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XREVRANGE

Introduction​

In Dragonfly, as well as in Redis and Valkey, the XREVRANGE command is used to return a range of elements from a stream in reverse order. The directionality of this command makes it useful for scenarios where you need to process or display recent data entries before older ones, such as in messaging apps, event logging, or any application requiring reverse chronological order retrieval.

Syntax​

XREVRANGE key end start [COUNT count]

Parameter Explanations​

  • key: The key of the stream from which elements are retrieved.
  • end: The maximum ID of the stream entry to include in the output.
  • start: The minimum ID of the stream entry to include in the output.
  • COUNT count (optional): Limits the number of entries returned, fetching the first count elements in reverse order.

Return Values​

The command returns an array of entries, where each entry is itself an array consisting of an ID and a field-value pair list. The entries are ordered from the highest ID to the lowest ID, within the specified range.

Code Examples​

Basic Example​

Retrieve entries from a stream within a specified ID range in reverse order:

# Adding entries to a stream.
dragonfly$> XADD mystream * field1 value1
"1617825600000-0"
dragonfly$> XADD mystream * field1 value2
"1617826800000-0"
dragonfly$> XADD mystream * field1 value3
"1617828000000-0"

# Retrieve entries in reverse order.
dragonfly$> XREVRANGE mystream 1617828000000-0 1617825600000-0
1) 1) "1617828000000-0"
   2) 1) "field1"
      2) "value3"
2) 1) "1617826800000-0"
   2) 1) "field1"
      2) "value2"
3) 1) "1617825600000-0"
   2) 1) "field1"
      2) "value1"

Using COUNT Option​

Retrieve a limited number of entries in reverse order using the COUNT option:

# Adding more entries to the stream.
dragonfly$> XADD mystream * field1 value4
"1617829200000-0"

# Retrieve only the two most recent entries.
dragonfly$> XREVRANGE mystream + - COUNT 2
1) 1) "1617829200000-0"
   2) 1) "field1"
      2) "value4"
2) 1) "1617828000000-0"
   2) 1) "field1"
      2) "value3"

Applications in Event Logging​

Consider a log stream where you want to fetch the most recent events up to a specific moment:

# Adding log entries.
dragonfly$> XADD logs * event login
"1617830000000-0"
dragonfly$> XADD logs * event logout
"1617831000000-0"

# Retrieve events in reverse order up to a specific ID.
dragonfly$> XREVRANGE logs 1617831000000-0 1617830000000-0
1) 1) "1617831000000-0"
   2) 1) "event"
      2) "logout"
2) 1) "1617830000000-0"
   2) 1) "event"
      2) "login"

Best Practices​

  • Utilize COUNT to limit the number of entries returned, optimizing performance and reducing memory usage when processing large streams.
  • Ensure that your specified end and start IDs correctly encapsulate the desired time or event window to avoid unexpected results.

Common Mistakes​

  • Misinterpreting the order of start and end: since XREVRANGE operates in reverse, end should be greater than start.
  • Overlooking the fact that stream IDs are ordered lexicographically, which can affect how ranges are defined and queried.

FAQs​

What happens if the stream key does not exist?​

If the stream key does not exist, XREVRANGE returns an empty array.

Can negative indexes be used for end and start parameters?​

Stream IDs are lexicographic and do not support negative indexes. Instead, ranges define specific or approximate position and order within the stream based on IDs.