Comparison of Service Invoke and callout

The Service Invoke mediation primitive can be compared to having a callout, callout response, and callout fault, embedded in the mediation flow. The following figure shows the similarity between the Service Invoke terminals and the callout terminals:

Similarities between the Service Invoke mediation primitive and the callout:

• The in terminal of the Service Invoke corresponds to the in terminal of the callout.
• The out terminal of the Service Invoke corresponds to the out terminal of the callout response.
• The fail terminal of the Service Invoke corresponds to the fail terminal of the callout response (for an unmodeled fault).
• A modeled fault output terminal of the Service Invoke, corresponds to a modeled fault terminal of the callout fault.

Differences between the Service Invoke mediation primitive and the callout:

• The Service Invoke mediation primitive does not switch from request flow to response flow.
• The Service Invoke mediation primitive does not modify either the transient context or the correlation context.

Although, there is a functional similarity between the Service Invoke mediation primitive and the callout node, the Service Invoke mediation primitive does not replace the callout node. Consider the following when deciding which combination of Service Invoke mediation primitive and callout to use:

• If you need to call a service and return the response, consider using the Service Invoke mediation primitive.
• If you need to interact with multiple services, and produce output that combines service responses, consider using multiple Service Invoke mediation primitives. In this type of configuration a callout node might not be necessary; you might be able to configure all the Service Invoke mediation primitives in the request flow.
• If you need to call an intermediate service, you can use a Service Invoke mediation primitive. For example, you might use an intermediate service to adjust a message, or validate a message externally. The mediation flow would contain a Service Invoke mediation primitive and a callout to the final service provider.
• If you need to mediate a message (without calling an intermediate service) and then call a service provider, the use of a callout is appropriate. The callout provides the simplest model for this configuration.