A client needs to expose a "calculate the loyalty rebate for this customer" operation. It reads three Dataverse tables, applies some business rules, writes a result. Every consumer - the Dynamics web app, a Power Automate flow, an external integration - should call the same operation.
Three places we could put it. Three different cost, latency, and scale profiles. Here is the matrix we now run on every "new operation" request.
The three options
Custom Action (legacy): a process defined in Dataverse that can be invoked through the SDK. Steps are Workflow Activity Actions. Old-school but still widely deployed.
Custom API (modern): the successor to custom actions. Defined as Dataverse entity rows (customapi, customapiRequestParameter, customapiResponseProperty), backed by a plugin that implements the logic. Exposed through the Web API with a typed OpenAPI schema.
