Shared Subscription

Shared Subscription

Shared subscription is a subscription mode to implement load balancing among multiple subscribers. EMQX has supported shared subscriptions since MQTT 3.1.1, and now it has become part of the MQTT 5.0 protocol.

Use cases:

In cluster mode: If the node where the subscriber is located malfunctions, the publisher’s messages will be lost (QoS 0 messages) or accumulated in the node (QoS 1 and QoS 2 messages). Though we can solve this problem by adding subscription nodes, this may affect the performance with lots of repeated messages and increase the complexity of the business.
When the publisher’s production capacity is strong, the subscriber’s consumption capacity may not be able to keep up in time. In this case, we have to rely on the load-balancing capability of the subscriber, which again increases the development cost.

How It Works

We can add a $share prefix to the original topic to enable shared subscriptions for a group of subscribers.

In the above diagram, three subscribers subscribe to the same topic $share/g/topic using a shared subscription method, where topic is the real topic name they subscribed to, and $share/g/ is a shared subscription prefix.

Example	Prefix	Topic Name
$share/abc/t/1	$share/abc/	t/1

Shared Subscriptions in Group

Shared subscriptions prefixed with $share/<group-name> are shared subscriptions with groups. The group name can be any string. Subscribers who belong to the same group will receive messages with load balancing, but EMQX will broadcast messages to different groups at the same time.

For example, if subscribers s1, s2, and s3 are members of group g1, subscribers s4 and s5 are members of group g2, and all subscribers subscribe to topic t1. When EMQX publishes a message msg1 to topic t1:

EMQX will send msg1 to both groups g1 and g2
Only one of s1, s2, s3 will receive msg1
Only one of s4 and s5 will receive msg1

Load Balancing Policy and Dispatch ACK Configuration

In MQTT specification, balancing strategy is not covered. EMQX supports a few different balancing strategies with the help from configuration.

Balancing strategies can be specified globally or per-group.

Global strategy can be set in broker.shared_subscription_strategy configuration.

# etc/emqx.conf
# Load Balance
broker.shared_subscription_strategy = random
# When the client is offline, or the message level is QoS1 or QoS2, and the device does not reply to the ACK for any reason, the message will send to the other devices in the group.
broker.shared_dispatch_ack_enabled = false

Load Balance	Description
`random`	Random selection among all subscribers
`round_robin`	Select the subscribers in a round-robin manner
`round_robin_per_group`	Select the subscribers in round-robin fashion within each shared subscriber group
`local`	Select a random local subscriber, if cannot be found, then select a random subscriber within the cluster
`sticky`	Always send to the last selected subscriber until the subscriber disconnects
`hash_clientid`	Select the subscribers by hashing the `clientIds`
`hash_topic`	Select the subscribers by hashing the source topic

Discussion on Message Loss

EMQX sends messages to subscribers’ sessions.

When session is persisted (clean_session=false) the subscriber can recover the data stream right after reconnecting without losing messages.

This is a bit contradicting with the ‘load balancing’ idea, since often when shared subscription is in use, if a subscriber is offline, the other subscribers in the group are expected to take over the data stream. Otherwise if the subscriber is offline for long enough the session message buffer will eventually overflow and result in message loss.

Due to the above reasons, persisted sessions are usually not common for shared subscribers, but you can still use it.

The configuration broker.shared_dispatch_ack_enabled is introduced to improve load sharing in case of persisted sessions. When set to true, EMQX will try to dispatch messages to other members in the group if one is offline.