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Working with 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/topicand 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
$sharepart of the topic as an environment variable.
Currently four message distribution policies for shared subscriptions are supported:
prefer_online_before_local. Under the
randompolicy messages will be published to a random member of the shared subscription, if any exist. Under the
prefer_localpolicy 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. The
prefer_online_before_localpolicy works similar to
prefer_local, but will look for an online subscriber on a non-local node, if there are only offline subscribers on the local one. Under the
local_onlypolicy 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.
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. This also means that there is no shared offline message queue for all clients, but each client has its own offline message queue. MQTT v5 shared subscriptions thus have a different behaviour than e.g. Kafka where consumers read from a single shared message queue.
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/topicname
mosquitto_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
mosquito_pub -h mqtt.example.io -p 1883 -t topicname -m "This is a test message"
mosquito_pub -h mqtt.example.io -p 1883 -t topicname/group1 -m "This is a test message"