# Shared subscription
Shared subscription is a subscription method that achieves load balancing among multiple subscribers, EMQX Cloud adopts a random balance strategy, select randomly among all subscribers.
# Shared subscription prefixes formats
EMQX 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 formats | Example | Prefix | Real 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 EMQX 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 EMQX 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 EMQX 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 MQTTX
Simulate client subscriptions using MQTTX.
- s1, s2 subscribe topic
$share/g1/test - s3 subscribe topic
test
Create client P1 using MQTTX to send 3 messages to the topic test
s1 receive msg1, msg2, s2 receive msg3 and s3 receive all messages
