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XREAD

Introduction​

In Dragonfly, as well as in Redis and Valkey, the XREAD command is used to read data from one or more streams. It is a blocking command by nature and is commonly utilized for implementing message queues, real-time data processing, and event sourcing patterns.

Syntax​

XREAD [COUNT count] [BLOCK milliseconds] STREAMS key [key ...] ID [ID ...]

Parameter Explanations​

  • COUNT count: The maximum number of entries to return per stream. Optional, defaults to return all available entries.
  • BLOCK milliseconds: The maximum number of milliseconds the command will block if no messages are available. Optional, defaults to non-blocking behavior.
  • STREAMS key [key ...]: One or more stream keys to read from.
  • ID [ID ...]: One or more specific entry IDs to start reading from, or use $ to start from the latest message.

Return Values​

The command returns a list of streams and the corresponding entries that were read from them, or an empty list if the BLOCK option is used and no messages are available within the specified time.

Code Examples​

Basic Example​

Read from a single stream starting from the first message:

dragonfly> XADD mystream * name Alice age 30
"1609945144079-0"
dragonfly> XADD mystream * name Bob age 25
"1609945144079-1"

dragonfly> XREAD STREAMS mystream 0
1) 1) "mystream"
   2) 1) 1) "1609945144079-0"
         2) 1) "name"
            2) "Alice"
            3) "age"
            4) "30"
      2) 1) "1609945144079-1"
         2) 1) "name"
            2) "Bob"
            3) "age"
            4) "25"

Using XREAD with COUNT​

Read a limited number of entries from a stream:

dragonfly> XREAD COUNT 1 STREAMS mystream 0
1) 1) "mystream"
   2) 1) 1) "1609945144079-0"
         2) 1) "name"
            2) "Alice"
            3) "age"
            4) "30"

Using XREAD with Block Option​

Block the command until new data arrives in the stream:

# Start a new terminal to add data after blocking
dragonfly> XREAD BLOCK 2000 STREAMS mystream $
(null)

# In another terminal, add a new entry
dragonfly> XADD mystream * name Charlie age 40
"1609945245092-0"

# Return to previous terminal and observe `XREAD` output
1) 1) "mystream"
   2) 1) 1) "1609945245092-0"
         2) 1) "name"
            2) "Charlie"
            3) "age"
            4) "40"

Best Practices​

  • Consider using the BLOCK option to efficiently poll streams for new entries, reducing the need for constant querying.
  • Use COUNT to prevent overwhelming your application with massive amounts of data if not necessary.

Common Mistakes​

  • Overlooking the starting ID for the stream, which may result in missing entries if set incorrectly.
  • Using blocking operations without a reasonable timeout can lead to application hangs if streams are quiet.

FAQs​

What happens if a stream doesn't exist?​

If a specified stream key does not exist, XREAD will not return entries from it and will move on to other specified streams.

Can XREAD be used for multiple streams?​

Yes, you can specify multiple streams to read simultaneously. The command will return data from them in the specified order.