The major problem we had in MB 2.2.0 is that message copying from Global queues to Node queues and vice versa. This consumes lots of time since each message goes through several database readings and writings before delivering.
As a solution to this problem slot based message delivery system was introduced to WSO2 MB 3.0.0. Slot is basically a chunk of messages in a message store which can only be owned by one node at a time. A queue is mapped to a row in message store and queue can be divided into several slots. Below diagram shows how slots are divided for a queue name foo. All the messages for the queue foo is stored in one row in message store. 
Architecture
In new message delivery model slot manager acts as the coordinator of slot distribution among subscribers. Above use case diagram shows the services provided by the slot manager. At publisher’s side publisher returns its last message ID to slot manager after every 1000 messages (this value is configurable) or after a timeout. Slot manager keeps these IDs in a hazelcast distributed map and use it to generate slots when a subscriber asks.
At subscriber’s side, subscriber talks to slot manager at several times.
- Get a slot
When a subscriber arrives, subscriber asks for a slot from slot manager. If there are free slots which are not allocated to any node, the slot manager returns a slot to subscriber node and update the slot assignment map . Slot assignment map is kept to trace currently assigned non-empty slots.
- Delete slot
When all the messages read from slot are sent and all the acks are received, member node asks the slot manager to delete the slot. At slot manager’s side, when a slot delete call is triggered, slot manager removes the entry from slot assignment map.
- Re-assign slot when last subscriber leaves
This method is called when the last subscriber of a particular node leaves the cluster. When this method is triggered, slot manager reassign the non-empty slots which were belonged to callee node, to free slots pool.
Other than these functions, when a member node leaves the cluster, slot manager reassign the non-empty slots which were belonged to the leaving node, to free slots pool.
Message Publishing
When publishing messages, a row in message store is dedicated to each queue. Row key is equal to queue name as shown in the image. All the publishers for that queue will store messages in that row.
Message Delivering
Message delivering is done by a thread known as Slot Delivery Worker(SDW). There can be more than one Slot Delivery Workers and each slot delivery worker is assigned with set of queues. Each slot delivery worker ask for a slot from slot manager when a subscriber arrives. If the slot delivery worker gets a non-empty slot, it reads all the messages in the slot in one row and deliver it to any subscriber in round robin manner. If a message delivery is failed or rejected those messages will be buffered in a queue at Message Flusher which is responsible of delivering messages subscribers. These messages are buffered queue wise in Message Flusher. Slot Delivery Worker read the messages from the slot and pass it to the Message Flusher. Message Flusher passes these messages to its queue wise separated message buffers. Message slot state diagram is shown in the below diagram.
Message Flow state diagram
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