Anapplication server is aserver that hosts applications^[1] orsoftware that delivers abusiness application through acommunication protocol.^[2] For a typicalweb application, the application server sits behind theweb servers.

An applicationserver framework is a service layer model. It includessoftware components available to asoftware developer through anapplication programming interface. An application server may have features such asclustering,fail-over, andload-balancing. The goal is for developers to focus on thebusiness logic.^[3]

Jakarta EE (formerly Java EE or J2EE) defines the core set of API and features ofJava application servers.

The Jakarta EE infrastructure is partitioned into logical containers.

• EJB container:Enterprise Beans are used to manage transactions. According to theJava BluePrints, thebusiness logic of an application resides inEnterprise Beans—a modular server component providing many features, including declarative transaction management, and improving applicationscalability.
• Web container: the web modules includeJakarta Servlets andJakarta Server Pages (JSP).
• JCA container (Jakarta Connectors)
• JMS provider (Jakarta Messaging)

Microsoft's .NET positions their middle-tier applications and services infrastructure in theWindows Server operating system and the.NET Framework technologies in the role of an application server.^[4] The Windows Application Server role includesInternet Information Services (IIS) to provide web server support, the .NET Framework to provide application support,ASP.NET to provideserver side scripting, COM+ for application component communication, Message Queuing for multithreaded processing, and theWindows Communication Foundation (WCF) for application communication.^[5]

PHP application servers run and managePHP applications.

• Zend Server, built byZend, provides application server functionality for the PHP-based applications.
• RoadRunner, built by Spiral Scout is a high-performance PHP application server, load-balancer, and process manager written inGo.

• Mono (a cross platform open-source implementation of .NET supporting nearly all its features, with the exception of Windows OS-specific features), sponsored byMicrosoft and released under theMIT License

Mobile application servers provide data delivery to mobile devices.

Core capabilities of mobile application services include

• Data routing– data is packaged in smaller (REST) objects with some business logic to minimize demands on bandwidth and battery
• Orchestration– transactions and data integration across multiple sources
• Authentication service– secure connectivity to back-end systems is managed by the mobile middleware
• Off-line support– allows users to access and use data even though the device is not connected
• Security– data encryption, device control, SSL, call logging

Although most standards-basedinfrastructure (includingSOAs) are designed to connect to any independent of any vendor, product or technology, most enterprises have trouble connecting back-end systems to mobile applications, because mobile devices add the following technological challenges:^[6]

• Limited resources – mobile devices have limited power and bandwidth
• Intermittent connectivity – cellular service and wifi coverage is often not continuous
• Difficult to secure^[7] – mobility andBYOD practices make it hard to secure mobile devices

An application server can be deployed:

• On premises
• Cloud
• Private cloud
• Platform as a service (PaaS)

• Application service provider
• List of application servers

{Table Web Interfaces}