What is the right way to use ThreadLocal in grpc interceptor?

[Chen Song]

We have a gPRC service that needs to set auth/identity information in a ThreadLocal variable in a class for it to correctly call another service. The gPRC service gets the auth/identiy information from request so I am thinking to use interceptor.

To start, I have some code looking as follows.

public class ImpersonationInterceptor {

    public <ReqT, RespT> interceptCall(

        ServerCall<ReqT, RespT> serverCall, Metadata headers, ServerCallHandler<ReqT, RespT> next) {

        Principal principal = ... // get the identify from the request

        

        AuthContext.setPrincipal(principal); // underneath it uses a ThreadLocal.

        return next.startCall(

            new SimpleForwardingServerCall<>(call) {

                public void close(Status status, Metadata trailers) {

                    AuthContext.setPrincipal(null); // clear the identity

                }

            }

        )

    }

}

Questions.

* The execution of the service method itself may not lie on the same thread to execute the interceptor, is that right?

* If true, the above is not gonna work, then the question is, what is the canonical way to set ThreadLocal variable in gRPC world? I know gRPC has Context support since 0.12, but in my case, I have to use the AuthContext's ThreadLocal mechanism.

Thanks very much in advance.

Chen

[Penn (Dapeng) Zhang]

>The execution of the service method itself may not lie on the same thread to execute the interceptor, is that right?

Yes.

I only see you set the principal, but don't see where/in what thread you read the principal, so I can not give you an answer. What's not possible for you to achieve your goal with ThreadLocal<AtomicReference<Principal>> as below?

static final ThreadLocal<AtomicReference<Principal>> threadLocal = new ThreadLocal<>();

public <ReqT, RespT> interceptCall(

        ServerCall<ReqT, RespT> serverCall, Metadata headers, ServerCallHandler<ReqT, RespT> next) {
    Principal principal = ... // get the identify from the request

    final AtomicReference<Principal> principalRef = new AtomicReference<Principal>(principal);
    threadLoacal.set(principalRef);

  return next.startCall(
      new SimpleForwardingServerCall<>(call) {
          public void close(Status status, Metadata trailers) {

            principalRef.set(null); // clear the identity
          }
      },
      headers);
}

[Derek Perez]

My understanding is that a request and all associated interceptors are executed on the same thread, per request. 

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