified topic, and does not need to know the publisher that sends the message. Message Queue for Apache Kafka uses the publish/subscribe pattern. For more information, see Publish/subscribe pattern of Message Queue for Apache Kafka.
Subscription relationship
The subscription relationship between consumer groups and topics. In Message Queue for Apache Kafka , you can query the status of online consumer groups that have subscribed to a specified topic. The status of the offline consumer groups cannot be queried.
Producer
An application that sends messages to Message Queue for Apache Kafka .
Consumer
An application that receives messages from Message Queue for Apache Kafka .
Consumer Group
A group of consumers that have the same group ID. When a topic is consumed by multiple consumers in the same consumer group, each message in the topic is delivered only to one consumer. This balances the load over consumers and allows messages in a topic to be concurrently consumed.
Topic
A message topic that is used to classify messages.
Topic traffic rebalance
An operation that evenly redistributes the topic traffic after a Message Queue for Apache Kafka cluster is scaled out. For more information, see Rebalance the topic traffic.
Partition
A message partition that is used to store messages. A topic consists of one or more partitions. Messages in each partition are stored on one or more brokers.
Offset
A sequence number that is assigned to a message when the message arrives at a partition.
Minimum offset
The minimum offset in a partition. This is the offset for the first message in the current partition. For more information about how to query the minimum offset in the current partition, see View partition status.
Maximum offset
The maximum offset in a partition. This is the offset for the latest message in the current partition. For more information about how to query the maximum offset in the current partition, see View partition status.
Consumer offset
The maximum offset of messages that are consumed in a partition. For more information about how to query the consumer offset, see View consumption status.
Latest consumption time
The time when the latest message consumed by a consumer group was published to and stored on the Message Queue for Apache Kafka broker. If no message accumulation occurs, the time is close to the message sending time.
Message accumulation
The total number of accumulated messages in the current partition. The value is equal to the maximum offset minus the consumer offset. Message accumulation is a key metric. If a large number of messages are accumulated, consumers may be blocked, or the consumption speed cannot keep up with the production speed. In this case, you must analyze the running status of consumers and improve the consumption speed. You can clear all accumulated messages, and start to consume from the maximum offset or reset consumer offsets based on time points. For more information, see Reset consumer offsets.
Local storage
A storage engine that uses the In-Sync Replicas (ISR) algorithm of native Apache Kafka. If you have special requirements, such as compact, idempotence, transactions, and partitionally ordered messages, we recommend that you use local storage. For more information, see Storage engine comparison.
Cloud storage
A storage engine that uses the algorithm for Alibaba Cloud disks. Cloud storage has the benefits of the underlying storage of Alibaba Cloud. Compared with local storage, cloud storage provides better performance in auto scaling, reliability, availability, and cost-effectiveness. Therefore, in most cases, we recommend that you use cloud storage. For more information, see Storage engine comparison.
cleanup.policy
A log cleanup policy. Configure a log cleanup policy only if you use local storage. Message Queue for Apache Kafka supports the following log cleanup policies:
Normal message
By default, messages of the same key are stored in the same partition in the order in which they are sent. A small number of messages are out of order during cluster restart or downtime. For more information, see Storage engine comparison.
Partitionally ordered messages
By default, messages of the same key are stored in the same partition in the order in which they are sent. When a cluster failure occurs, the messages are still in order. However, messages in some partitions cannot be sent until the failed instance is restored. For more information, see Storage engine comparison.
Connector
A component in Message Queue for Apache Kafka , which is used to synchronize data between Message Queue for Apache Kafka and other Alibaba Cloud services. For more information, see Overview.
Tag
A tag that is used to identify Message Queue for Apache Kafka resources. You can use tags to classify Message Queue for Apache Kafka resources based on resource features for easy resource search and aggregation. For more information, see Bind a tag.
RAM
A service provided by Alibaba Cloud to manage user identities and resource access permissions. You can only grant permissions to RAM users in the Message Queue for Apache Kafka console or by using the API operations. No matter whether RAM users are authorized or not, RAM users can use SDKs to send and receive messages. For more information, see Access control overview.
ACL
A service provided by Message Queue for Apache Kafka to manage the permissions of SASL users and clients to send and receive messages by using SDKs. This feature is consistent with that in open source Apache Kafka. The ACL feature is only applicable to scenarios where you want to implement access control for users that use SDKs to send and receive messages. This feature is not applicable to scenarios where you want to implement access control for users that send and receive messages in the Message Queue for Apache Kafka console or by using API operations. For more information, see Access control overview.