Hello,
I'm trying to get my head around how to use request and response metadata
while working with grpc stubs in multithreaded environment ( i.e. multiple
threads hitting same stub at same time ), I found this example in one of
gRPC tests:
AtomicReference<Metadata> trailersCapture = new
AtomicReference<Metadata>();
AtomicReference<Metadata> headersCapture = new
AtomicReference<Metadata>();
stub = MetadataUtils.captureMetadata(stub, headersCapture,
trailersCapture);
assertNotNull(stub.emptyCall(EMPTY));
// Assert that our side channel object is echoed back in both headers
and trailers
Assert.assertEquals(contextValue,
headersCapture.get().get(METADATA_KEY));
Assert.assertEquals(contextValue,
trailersCapture.get().get(METADATA_KEY));
Looking into Metadata utils, I can see it creates an interceptor, which
sets the state of headersCapture/trailersCapture upon headers coming:
public static ClientInterceptor newCaptureMetadataInterceptor(final
AtomicReference<Metadata> headersCapture, final AtomicReference<Metadata>
trailersCapture) {
return new ClientInterceptor() {
public <ReqT, RespT> ClientCall<ReqT, RespT>
interceptCall(MethodDescriptor<ReqT, RespT> method, CallOptions callOptions,
Channel next) {
return new SimpleForwardingClientCall(next.newCall(method,
callOptions)) {
public void start(final Listener<RespT> responseListener,
Metadata headers) {
headersCapture.set((Object)null);
trailersCapture.set((Object)null);
super.start(new
SimpleForwardingClientCallListener(responseListener) {
public void onHeaders(Metadata headers) {
headersCapture.set(headers);
super.onHeaders(headers);
[...]
}
As far as I'm concerned, this might work well when stub is called by one
thread, but because we share headersCapture state, it will get overwritten
( or it won't and I'm not seeing something? )
My main question is : How can i setup a stub, that will be thread-safe, and
able to return correct metadata headers back to the each caller?
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