With the surging popularity of web-scale applications, there can be a large number of requests from different parts of the world for those applications. The user and data loads are generally unpredictable. The task/operation complexity is also unpredictable. Because of heavy loads, cloud applications find it difficult to process every request and deliver the reply within the stipulated timeline. One option is to add new server instances. There are some practical difficulties in clustered and load-balanced environments too. However, these consumers must be coordinated to ensure that each message is only delivered to a single consumer. The workload also needs to be load balanced across consumers to prevent an instance from becoming a bottleneck.

An overwhelming solution approach here is to use a messaging system (message queue or broker) in between any requesting applications/users and the processing applications. A message-oriented middleware (MOM) is a way forward for meeting a large number of concurrent consumers. This middleware approach supports asynchronous communication and processing, thereby the massive number of requests can be answered quickly.

A message queue/broker/bus is used to establish the communication channel between the application and the instances of the consumer service. The application posts requests in the form of messages to the queue and the consumer service instances receive messages from the queue and process them. This approach enables the same pool of consumer service instances to handle messages from any instance of the application. The following figure illustrates this architecture:

This pattern enables multiple concurrent consumers to process messages received on the same messaging channel. This pattern enables a system to process multiple messages concurrently to optimize throughput, to improve scalability and availability, and to balance the workload.