grpc metadata java

gRPC Metadata in Java

Introduction

gRPC is a high-performance, open-source framework for building distributed systems. It uses Protocol Buffers as the interface definition language and supports various programming languages, including Java. In gRPC, metadata allows clients and servers to send additional information with each request or response. This metadata can be used for authentication, tracing, and other purposes.

In this article, we will explore how to work with metadata in gRPC Java. We will cover how to set and retrieve metadata in the client and server code, and also how to use interceptors to manipulate metadata.

Setting Metadata in the Client

To set metadata in the client, we need to create a Metadata object and add key-value pairs to it. The key is a Metadata.Key object, and the value can be any Object. Here's an example:

import io.grpc.Metadata;
import io.grpc.Metadata.Key;
import io.grpc.stub.MetadataUtils;

// Create a Metadata object
Metadata metadata = new Metadata();

// Define a key
Key<String> key = Key.of("Authorization", Metadata.ASCII_STRING_MARSHALLER);

// Add a value to the metadata
metadata.put(key, "Bearer my_token");

// Use the Metadata in the gRPC call
MyServiceGrpc.MyServiceBlockingStub stub = MyServiceGrpc.newBlockingStub(channel);
stub = MetadataUtils.attachHeaders(stub, metadata);

In this example, we create a Metadata object and define a key named "Authorization". We add a value "Bearer my_token" to the metadata. Then, we attach the metadata to the gRPC call using the MetadataUtils.attachHeaders method.

Retrieving Metadata in the Server

In the server code, we can retrieve the metadata sent by the client. We can access the metadata in the gRPC method implementation by using the ServerCall object. Here's an example:

import io.grpc.Metadata;
import io.grpc.ServerCall;
import io.grpc.ServerCallHandler;
import io.grpc.ServerInterceptor;
import io.grpc.ServerInterceptors;
import io.grpc.ServerServiceDefinition;
import io.grpc.stub.MetadataUtils;

// Create a ServerInterceptor
ServerInterceptor interceptor = new ServerInterceptor() {
    @Override
    public <ReqT, RespT> ServerCall.Listener<ReqT> interceptCall(ServerCall<ReqT, RespT> call, Metadata headers, ServerCallHandler<ReqT, RespT> next) {
        // Get the metadata
        String authorization = headers.get(Metadata.Key.of("Authorization", Metadata.ASCII_STRING_MARSHALLER));

        // Do something with the metadata

        // Call the next interceptor or the actual method implementation
        return next.startCall(call, headers);
    }
};

// Create a ServerServiceDefinition with the interceptor
ServerServiceDefinition serviceDefinition = ServerInterceptors.intercept(service, interceptor);

In this example, we create a ServerInterceptor object that intercepts the gRPC calls. In the interceptCall method, we can access the metadata sent by the client using the headers parameter. We retrieve the value of the "Authorization" key and perform some actions based on it.

Manipulating Metadata with Interceptors

Interceptors in gRPC Java can be used not only to retrieve metadata but also to manipulate it. We can add, remove, or modify metadata using interceptors. Here's an example of adding a custom header to the metadata:

import io.grpc.Metadata;
import io.grpc.ServerCall;
import io.grpc.ServerCallHandler;
import io.grpc.ServerInterceptor;
import io.grpc.ServerInterceptors;
import io.grpc.ServerServiceDefinition;
import io.grpc.stub.MetadataUtils;

// Create a ServerInterceptor
ServerInterceptor interceptor = new ServerInterceptor() {
    @Override
    public <ReqT, RespT> ServerCall.Listener<ReqT> interceptCall(ServerCall<ReqT, RespT> call, Metadata headers, ServerCallHandler<ReqT, RespT> next) {
        // Add a custom header to the metadata
        headers.put(Metadata.Key.of("X-Custom-Header", Metadata.ASCII_STRING_MARSHALLER), "custom_value");

        // Call the next interceptor or the actual method implementation
        return next.startCall(call, headers);
    }
};

// Create a ServerServiceDefinition with the interceptor
ServerServiceDefinition serviceDefinition = ServerInterceptors.intercept(service, interceptor);

In this example, we add a custom header "X-Custom-Header" with the value "custom_value" to the metadata. This header will be sent with each gRPC call.

Conclusion

Metadata in gRPC Java allows clients and servers to exchange additional information with each request or response. We can set and retrieve metadata in the client and server code, and manipulate metadata using interceptors. Metadata is a powerful feature of gRPC that enables various use cases such as authentication and tracing.

In this article, we covered the basics of working with metadata in gRPC Java. We saw how to set and retrieve metadata in the client and server, and also how to manipulate metadata using interceptors. Understanding how to use metadata effectively can enhance the functionality and security of your gRPC applications.

Give it a try and explore the possibilities of metadata in gRPC Java!

References

• [gRPC Java Documentation](