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XRANGE

Introduction​

In Dragonfly, as well as in Redis and Valkey, the XRANGE command is used to return a range of elements in a stream. Streams are powerful data types for handling ordered logs and time-series data, making XRANGE especially useful for retrieving entries within specified time frames or other ranges.

Syntax​

XRANGE key start end [COUNT count]

Parameter Explanations​

  • key: The key of the stream from which entries are fetched.
  • start: The minimum ID of the range, inclusive.
  • end: The maximum ID of the range, inclusive.
  • COUNT count (optional): Limits the number of entries returned to 'count'.

Return Values​

The command returns a list of stream entries that correspond to the specified range. Each entry is represented by a two-element array with an ID and a subsequent map of field-value pairs.

Code Examples​

Basic Example​

Retrieve all entries within a specified range:

# Adding entries to the stream
dragonfly> XADD mystream * foo "1" bar "data1"
"1609459200000-0"
dragonfly> XADD mystream * foo "2" bar "data2"
"1609459200010-0"

# Retrieving entries from '1609459200000-0' to '1609459200010-0'
dragonfly> XRANGE mystream 1609459200000-0 1609459200010-0
1) 1) "1609459200000-0"
   2) 1) "foo"
      2) "1"
      3) "bar"
      4) "data1"
2) 1) "1609459200010-0"
   2) 1) "foo"
      2) "2"
      3) "bar"
      4) "data2"

Using COUNT Option​

Retrieve a limited number of entries starting from a specific ID:

# Adding another entry
dragonfly> XADD mystream * foo "3" bar "data3"
"1609459200020-0"

# Retrieve only one entry starting from '1609459200000-0'
dragonfly> XRANGE mystream 1609459200000-0 1609459200020-0 COUNT 1
1) 1) "1609459200000-0"
   2) 1) "foo"
      2) "1"
      3) "bar"
      4) "data1"

Best Practices​

  • When processing large streams, using the COUNT parameter can help limit the amount of data transferred and improve performance.
  • Use precise start and end IDs to avoid retrieving unnecessary entries and to maintain efficient operations.

Common Mistakes​

  • Providing start or end in incorrect formats can lead to errors; ensure the IDs follow the correct stream ID format (e.g., <timestamp>-<sequence>).
  • Not specifying an end range if the intent is to read bounded data. Omitting this parameter leads to fetching entries up to the maximum sequence for the start's timestamp.

FAQs​

What happens if the stream key does not exist?​

If the stream key does not exist, XRANGE returns an empty list.

How are stream IDs formatted?​

Stream IDs are formatted as <milliseconds-time>-<sequence-number>, where the sequence number starts at zero and increases with each entry at the same timestamp.