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.
Currently three message distribution policies for shared subscriptions are supported:
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.
shared_subscription_policy = prefer_local
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. Notice that the shared subscription policy is applied before considering online or offline status of clients.
Subscriptions Note: When subscribing to a shared topic, make sure to escape the
So, for dash or bash shells
mosquitto_sub -h mqtt.example.io -p 1883 -q 2 -t \$share/group/topicnamemosquitto_sub -h mqtt.example.io -p 1883 -q 2 -t \$share/group/topicname/#
Publishing Note: When publishing to a shared topic, do not include the prefix
$share/group/ as part of the publish topic name
mosquitto_pub -h mqtt.example.io -p 1883 -t topicname -m "This is a test message"mosquitto_pub -h mqtt.example.io -p 1883 -t topicname/group1 -m "This is a test message"