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.
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.
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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