What is the difference between SOA, ESB and EAI Architecture?

These ideas fit one inside another like a set of Russian stacking dolls.

The outside doll is Enterprise Application Integration or EAI. This is the use of software and computer systems architectural principles to integrate enterprise-grade applications. These principles include the use of service-oriented architecture to facilitate the flow of information between systems.

The middle doll is Service Oriented Architecture or SOA, which is a style of software design where services are provided over a network. The idea behind SOA is that a service user need not know the details about how the service is provided. REST and SOAP are two ways to implement a SOA.

The inner doll is an Enterprise Service Bus or ESB, which is one of several ways to implement SOA. An ESB typically employs a publish-subscribe pattern.

In EAI, managing the flow of information between systems in ways that ensure information delivery is secure, robust and authoritative is crucial. ESB systems provide infrastructure for authentication, message queuing for robustness and a publication process for correctness. ESB’s also provide message translation services, which facilitates connecting systems that support differing format standards.

In summary, an ESB is one way to implement an SOA. SOA is one way to accomplish EAI.