Spring Boot Passthrough JWT with RestTemplate

In a microservice environment it is often the case, that calls from a client to a service result in further calls to other services. One possible scenario is a call to a GraphQL service which gathers information from different backend (REST) services and present it as a cohesive data graph.

In this scenario the user is authenticated to the backend services via OAuth2 (e.g., Keycloak or a Spring Boot OAuth2 server) and the GraphQL service should passthrough the authentication header (a JWT bearer) of incoming requests to the backend services. This way the authentication has to be validated only once in the backend services and as “near” as possible to the (REST) resources.

This is not meant as a replacement for service-to-service authentication, but as an addition if you do not use the full OpenID connect standard with a separate identity token to pass on, but still want to serve verifiable user data to your backend service. In contrast, you may use this to pass through any header (including a identity token). This is just a scenario that I faced.

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