gRPC provides simple APIs to implement and install interceptors on a per ClientConn/Server basis. Interceptor intercepts the execution of each RPC call. Users can use interceptors to do logging, authentication/authorization, metrics collection, and many other functionality that can be shared across RPCs.
go run server/main.go
go run client/main.go
In gRPC, interceptors can be categorized into two kinds in terms of the type of RPC calls they intercept. The first one is the unary interceptor, which intercepts unary RPC calls. And the other is the stream interceptor which deals with streaming RPC calls. See here for explanation about unary RPCs and streaming RPCs. Each of client and server has their own types of unary and stream interceptors. Thus, there are in total four different types of interceptors in gRPC.
UnaryClientInterceptor is the type for client-side unary interceptor. It is essentially a function type with signature: func(ctx context.Context, method string, req, reply interface{}, cc *ClientConn, invoker UnaryInvoker, opts ...CallOption) error. An implementation of a unary interceptor can usually be divided into three parts: pre-processing, invoking RPC method, and post-processing.
For pre-processing, users can get info about the current RPC call by examining the args passed in, such as RPC context, method string, request to be sent, and CallOptions configured. With the info, users can even modify the RPC call. For instance, in the example, we examine the list of CallOptions and see if call credential has been configured. If not, configure it to use oauth2 with token “some-secret-token” as fallback. In our example, we intentionally omit configuring the per RPC credential to resort to fallback.
After pre-processing is done, use can invoke the RPC call by calling the invoker.
Once the invoker returns the reply and error, user can do post-processing of the RPC call. Usually, it's about dealing with the returned reply and error. In the example, we log the RPC timing and error info.
To install a unary interceptor on a ClientConn, configure Dial with DialOption WithUnaryInterceptor.
StreamClientInterceptor is the type for client-side stream interceptor. It is a function type with signature: func(ctx context.Context, desc *StreamDesc, cc *ClientConn, method string, streamer Streamer, opts ...CallOption) (ClientStream, error). An implementation of a stream interceptor usually include pre-processing, and stream operation interception.
For pre-processing, it's similar to unary interceptor.
However, rather than doing the RPC method invocation and post-processing afterwards, stream interceptor intercepts the users' operation on the stream. First, the interceptor calls the passed-in streamer to get a ClientStream, and then wraps around the ClientStream and overloading its methods with intercepting logic. Finally, interceptors returns the wrapped ClientStream to user to operate on.
In the example, we define a new struct wrappedStream, which is embedded with a ClientStream. Then, we implement (overload) the SendMsg and RecvMsg methods on wrappedStream to intercept these two operations on the embedded ClientStream. In the example, we log the message type info and time info for interception purpose.
To install the stream interceptor for a ClientConn, configure Dial with DialOption WithStreamInterceptor.
Server side interceptor is similar to client side, though with slightly different provided info.
UnaryServerInterceptor is the type for server-side unary interceptor. It is a function type with signature: func(ctx context.Context, req interface{}, info *UnaryServerInfo, handler UnaryHandler) (resp interface{}, err error).
Refer to client-side unary interceptor section for detailed implementation explanation.
To install the unary interceptor for a Server, configure NewServer with ServerOption UnaryInterceptor.
StreamServerInterceptor is the type for server-side stream interceptor. It is a function type with signature: func(srv interface{}, ss ServerStream, info *StreamServerInfo, handler StreamHandler) error.
Refer to client-side stream interceptor section for detailed implementation explanation.
To install the unary interceptor for a Server, configure NewServer with ServerOption StreamInterceptor.