Middleware and its importance

Middleware is a software layer placed between the operating system (and network) of each node and the distributed applications. It provides a uniform programming model and common services such as communication, naming, security, transactions and replication, so that a heterogeneous collection of computers appears as one system. Examples: CORBA, Java RMI, DCOM, Sun RPC, message queues.

 +-------------+  +-------------+  +-------------+
 | Application |  | Application |  | Application |
 +-------------+  +-------------+  +-------------+
 +-------------------------------------------------+
 |            MIDDLEWARE (common API)              |
 | RPC/RMI | naming | security | transactions      |
 +-------------------------------------------------+
 +--------+     +--------+     +--------+
 |Windows |     | Linux  |     |  Unix  |   local OS
 +--------+     +--------+     +--------+
 ======== Network (TCP/IP) ===================

Importance of middleware

1. Hides heterogeneity: differences in hardware, OS, network and programming language are masked; for example CORBA IDL allows a C++ client to call a Java server.
2. Provides transparency: location, access, migration, replication and failure transparency, so programmers write code as for a single computer.
3. Simplifies programming: ready-made communication (RPC, RMI, messaging) means programmers do not write socket code.
4. Common services: naming, directory, security, persistence, transactions and event services are reused by all applications.
5. Interoperability and openness: standard interfaces and protocols (IIOP, SOAP) let systems from different vendors work together.
6. Portability and scalability: applications move to other platforms unchanged; load balancing and replication support growth.
7. Fault tolerance support: failure detection, retry and replication hidden from the application.