Middleware in distributed applications
The term is most commonly used for software that enables communication and management of data in distributed applications. An IETF workshop in 2000 defined middleware as "those services found above the transport (i.e., over TCP/IP) layer set of services but below the application environment (i.e., below application-level APIs). In this more specific sense middleware can be described as the dash (“-”) in client-server, or the -to- in peer-to-peer. Middleware includes web servers, application servers, content management systems, and similar tools that support application development and delivery.
ObjectWeb defines middleware as: "The software layer that lies between the operating system and applications on each side of a distributed computing system in a network." Services that can be regarded as middleware include enterprise application integration, data integration, message oriented middleware (MOM), object request brokers (ORBs), and the enterprise service bus (ESB).
Database access services are often characterised as middleware. Some of them are language specific implementations and support heterogeneous features and other related communication features. Examples of database-oriented middleware include ODBC, JDBC and transaction processing monitors.
Distributed computing system middleware can loosely be divided into two categories – those that provide human-time services (such as web request servicing) and those that perform in machine-time. This latter middleware is somewhat standardized through the Service Availability Forum and is commonly used in complex, embedded systems within telecom, defense and aerospace industries.
Other examples of middleware
The term middleware is used in other contexts as well. Middleware is sometimes used in a similar sense to a software driver, an abstraction layer that hides detail about hardware devices or other software from an application.
- The mer software distribution is a middleware: it lacks the Linux kernel and it also lacks a UI. Mer is targeted at hardware vendors' mobile-oriented operating systems.
- The Android operating system uses the Linux kernel at its core, and also provides an application framework that developers incorporate into their applications. In addition, Android provides a middleware layer including libraries that provide services such as data storage, screen display, multimedia, and web browsing. Because the middleware libraries are compiled to machine language, services execute quickly. Middleware libraries also implement device-specific functions, so applications and the application framework need not concern themselves with variations between various Android devices. Android's middleware layer also contains the Dalvik virtual machine and its core Java application libraries.
- Game engine software such as Gamebryo and Renderware are sometimes described as middleware, because they provide many services to simplify game development.
- In simulation technology, middleware is generally used in the context of the high level architecture (HLA) that applies to many distributed simulations. It is a layer of software that lies between the application code and the run-time infrastructure. Middleware generally consists of a library of functions, and enables a number of applications—simulations or federates in HLA terminology—to page these functions from the common library rather than re-create them for each application.
- Wireless networking developers can use middleware to meet the challenges associated with a wireless sensor network (WSN). Implementing a middleware application allows WSN developers to integrate operating systems and hardware with the wide variety of various applications that are currently available.
- The QNX operating system offers middleware for providing multimedia services for use in automobiles, aircraft and other environments.
- Multimedia Home Platform (DVB-MHP) is an open middleware system standard designed by the Digital Video Broadcasting (DVB) project for interactive digital television. The MHP enables the reception and execution of interactive, Java-based applications on a television set.
- Universal Home API, or UHAPI, is an application programming interface (API) for consumer electronics appliances, created by the UHAPI Forum. The objective of UHAPI is to enable standard middleware to run on audio/video streaming platforms via a hardware-independent industry standard API.
- The Miles Sound System provided a middleware software driver allowing developers to build software that worked with a range of different sound cards, without concerning themselves with the details of each card.
- Radio-frequency identification software toolkits provide middleware to filter noisy and redundant raw data.
- ILAND is a service-based middleware dedicated to real-time applications. It offers deterministic reconfiguration support in bounded time.
- Television, media and set-top-box software often refers to middleware. For instance, OpenTV 5 middleware (sometimes called Connectware) is a Linux and HTML5 distribution for set-top-box including a default TV navigation guide and provides open APIs to tailored TV user experience that can leverage the home cloud, broadcast video and web services for Pay-TV operators. Other examples of Television middleware also known as OTT middleware or IPTV middleware are Mware Solutions.
The distinction between operating system and middleware functionality is, to some extent, arbitrary. While core kernel functionality can only be provided by the operating system itself, some functionality previously provided by separately sold middleware is now integrated in operating systems. A typical example is the TCP/IP stack for telecommunications, nowadays included in virtually every operating system. At the other end of the scale, the boundary between middleware and application has also moved. Digital TV middleware for example usually provides enough functionality so that no "application" is needed for end-users to be able to use the TV services.
Middleware gained popularity in the 1980s as a solution to the problem of how to link newer applications to older legacy systems, although the term had been in use since 1968. It also facilitated distributed processing, the connection of multiple applications to create a larger application, usually over a network.
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