Clusters.most times message queues are the primary communication mechanisms. Therefore, when a message queue fails applications can not communicate. To increase the reliability of such a system software can resort to the use of message queues clusters, which means it will be available more message queues are several cars. The way it works is dependent on a cluster implementation, but in principle all implementations are similar. Such a cluster consists of several servers tails. Each server contains the same set of queues and distribution of these tails between servers is queue simulation software transparent to the client application. Customers behave as if they have just one server. When a client sends a message to one of the queues is selected, the message is stored in the queue. Subscriber-ii listens messages that are sent to a topic which interests them. As with message queues and publish-subscribe model implies a server whose role is to distribute incoming messages to a specific topic or to all clients registered for that topic fig.. From the point of view of decoupling, publish-subscribe model has two important properties. The sender queue management software and recipient are, none of them having no other information. The second property is the fact that each topic can be more applications. So the reception, a queue is selected and removed from her message. Targeting specific application to a queueis the responsibility of the cluster queue. This targeting can be static when the customer makes the first query or dynamically for each query this mode is not usable for an application that must receive messages in the order they were sent. Using clusters has two advantages: if a server fails then the other server lines will be available, and the second advantage is that it can make a load balancing servers queue, leading to improved system performance software..message queues bidirectional although based technologies message queues are decoupled and asynchronous by definition can also be used for developing software system that requires synchronous communication and customer queue management system a higher degree of coupling. In such situations' message queues are used to send messages to a recipient. The message contains information identifying the tail to be written reply to the message. Receiving the message sent message sender waiting queue specified response. Some reasons for resorting to the use of message queues for synchronous communication are listed below:- message queues based technologies can be used to link existing applications with a low cost and minimal risk; adapters performing interfacing between different technologies based on message queues are available or can be made legacy applications with little effort; applications do not have to be rewritten in order to be integrated in a large scale system;- message queue-based technologies are available on most platforms; this facilitates integration with legacy applications or those of partner companies.the pub- subscribe publish-subscribe technologies based middle ware message queues are very effective in terms of developing loosely coupled software systems, but obviously there are limits.
The main limitation is that middle ware technologies are technologies that allow only one-to-one communication. Thus, a single sender send a single message to a single queue and only one recipient extracts the message from the queue. To eliminate this limitation has been proposed another model based on messages namely public-subscribe model. Fig.. The publish-subscribe. Register/ subscribe publisher subject subscriber post/ postthe public communication-subscribe expand communication model based on queues to enable the implementation of all types of communication:to many, many to many and many to one. A publisher see more sends one copy of a message to a topic. The subject is a logical name for public-subscribe model, being the equivalent of tail if communication model based on message queues.