Redis流中的消费者群是什么？

消费者组通过允许多个消费者协作读取同一数据流解决消息重复处理问题。1. 消费者组确保每条消息仅被组内一个消费者处理一次；2. 消费者可确认成功处理，未确认消息可重新分配；3. 使用XGROUP CREATE创建组，XREADGROUP读取消息，XACK确认处理完成；4. 未确认消息存于PEL列表，可通过XPENDING查看或XCLAIM转移处理。

A consumer group in Redis Streams is a way to organize multiple consumers so they can work together reading data from the same stream, without stepping on each other’s toes. Think of it like a team working on processing tasks from a shared to-do list — each person (consumer) takes a task and works on it independently.

What problem does a consumer group solve?

Without consumer groups, if you have multiple consumers reading from the same stream, they’ll all get the same messages. That’s not ideal if you want to split the workload — imagine three people doing the exact same task at the same time.

With a consumer group:

• Each message in the stream is processed only once by one consumer in the group.
• Consumers can acknowledge successful message processing.
• If something goes wrong or a consumer crashes, unacknowledged messages can be reassigned.

This makes consumer groups perfect for building reliable and scalable message processing systems.

How do consumer groups work in practice?

When using a consumer group, there are a few key concepts to understand:

• Stream: The source of the data.
• Group: A named group of consumers.
• Consumer: An individual process or client that reads messages from the stream as part of a group.
• Pending Entries List (PEL): Tracks which messages have been delivered to which consumer but haven’t been acknowledged yet.

Here’s a simplified flow:

• One consumer in the group reads a message using XREADGROUP.
• The message is marked as being in progress.
• Once the consumer finishes processing, it sends an XACK to confirm completion.
• If the message isn’t acknowledged within a certain time, it can be delivered to another consumer.

How to create and use a consumer group

To start with consumer groups, first you need to create a group associated with a stream using the XGROUP CREATE command. Here's how:

XGROUP CREATE mystream mygroup $

The $ symbol means the group starts reading from the latest message onward. If you want to process all existing messages too, omit the $.

Once the group exists, a consumer can start pulling messages:

XREADGROUP GROUP mygroup consumer1 COUNT 1 STREAMS mystream >

This tells Redis to give up to 1 unprocessed message from mystream to consumer1 in the mygroup group.

After processing, the consumer should send:

XACK mystream mygroup <message-id>

This removes the message from the pending list, telling Redis it was handled successfully.

Things to watch out for

There are a few gotchas when working with consumer groups:

• Unacknowledged messages stay in the PEL until confirmed. You can check them with XPENDING.
• Consumers can crash, leaving messages unacknowledged. Use XCLAIM to move those messages to another consumer.
• Consumer names don't need to be unique, but it helps track who's doing what.
• Message retention depends on your use case — Redis won’t delete messages automatically unless you set a policy.

For example, if you're debugging or recovering, this is useful:

XPENDING mystream mygroup

It shows how many messages are still waiting to be acknowledged, along with details like which consumer has them and how long they've been pending.

So while consumer groups handle most of the coordination, you still need to manage acknowledgments and possible retries yourself.

基本上就这些。

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