There are several categories of services. Many leading SOA vendors and thinkers typically break them down into Business and Utility types. A Business service represents a business capability that is needed to complete a step within a larger business process. Examples may include retrieving customer information, making payments, or checking order status. Utility services represent a technical capability that is business process agnostic. Examples are e-mail, logging, or authentication services.
Services can be combined together to create composite services. This is called orchestration. An example of this can be a Money Transfer service that needs to debit one account and deposit money into another one. Composite services can also be categorized as Business and Utility. Best practices and general orchestration guidelines as related to orchestrations, atomic services, and their relationships will be discussed separately.
Regardless of the type, a service is comprised of three components.
- This defines how services are exposed and can be accessed by its consumers.
- Interfaces are not limited to Web Services and can be represented by any remote messaging protocol.
- This defines what services expect during the interaction with the consumer. Message structures, relevant policies, and related security mechanisms are all part of a contract.
- Contract defines a “legal” agreement on how the service and its consumers should interact.
- This is the actual service code.
A service may also have multiple contracts. I have recommended in the past that for a service to be maximally reusable, it needs to implement the Service Façade pattern (see “SOA Façade Pattern” post and “Service Façade” pattern). This pattern recommends that multiple different interfaces and contracts for the same service be created. The Concurrent Contracts pattern also addresses this issue.
It is important to understand that while the interface, contract, and implementation describe a service as a whole, they are not closely tied together. An interface is not dependent on the contract details and should not be tied to the specific messaging structure. The opposite is also true – the contract should not be tied to any specific communication protocols. Additionally, the implementation should be flexible enough to accommodate the potential for multiple interfaces and contracts. Ideally, however, I would recommend that a service expose only a single contract and interface and the ESB would take care of exposing additional endpoints and facades as necessary.