Fetching data from a stream via a consumer group, and not acknowledging such data, has the effect of creating pending entries. This is well explained in the XREADGROUP command, and even better in our introduction to Redis Streams. The XACK command will immediately remove the pending entry from the Pending Entry List (PEL) since once a message is successfully processed, there is no longer need for the consumer group to track it and to remember the current owner of the message.
The XPENDING command is the interface to inspect the list of pending messages, and is as thus a very important command in order to observe and understand what is happening with a streams consumer groups: what clients are active, what messages are pending to be consumed, or to see if there are idle messages. Moreover this command, together with XCLAIM is used in order to implement recovering of consumers that are failing for a long time, and as a result certain messages are not processed: a different consumer can claim the message and continue. This is better explained in the streams intro and in the XCLAIM command page, and is not covered here.
*Summary form of XPENDING
When XPENDING is called with just a key name and a consumer group name, it just outputs a summary about the pending messages in a given consumer group. In the following example, we create a consumer group and immediately create a pending message by reading from the group with XREADGROUP.
> XGROUP CREATE mystream group55 0-0 OK > XREADGROUP GROUP group55 consumer-123 COUNT 1 STREAMS mystream > 1) 1) "mystream" 2) 1) 1) 1526984818136-0 2) 1) "duration" 2) "1532" 3) "event-id" 4) "5" 5) "user-id" 6) "7782813"
We expect the pending entries list for the consumer group
have a message right now: consumer named
consumer-123 fetched the
message without acknowledging its processing. The simple XPENDING
form will give us this information:
> XPENDING mystream group55 1) (integer) 1 2) 1526984818136-0 3) 1526984818136-0 4) 1) 1) "consumer-123" 2) "1"
In this form, the command outputs the total number of pending messages for this consumer group, which is one, followed by the smallest and greatest ID among the pending messages, and then list every consumer in the consumer group with at least one pending message, and the number of pending messages it has.
This is a good overview, but sometimes we are interested in the details. In order to see all the pending messages with more associated information we need to also pass a range of IDs, in a similar way we do it with XRANGE, and a non optional count argument, to limit the number of messages returned per call:
> XPENDING mystream group55 - + 10 1) 1) 1526984818136-0 2) "consumer-123" 3) (integer) 196415 4) (integer) 1
In the extended form we no longer see the summary information, instead there are detailed information for each message in the pending entries list. For each message four attributes are returned:
- The ID of the message.
- The name of the consumer that fetched the message and has still to acknowledge it. We call it the current owner of the message.
- The number of milliseconds that elapsed since the last time this message was delivered to this consumer.
- The number of times this message was delivered.
The deliveries counter, that is the fourth element in the array, is incremented when some other consumer claims the message with XCLAIM, or when the message is delivered again via XREADGROUP, when accessing the history of a consumer in a consumer group (see the XREADGROUP page for more info).
Finally it is possible to pass an additional argument to the command, in order to see the messages having a specific owner:
> XPENDING mystream group55 - + 10 consumer-123
But in the above case the output would be the same, since we have pending messages only for a single consumer. However what is important to keep in mind is that this operation, filtering by a specific consumer, is not inefficient even when there are many pending messages from many consumers: we have a pending entries list data structure both globally, and for every consumer, so we can very efficiently show just messages pending for a single consumer.
The command returns data in different format depending on the way it is called, as previously explained in this page. However the reply is always an array of items.