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XACK

Introduction​

In Dragonfly, as well as in Redis and Valkey, the XACK command is used to acknowledge one or more messages in a stream that have been successfully processed. It is useful in stream processing systems where consumers must signal that a message has been handled, helping to manage message delivery and retention more efficiently within a consumer group. After successfully processing a message, a consumer should call XACK to prevent reprocessing and remove the message from the Pending Entries List (PEL).

Syntax​

XACK key group id [id ...]
  • Time complexity: O(1) for each message ID processed.
  • ACL categories: @write, @stream, @fast

Parameter Explanations​

  • key: The name of the stream.
  • group: The name of the consumer group.
  • id: The ID of the message to acknowledge. Multiple IDs can be specified.

Return Values​

  • The command returns an integer indicating the number of messages that were successfully acknowledged.
  • Message IDs that are not part of the PEL (i.e., already been acknowledged) will not be considered successful acknowledgments.
  • If the stream or the consumer group does not exist, the command simply returns 0.

Code Examples​

Basic Acknowledgement Example​

Acknowledge a single message that has been processed:

dragonfly$> XADD mystream * name Alice age 20
"1609097574170-0"

dragonfly$> XGROUP CREATE mystream mygroup 0 MKSTREAM
OK

dragonfly$> XREADGROUP GROUP mygroup consumer-1 COUNT 1 STREAMS mystream >
1) 1) "mystream"
   2) 1) 1) "1609097574170-0"
         2) 1) "name"
            2) "Alice"
            3) "age"
            4) "20"

dragonfly$> XACK mystream mygroup "1609097574170-0"
(integer) 1

Acknowledging Multiple Messages​

Acknowledge multiple messages after processing:

dragonfly$> XADD mystream * name Bob age 25
"1609097574171-0"

dragonfly$> XADD mystream * name Charlie age 30
"1609097574172-0"

# The consumer reads the 2 new messages.
dragonfly$> XREADGROUP GROUP mygroup consumer-1 COUNT 2 STREAMS mystream >
# ...

# Acknowledge the 2 messages for the consumer group.
dragonfly$> XACK mystream mygroup "1609097574171-0" "1609097574172-0"
(integer) 2

Handling Non-existent IDs​

Acknowledge attempt on a non-existent message ID:

dragonfly$> XACK mystream mygroup "1609097574173-0"
(integer) 0  # No acknowledgment since the ID does not exist.

Best Practices​

  • Use XACK to signal message processing completion to avoid reprocessing and free up resources.
  • Implement proper error handling to manage message IDs that may no longer exist or are not yet acknowledged.

Common Mistakes​

  • Attempting to acknowledge messages using IDs that were never read by the group.

FAQs​

What happens if the message ID is not found?​

If the message ID does not exist in the pending entries list of the consumer group, XACK returns 0.

Can XACK be used for automatic acknowledgement?​

No, XACK requires explicit calls to acknowledge messages after successful processing by consumers.