XREADGROUP is a Redis command used to read data from a stream with a specific consumer group. This is particularly useful in scenarios where you have multiple readers (consumers) for the same stream and want to ensure that every message is processed exactly once by any one of the consumers.
First, let's assume you have a Redis connection instantiated:
Now let's create a stream and a group:
Here's how to use
XREADGROUP to read data from the stream:
In this example, we are using
XREADGROUP to read one message (
$count) from the stream 'mystream'. It'll block for 2 seconds (
$block) if there's no new data available.
COUNToption sparingly and wisely; fetching too many messages at once could lead to high memory usage.
XREADGROUPcan return an empty array if there are no new messages in the stream.
Q: What happens when two consumers read the same stream with XREADGROUP?
A: If two consumers belong to the same group, they will not receive the same messages from the stream. Redis ensures that each message is delivered to a single consumer in a group.
Q: What does the '>' symbol mean in the streams array?
A: The '>' symbol tells Redis to return new messages that the consumer hasn't seen yet. If you provide a message ID instead, it'll return messages with IDs greater than the provided one.