# Shared Subscription

Shared subscription is a subscription method that achieves load balancing among multiple subscribers.

                                                   [subscriber1] got msg1
             msg1, msg2, msg3                    /
[publisher]  ---------------->  "$share/g/topic"  -- [subscriber2] got msg2
                                                 \
                                                   [subscriber3] got msg3

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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, and subscribers s4 and s5 are members of group g2, and all subscribers subscribe to topic t1. When EMQX Broker 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

                                       [s1]
           msg1                      /
[emqx]  ------>  "$share/g1/topic"    - [s2] got msg1
         |                           \
         |                             [s3]
         | msg1
          ---->  "$share/g2/topic"   --  [s4]
                                     \
                                      [s5] got msg1

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# 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.
Configure broker.shared_subscription_group.$group_name.strategy for per-group strategy

# 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

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Load Balance	Description
random	Random selection among all subscribers
round_robin	In order of subscription
sticky	Always send to the last selected subscriber
hash	Hash by publisher ClientID

# 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 reconnect 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 above reasons, persisted sessions are usually not common for shared subscribers, but there is nothing stopping you from doing 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.