Shared subscriptions

A shared subscription is a mechanism for distributing messages to a set of subscribers to shared subscription topic, such that each message is received by only one subscriber. This contrasts with normal subscriptions where each subscriber will receive a copy of the published message.
A shared subscription is on the form $share/sharename/topic and subscribers to this topic will receive messages published to the topic topic. The messages will be distributed according to the defined distribution policy.
The MQTT spec only defines shared subscriptions for protocol version 5. VerneMQ supports shared subscription for v5 (as per the specification) and for v3.1.1 (backported feature).
When subscribing to a shared subscription using command line tools remember to quote the topic as some command line shells, like bash, will otherwise expand the $share part of the topic as an environment variable.
Configuration
Currently three message distribution policies for shared subscriptions are supported: prefer_local, random and local_only. Under the random policy messages will be published to a random member of the shared subscription, if any exist. Under the prefer_local policy messages will be delivered to a random node-local member of the shared subscription, if none exist, the message will be delivered to a random member of the shared subscription on a remote cluster node. Under the local_only policy message will be delivered to a random node-local member of the shared subscription.
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shared_subscription_policy = prefer_local
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When a messages is being delivered to subscribers of a shared subscription, the message will be delivered to an online subscriber if possible, otherwise the message will be delivered to an offline subscriber.
Random Message Routing for Shared Subscribers
Local Only Message Routing for Shared Subscribers
Prefer_local Message Routing for Shared Subscribers
Note that Shared Subscriptions still fully operate under the MQTT specification (be it MQTT 5.0 or backported to older protocol versions). Be aware of this, especially regarding QoS and clean_session configurations.
Examples
Subscriptions Note: When subscribing to a shared topic, make sure to escape the $
So, for dash or bash shells
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mosquitto_sub -h mqtt.example.io -p 1883 -q 2 -t \$share/group/topicname
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mosquitto_sub -h mqtt.example.io -p 1883 -q 2 -t \$share/group/topicname/#
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Publishing Note: When publishing to a shared topic, do not include the prefix $share/group/ as part of the publish topic name
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mosquito_pub -h mqtt.example.io -p 1883 -t topicname -m "This is a test message"
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mosquito_pub -h mqtt.example.io -p 1883 -t topicname/group1 -m "This is a test message"
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