Middleware is a type of software that connects two otherwise separate applications or separate systems. Middleware provides a way for the to be connected systems to exchange information even though they might have very different interfaces. Messaging is a critical way for middleware to be implemented. Middleware helps applications work across multiple platforms or OS environments.

Middleware is used in situations such as connecting company LAN with its legacy systems, exchanging messages between two mail systems, allowing web client to exchange information with database servers, etc. Common middleware categories include:

• ESBs (Enterprise Service Bus): ESB is an open standards-based distributed synchronous or asynchronous messaging middleware that provides secure interoperability between enterprise applications via XML, Web services interfaces and standardized rules-based routing of documents.
• TP (Transaction processing) monitors: provides monitoring and control for processes occurring between objects to ensure that operations complete successfully.
• DCE (Distributed Computing Environment): a suite of technology services for creating distributed applications that run on different platforms.
• RPC (Remote Procedure Call): standards for clients making requests to a server to run certain procedures.
• Object Request Brokers (ORBs): provides an interface that objects can use to communicate with other objects in distributed network environments.
• Database access middleware: lets clients access database from various operating systems or applications. SQL is one of such middleware.
• Message Passing: Email system is an example of such type of middleware
• XML-based middleware: XML allows developer to create documents for exchanging structured information across the Internet.

Related Terms: XML, Object Request Broker, RPC, Distributed Computing Environment, nterprise Service Bus, Transaction processing