Basic Concepts

Messages are JavaScript object notation (JSON) objects used for transmitting information. They can be sent one by one or in batches. Sending messages in batches can be achieved only through calling DMS APIs.

Each message can contain up to 512 KB of data.

Each message can optionally have one or more attributes. Each message attribute consists of a name and a value in the key-value format. Attributes are part of the message and are sent and retrieved along with it.

Attributes allow consumers to decide how to process the messages before processing the message body.

A message queue is a container that receives and stores message files. By default, 30 queues can be created under a project. Different messages in one queue can be retrieved by multiple consumers at the same time.

• Standard queue: The queue mode can be partition-level first in first out (FIFO) or global FIFO. Partition-level FIFO queues have higher concurrency performance than global FIFO queues, but do not guarantee the FIFO order of all messages.
• Advanced queue: Open-source Kafka clients can be used to create and retrieve messages. The queue mode can be high throughput or high reliability. In high throughput mode, all message replicas are flushed to a disk asynchronously to ensure high performance. In high reliability mode, all message replicas are flushed to a disk synchronously to ensure message reliability.
• RabbitMQ queue: RabbitMQ queues are created in RabbitMQ instances, which are physically isolated. You can customize the computing capabilities and storage space of RabbitMQ instances based on service requirements.

Dead letter messages are messages that cannot be correctly retrieved.

DMS can process messages that are not correctly retrieved. If a message fails to be retrieved for a preset number of times, it will be sent to the dead letter queue and retained for at least 72 hours. You can then retrieve the message from the dead letter queue.

Messages in the dead letter queue are specific to individual consumer groups. Consumers in a consumer group cannot retrieve dead letter messages of other consumer groups.

Dead letter messages from standard queues in global FIFO mode are sent to the dead letter queue in the FIFO order.

A producer, also called a message sender, sends messages to specific queues.

A consumer, also called a message receiver, retrieves messages from queues. After acknowledging a message, a consumer cannot retrieve the same message again.

Consumer groups classify different types of message consumers. A maximum of three consumer groups can be created in each queue.

Messages in a queue can be retrieved once by each consumer group. Messages acknowledged by a consumer group are no longer available to that consumer group but still available to other consumer groups.

Consumers in the same consumer group can retrieve different messages from a queue at the same time.

You can reset the position from which to sequentially retrieve messages sent from producers to DMS. You can choose to start retrieving messages from the beginning of a queue, from the end of a queue, or from a specified point in time within the message retention period (by default, 72 hours for standard queues).

DMS allows you to use labels to filter the messages you want to retrieve from the chosen queue.

Messages can be sent to the chosen queues regardless of whether the messages have labels or not.

If the Message Labels field in the Retrieve Message dialog box is left unspecified, all types of messages in the chosen queue will be retrieved. If this field is assigned a non-empty value, only messages with the specified message labels will be retrieved.

Messages can be delivered after a specified delay time.