While web services are an appropriate technology for SOA, some other aspects need to be considered:
In most enterprises, web services are not the only middleware solution used. Usually enterprises already have one or more middleware products, such as messaging systems and ORBs. They cannot afford to replace them overnight with web services. Therefore, there is a need to integrate different middleware products, and provide interoperability with web services.
In order to provide connectivity between services, the use of SOAP in complex environments is not adequate. In such environments, we need ways to connect, mediate, manage, and control the services and particularly the communication between them.
SOAP over HTTP might not be robust enough for heavy enterprise use. Enterprise information systems require dependable, robust, and secure service infrastructure.
The Enterprise Service Bus (ESB) is the software infrastructure, acting as an intermediary layer of middleware that addresses the above-mentioned requirements. An ESB adds flexibility to communication between services and simplifies the integration and reuse of services. An ESB makes it possible to connect services implemented in different technologies (such as EJBs, messaging systems, CORBA components, and legacy applications) in an easy way. An ESB can act as a mediator between different, often incompatible protocols and middleware products.
The ESB provides a robust, dependable, secure and scalable communication infrastructure between services. It also provides control over the communication and control over the use of services, including:
Message interception capabilities: This allows us to intercept requests to services and responses from services and apply additional processing to them. In this manner, the ESB acts as an intermediary.
Routing capabilities: This allows us to route the messages to different services based on their content, origin, or other attributes.
Transformation capabilities: These allow us to transform messages before they are delivered to services. For XML formatted messages, such transformations are usually done using XSLT (Extensible Stylesheet Language for Transformations) or XQuery engines.
Control over the deployment, usage, and maintenance of services: This allows logging, profiling, load balancing, performance tuning, charging for use of services, distributed deployment, on-the-fly reconfiguration, etc.
Other important management features include the definition of correlation between messages, definition of reliable communication paths, definition of security constraints related to messages and services, etc.
Currently there are several products on the market that claim to provide ESB functionality. A good ESB should provide at least quality‑of‑service support of enterprise level, including reliability, fault-tolerance, and security. If provided by an ESB, services can depend on these features and do not need to implement them themselves. The ESB should also allow configuring any combination of these quality‑of‑service features and provide flexibility.
An ESB should provide support for a variety of technologies on which services are implemented. In addition to web services, an ESB should provide connectors for a broad range of technologies, such as J2EE and .NET components, messaging middleware, legacy applications, and TP monitors. The ESB needs to provide flexibility to bind any combination of services without technological constraints. It should also support a combination of different interaction models, such as queuing, routing, etc., without changing the services or requiring writing code.
An ESB should make services broadly available. This means that it should be easy to find, connect, and use a service irrespective of the technology it is implemented in. With broad availability of services, an ESB can increase reuse and can make the composition of services easier. Finally, an ESB should provide management capabilities, such as message routing, interaction, and transformation, which we have already described.
An ESB that provides these features becomes an essential part of the SOA. It provides several benefits, including increased flexibility, reduced deployment, development, and maintenance costs, and increased reliability and manageability. Therefore the ESB is an essential part of SOA, which we will discuss in the next section.