Recently I answered a question on stackoverflow.com asking can a SOA be designed with REST? I’m cross-posting the answer here.

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At a high Level the answer is Yes, however not completely.

SOA requires thinking about the system in terms of

• Services (well-defined business functionality)
• Components (discrete pieces of code and/or data structures)
• Processes (Service orchestrations. Generally using BPEL)

Being able to compose new higher level services or business processes is a basic feature of a good SOA. XML, SOAP based Web Services and related standards are good fit for realizing SOA.

Also SOA has a few accepted principles – http://en.wikipedia.org/wiki/Service-oriented_architecture#Principles

• Standardized service contract – Services adhere to a communications agreement, as defined collectively by one or more service-description documents.
• Service Loose Coupling – Services maintain a relationship that minimizes dependencies and only requires that they maintain an awareness of each other.
• Service Abstraction – Beyond descriptions in the service contract, services hide logic from the outside world.
• Service reusability – Logic is divided into services with the intention of promoting reuse.
• Service autonomy – Services have control over the logic they encapsulate.
• Service granularity – A design consideration to provide optimal scope and right granular level of the business functionality in a service operation.
• Service statelessness – Services minimize resource consumption by deferring the management of state information when necessary.
• Service discoverability – Services are supplemented with communicative meta data by which they can be effectively discovered and interpreted.
• Service composability – Services are effective composition participants, regardless of the size and complexity of the composition.

A SOA based architecture is expected to have Service Definition. Since RESTful web services lack a definitive service definition (similar to wsdl), it is difficult for a REST based system to fulfill most of the above principles.

To achieve the same using REST, you’d need to have RESTful Web Services + Orchestration (possible using some lightweight ESB like MuleESB or Camel)

Please also see this resource – From SOA to REST

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Adding this part as clarification for below comment –

Orchestration is required to compose processes. That’s what provides the main benefit of SOA.

Say you have a order processing application with operations like
–

• addItem
• addTax
• calculateTotal
• placeOrder

Initially you created a process (using BPEL) which uses these operations in sequence. You have clients who use this Composed Service. After a few months a new client comes who has tax exemption, then instead of writing new service, you could just create a new process skipping the addTax operation. Thus you could achieve faster realization of business functionality just by re-using existing service. In practice there are mutiple such services.

Thus BPEL or similar (ESB or routing) technology is essential for SOA. Without business use, a SOA is not really a SOA.