Middleware is software that enables one or more kinds of communication or connectivity between applications or application components in a distributed network. By making it easier to connect applications that weren't designed to connect with one another, and providing functionality to connect them in intelligent ways, middleware streamlines application development and speeds time to market.

There are many types of middleware. Some, such as message brokers or transaction processing monitors, focus on one type of communication. Others, such as web application servers or mobile device middleware, provide the full range of communication and connectivity capabilities needed to build a particular type of application. Still others, such as a cloud-based integration platform as a service (iPaaS) offering or an enterprise service bus (EBS), function as a centralized integration hub for connecting all the components in an enterprise. (There's even middleware that lets developers build their own customized middleware.)

Middleware got its name because the first middleware typically acted as a mediator between an application front-end, or client, and a back-end resource - e.g., a database, mainframe application or specialized hardware device - from which the client might request data. 

But today's middleware operates well beyond this scope. Portal middleware, for example, encompasses the application front-end as well as tools for back-end connectivity; database middleware typically includes its own data store. And as you'll read below, an emerging class of middleware leverages container technology to help developers connect to resources distributed across multiple clouds.

At the most basic level, middleware enables developers to build applications without having to create a custom integration every time they need to connect to application components (services or microservices), data sources, computing resources or devices.

It does this by providing services that enable different applications and services to communicate using common messaging frameworks such as JSON (JavaScript object notation), REST (representational state transfer), XML (extensible markup language), SOAP (simple object access protocol), or web services. Typically, middleware also provides services that enable components written in multiple languages - such as Java, C++, PHP, and Python - to talk with each other.

In addition to providing this work-saving interoperability, middleware also includes services that help developers

• Configure and control connections and integrations. Based on information in a client or front-end application request, middleware can customize the response from the back-end application or service. In a retailer's ecommerce application, middleware application logic can sort product search results from a back-end inventory database by nearest store location, based on the IP address or location information in the HTTP request header.
  
  
• Secure connections and data transfer.  Middleware typically establishes a secure connection from the front-end application to back-end data sources using Transport Layer Security (TSL) or another network security protocol. And it can provide authentication capabilities, challenging front-end application requests for credentials (username and password) or digital certificates. 
  
  
• Manage traffic dynamically across distributed systems. When application traffic spikes, enterprise middleware can scale to distribute client requests across multiple servers, on premises or in the cloud. And concurrent processing capabilities can prevent problems when multiple clients try to access the same back-end data source simultaneously.

There are many different types of middleware. Some focus on specific types of connectivity, others on specific applications, application components and devices; some combine middleware capabilities for a specific development task. Some of the best-known and most commonly-used types of middleware software include:

Message-oriented middleware (MOM) enables application components using different messaging protocols to communicate to exchange messages. In addition to translating - or transforming - messages between applications, MOM manages routing of the messages so they always get to the proper components in the in the proper order. Examples of MOM include message queues and message brokers.

Remote procedure call (RPC) middleware enables one application to trigger a procedure in another application - running on the same computer or on a different computer or network - as if both were part of the same application on the same computer.

Data or database middleware simplifies access to, and interaction with, back-end databases. Typically database middleware is some form of SQL database server.

API (application programming interface) middleware provides tools developers can use to create, expose and manage APIs for their applications - so that other developers can connect to them. Some API middleware includes tools for monetizing APIs - enabling other organizations to use them, at cost. Examples of API middleware include API management platforms, API gateways and API developer portals.

Object request broker (ORB) middleware acts as broker between a request from one application object or component, and the fulfillment of that request by another object or component on the distributed network. ORBs operate with the Common Object Request Broker Architecture (CORBA), which enables one software component to make a request of another without knowing where other is hosted, or what its UI looks like - the "brokering" handles this information during the exchange.

Transactional middleware provides services to support the execution of data transactions across a distributed network. The best-known transactional middleware are transaction processing monitors (TPMs), which ensure that transactions proceed from one step to the next - executing the data exchange, adding/changing/deleting data where needed, etc. - through to completion.

Asynchronous data streaming middleware replicates a data stream in an intermediate store, enabling data sharing between multiple applications. Apache Kafka is one of the best-known examples of middleware for real-time data streaming.

Device middleware provides a focused set of integration and connectivity capabilities for developing apps for a specific mobile OS.

Portal middleware provides tools and resources for integrating content and capabilities from various related applications 'at the glass' - or on a single screen - to create a single, composite application.

Robotics middleware simplifies the process of integrating robotic hardware, firmware and software from multiple manufacturers and locations.

Enterprise application integration middleware

Enterprise application integration middleware lets an organization establish an enterprise integration hub - a standardized way to connect all applications, application components, business processes and back-end data sources in the extended enterprise.

Until about ten years ago, the most prevalent enteprise application integration middleware was the enterprise service bus (ESB), which served as the integration hub within a service-oriented architecture (SOA). Today iPaaS lets an organization connect applications, data, processes and services across on-premises, private cloud, and public cloud environments - without the work and expense of purchasing, installing, managing, and maintaining the integration middleware (and the hardware it runs on) within its own data center.

Platform middleware

Platform middleware (or application platform middleware) can further support application development and accelerate application delivery by providing a runtime hosting environment - such as a Java runtime environment (Java RE), or containers, or both - for application or business logic. Platform middleware can include or combine enterprise application servers, web servers, and content management systems, as well as the other middleware types listed above.

Cloud-native is an application development approach that leverages fundamental cloud computing technologies, with the goal of providing a consistent development, deployment and management across on-premises, private cloud or public cloud environments. 

Practically speaking, today cloud native-applications are applications built from microservices, and deployed in containers orchestrated using Kubernetes. Microservices are loosely-coupled application components that encompass their own stack, and can be deployed and updated independently of one another, and communicate with one another using a combination of REST APIs, message brokers and event streams. Containers are lightweight executables that package application code – such as microservices – together with just the OS libraries and dependencies needed to run that code on any traditional IT or cloud infrastructure.

Together these and related technologies create a powerful, develop-once-deploy-anywhere platform for delivering net-new hybrid cloud applications, and for modernizing traditional legacy systems for use in the cloud. But they also lead to a complex development environment that combines even more software applications, data sources, programming languages, tools and distributed systems. 

Middleware can resolve some of this complexity, but running containerized applications with conventional middleware can add complexities of its own, as well as the kind of infrastructure overhead that containers were designed to eliminate. For this reason, popular cloud application development platforms such as Cloud Foundry (link resides outside IBM) and Red Hat Open Shift evolved to include containerized middleware - middleware modularized so that just the required connectivity functions can be packaged in a container.