Consumer Groups

What is a consumer group

A consumer group lets multiple workers (consumers) read from the same stream, with each entry delivered to only ONE consumer in the group. Redis tracks which entries have been delivered but not yet acknowledged. This enables at-least-once processing.

Creating a group — XGROUP CREATE

127.0.0.1:6379> XADD jobs * type "email" to "[email protected]"
"1700000000000-0"
127.0.0.1:6379> XADD jobs * type "sms" to "+6681234567"
"1700000000001-0"
127.0.0.1:6379> XGROUP CREATE jobs workers $ MKSTREAM
OK

$ means “start from the latest entry” — only new messages added after the group is created will be delivered. Use 0 to read from the beginning of the stream. MKSTREAM creates the stream automatically if it does not exist yet.

XREADGROUP — reading as a consumer

127.0.0.1:6379> XADD jobs * type "email" to "[email protected]"
"1700000000002-0"
127.0.0.1:6379> XREADGROUP GROUP workers consumer-1 COUNT 1 STREAMS jobs >
1) 1) "jobs"
   2) 1) 1) "1700000000002-0"
            2) 1) "type"
               2) "email"
               3) "to"
               4) "[email protected]"

The special ID > means “give me new undelivered entries”. Once delivered, the entry is placed in the Pending Entries List (PEL) for consumer-1 until it is explicitly acknowledged.

XACK — acknowledging entries

127.0.0.1:6379> XACK jobs workers 1700000000002-0
(integer) 1

Once acknowledged, the entry is removed from the PEL. Only call XACK after your worker has successfully processed the message — acknowledging too early risks data loss if the worker crashes mid-processing.

XPENDING — inspecting unacknowledged entries

127.0.0.1:6379> XPENDING jobs workers - + 10
1) 1) "1700000000002-0"
   2) "consumer-1"
   3) (integer) 4321
   4) (integer) 1

Each row shows: the entry ID, the consumer holding it, how long it has been idle in milliseconds, and how many times it has been delivered. This is your primary tool for detecting stuck or slow consumers.

XADD jobs * type "email" to "[email protected]"
XADD jobs * type "sms" to "+6681234567"
XGROUP CREATE jobs workers 0 MKSTREAM
XREADGROUP GROUP workers consumer-1 COUNT 10 STREAMS jobs >
XACK jobs workers 1700000000000-0
XPENDING jobs workers - + 10

Copy sequence (update IDs from XADD output) ⧉

🟥 Consumer groups split a stream across workers with acknowledgements

Consumer groups give you at-least-once delivery. If a consumer crashes before calling XACK, the entry stays in the PEL and can be claimed by another worker using XCLAIM or XAUTOCLAIM. This makes Redis Streams a reliable foundation for distributed task queues.

What does `XREADGROUP GROUP g consumer STREAMS s >` mean?

Deliver new undelivered entries to this consumer Re-deliver all pending entries from the beginning Acknowledge all entries in the group Delete the consumer group

What happens to a stream entry after XACK is called?

It is deleted from the stream entirely It is moved to a dead-letter stream It is removed from the Pending Entries List It is redelivered to another consumer

What does XPENDING show?

All entries ever written to the stream Entries that have been delivered but not yet acknowledged Entries waiting to be delivered The number of active consumer groups

In `XGROUP CREATE jobs workers 0 MKSTREAM`, what does `0` mean?

Only deliver zero entries at a time Start from the latest entry only Start reading from the beginning of the stream Create zero consumers initially

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