PUBSUB NUMSUB

Introduction and Use Case(s)

The PUBSUB NUMSUB command in Redis is used to check the number of subscribers for a given set of channels. This can be particularly useful for monitoring purposes, allowing you to gauge the popularity or activity level of various channels within your application.

Syntax

  1. PUBSUB NUMSUB [channel-1] [channel-2] ... [channel-N]

Parameter Explanations

  • channel-1, channel-2, …, channel-N: One or more channel names for which you want to retrieve the number of subscribers. These are optional, but at least one should be provided to get meaningful results.

Return Values

The command returns an array. Each pair in the array consists of:

  1. The channel name.
  2. The number of subscribers currently subscribed to that channel.

Example Output

  1. 1) "channel-1"
  2. 2) (integer) 10
  3. 3) "channel-2"
  4. 4) (integer) 20

Code Examples

Using CLI

  1. dragonfly> SUBSCRIBE mychannel
  2. Reading messages... (press Ctrl-C to quit)
  3. dragonfly> PUBLISH mychannel "hello"
  4. (integer) 1
  6. In another terminal:
  8. dragonfly> PUBSUB NUMSUB mychannel otherchannel
  9. 1) "mychannel"
  10. 2) (integer) 1
  11. 3) "otherchannel"
  12. 4) (integer) 0

Best Practices

  • Regularly monitor the subscriber count for critical channels to ensure your messaging infrastructure is functioning correctly.
  • Use this command in combination with other PUBSUB commands like PUBSUB CHANNELS and PUBSUB NUMPAT to get a comprehensive view of your Pub/Sub system’s status.

Common Mistakes

  • Not specifying any channels when using the command will lead to no meaningful information being returned. Always specify at least one channel to get the number of subscribers.
  • Misunderstanding the output structure: Remember that the result pairs each channel name with its subscriber count.

FAQs

What happens if I use PUBSUB NUMSUB without any channels?

If you don’t provide any channels, the command will not return any useful information about subscribers.

Does PUBSUB NUMSUB include pattern subscriptions?

No, PUBSUB NUMSUB only includes direct channel subscriptions. For pattern subscription counts, use PUBSUB NUMPAT.