Queues

A queue implements a one-way communication, where the sender places a message on the queue and a receiver will collect the message asynchronously. Features such as dead letter queues, paired namespaces, active/passive replication, and auto-forwarding to a chain queue that's part of the same name provide the rich feature set for message flowing between an application and providing a highly available solution.

A queue consists of three key elements:

• Sender: Sends the message to the receiver through a durable entity.
• Durable entity: Stores the received durable message and offers persistence. The messages are stored until they are collected by the receiver.
• Receiver: The final recipient of the message.

The key advantages of a queue are as follows:

• Queues operate on the principle of first in, first out (FIFO): For example, consider a simple queue where, at one end, you put messages, and on the other end you will receive them in the same respective order. For example, service bus queue implements the FIFO pattern.
• Point-to-point: The fundamental concept of Queues is, they are point-to-point messaging; even though there may be multiple senders of messages, there is only one receiver of the messages. 
• Asynchronous communication: This implies that endpoint addresses are connected directly. A static structure may exist where senders and receivers communicate through named channels. Asynchronous communication helps with building decoupled architecture and allows higher resilience to add and process messages when either the publisher or consumer of messages has downtime.
• Security: Due to the mutual knowledge of senders and receivers from the security point of view, senders know where the data will land, and it's easier to enforce security policies.

The following figure illustrates the preceding concept: