# Shared subscription

Shared subscription is a subscription method that achieves load balancing among multiple subscribers, EMQ X Cloud adopts a random balance strategy, select randomly among all subscribers.

# Shared subscription prefixes formats

EMQ X Cloud supports shared subscription prefixes in two formats: shared subscription with groups (prefixed with $share/<group-name>/) and shared subscription without group (prefixed with $queue/).

Examples of two shared subscription prefixes formats are as follows.

prefixes formatsExamplePrefixReal topic name
Shared subscription with groups$share/abc/t/1$share/abc/t/1
Shared subscription without group$queue/t/1$queue/t/1

# Shared subscription with groups

Shared subscriptions prefixed with $share/<group-name>/ are shared subscriptions with groups. group-name can be any string. Subscribers who belong to the same group will receive messages with load balancing, but EMQ X Cloud will broadcast messages to different groups.

# Example

  • subscribers s1, s2, and s3 belong to group g1, subscribe topic $share/g1/test
  • subscribers s4 and s5 belong to group g2, subscribe topic $share/g2/test
  • subscriber s6 subscribe topic test

Then when publisher publishes a message with topic test to the EMQ X Cloud (tip: this topic does not need to be prefixed).

  • Only one of s1, s2, s3 will receive msg
  • Only one of s4, s5 will receive msg
  • s6 will receive msg

# Shared subscription without group

Shared subscriptions prefixed with $queue/ are shared subscriptions without groups. It is a special case of $share subscription, which is quite similar to all subscribers in a subscription group.

# Example

  • subscribers s1, s2, and s3 subscribe topic $queue/test
  • subscriber s4 subscribe topic test

Then when publisher publishes a message with topic test to the EMQ X Cloud (tip: this topic does not need to be prefixed).

  • Only one of s1, s2, s3 will receive msg
  • s4 will receive msg

# Test shared subscription using MQTT X

Simulate client subscriptions using MQTT X.

  • s1, s2 subscribe topic $share/g1/test
  • s3 subscribe topic test

shared_subscription_1

Create client P1 using MQTT X to send 3 messages to the topic test

shared_subscription_2

s1 receive msg1, msg2, s2 receive msg3 and s3 receive all messages

shared_subscription_3