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The protocol engines' pending iterator are responsible for maintaining fairness within the connection.  They do this by maintaining state between invocations.  For instance if a connection had sessions A, B, C, all with tasks to producer and on this output cycle, the network stopped accepting bytes after A's tasks, on the next output cycle. B would be considered first, even if A had subsequently had more work. This fairness patten is repeated through each layer of the protocol.

Queues

Queue model objects provide the messaging queues.   There are several specialisations of Queue

• StandardQueue which provides a FIFO behaviour
• PriorityQueue which provides queuing ordered by a message's priority
• LVQQueue which provides a last-value or conflation queue.

Internally queues are implemented as a linked list (QueueEntryList).  The linked list is implemented from first principals.  It is thread safe and lock-less (it uses CAS operations).  

Enqueueing

When a message is enqueued (using the AbstractQueue#enqueue() method).     This adds the message to the tail of the queue and notifies a interested subscriber (consumer) about the new message.  This is described by Consumer-Queue-Interactions

Subscriptions

Each subscription keeps a "pointer" (QueueContext#_lastSeenEntry) into the list denoting the point at which that particular subscription has reached. A subscription will only take a message if it is the next AVAILABLE (MessageInstance.State.AVAILABLE) entry.

The diagram below shows point to point queue with three subscribers attached.

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Messages

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Management

The Broker exposes two management layers:

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