Available since: 5.0.0
Time complexity: O(N) with N being the number of elements returned, so asking for a small fixed number of entries per call is O(1). O(M), where M is the total number of entries scanned when used with the IDLE filter. When the command returns just the summary and the list of consumers is small, it runs in O(1) time; otherwise, an additional O(N) time for iterating every consumer.
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
will immediately remove the pending entry from the Pending Entries 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.
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
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.
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
> 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
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.
The summary provides 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 is detailed information for each message in the pending entries list. For each message four attributes are returned:
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).
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.
It is also possible to filter pending stream entries by their idle-time,
given in milliseconds (useful for
XCLAIMing entries that have not been
processed for some time):
> XPENDING mystream group55 IDLE 9000 - + 10 > XPENDING mystream group55 IDLE 9000 - + 10 consumer-123
The first case will return the first 10 (or less) PEL entries of the entire group
that are idle for over 9 seconds, whereas in the second case only those of
XPENDING command allows iterating over the pending entries just like
XREVRANGE allow for the stream's entries. You can do this by
prefixing the ID of the last-read pending entry with the
( character that
denotes an open (exclusive) range, and proving it to the subsequent call to the
Array reply, specifically:
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.
IDLEoption and exclusive range intervals.