XPENDING

Introduction and Use Case(s)

The XPENDING command in Redis is used to retrieve information about pending messages in a stream’s consumer group. It is particularly useful for monitoring message processing and identifying unacknowledged messages within the stream, which can be critical for debugging and ensuring reliable message delivery.

Syntax

XPENDING key group [start end count] [consumer]

Parameter Explanations

• key: The name of the stream.
• group: The name of the consumer group.
• start (optional): The lower bound ID for pending messages. Use ‘-‘ to start from the earliest message.
• end (optional): The upper bound ID for pending messages. Use ‘+’ to end at the latest message.
• count (optional): Limits the number of pending messages returned.
• consumer (optional): Filters pending messages by a specific consumer.

Return Values

The XPENDING command returns different outputs depending on its usage:

1. Without additional arguments:

   dragonfly> XPENDING mystream mygroup
   1) (integer) total_pending
   2) smallest_pending_id
   3) largest_pending_id
   4) 1) 1) "consumer1"
          2) (integer) pending_count_for_consumer1
      2) 1) "consumer2"
          2) (integer) pending_count_for_consumer2

2. With start, end, count, and optionally consumer:

   dragonfly> XPENDING mystream mygroup - + 10
   1) 1) message_id
      2) consumer
      3) milliseconds_since_delivered
      4) delivery_count

Code Examples

dragonfly> XADD mystream * name John
"1627815448523-0"
dragonfly> XGROUP CREATE mystream mygroup 0
OK
dragonfly> XREADGROUP GROUP mygroup consumer1 COUNT 1 STREAMS mystream >
1) 1) "mystream"
   2) 1) "1627815448523-0"
      2) 1) "name"
         2) "John"
dragonfly> XPENDING mystream mygroup
1) (integer) 1
2) "1627815448523-0"
3) "1627815448523-0"
4) 1) 1) "consumer1"
      2) (integer) 1
dragonfly> XPENDING mystream mygroup - + 10
1) 1) "1627815448523-0"
   2) "consumer1"
   3) (integer) 12345
   4) (integer) 1

Best Practices

• Regularly monitor the pending entries list (PEL) using XPENDING to ensure no messages remain unacknowledged for too long, which could indicate issues with consumers or message processing.

Common Mistakes

• Failing to properly handle unacknowledged messages can lead to data loss or inconsistencies. Always ensure that your application logic includes robust error handling and retry mechanisms for message processing.

FAQs

What happens if I don’t specify any range or consumer?

Without specifying start, end, count, or consumer, XPENDING provides an overview of all pending messages in the consumer group, including the total count and information per consumer.

Can I use XPENDING to get details of specific pending messages?

Yes, by providing start, end, and count parameters, you can retrieve details of specific pending messages, and filter further using the consumer parameter if needed.