You want to publish a single GraphQL API in APIM. For inventory data, you already have a GraphQL backend and want APIM to proxy it. For orders, you only have REST endpoints and need to resolve GraphQL fields by calling those REST services and shaping the response. What should you do?

The correct approach is to configure a GraphQL API in APIM. For the existing GraphQL backend (inventory), APIM can be set up for passthrough, directly forwarding requests to the upstream GraphQL service. For the order data, which only exists as REST endpoints, APIM's GraphQL resolver policies are the appropriate solution. These policies allow you to define how specific GraphQL fields are resolved. Within a resolver policy, you can make calls to your REST backends, retrieve the necessary data, and then transform (map) that data into the shape expected by your GraphQL schema before returning it to the client. This combines existing services under a single GraphQL endpoint without requiring a full rewrite of the REST services.