Summary

Simplified Python gRPC interceptors.

The Python grpc package provides service interceptors, but they're a bit hard to use because of their flexibility. The grpc interceptors don't have direct access to the request and response objects, or the service context. Access to these are often desired, to be able to log data in the request or response, or set status codes on the context.

Installation

To just get the interceptors (and probably not write your own):

$ pip install grpc-interceptor

To also get the testing framework, which is good if you're writing your own interceptors:

$ pip install grpc-interceptor[testing]

Usage

Server Interceptor

To define your own interceptor (we can use ExceptionToStatusInterceptor as an example):

from grpc_interceptor import ServerInterceptor
from grpc_interceptor.exceptions import GrpcException

class ExceptionToStatusInterceptor(ServerInterceptor):
    def intercept(
        self,
        method: Callable,
        request: Any,
        context: grpc.ServicerContext,
        method_name: str,
    ) -> Any:
        """Override this method to implement a custom interceptor.
         You should call method(request, context) to invoke the
         next handler (either the RPC method implementation, or the
         next interceptor in the list).
         Args:
             method: The next interceptor, or method implementation.
             request: The RPC request, as a protobuf message.
             context: The ServicerContext pass by gRPC to the service.
             method_name: A string of the form
                 "/protobuf.package.Service/Method"
         Returns:
             This should generally return the result of
             method(request, context), which is typically the RPC
             method response, as a protobuf message. The interceptor
             is free to modify this in some way, however.
         """
        try:
            return method(request, context)
        except GrpcException as e:
            context.set_code(e.status_code)
            context.set_details(e.details)
            raise

Then inject your interceptor when you create the grpc server:

interceptors = [ExceptionToStatusInterceptor()]
server = grpc.server(
    futures.ThreadPoolExecutor(max_workers=10),
    interceptors=interceptors
)

To use ExceptionToStatusInterceptor:

from grpc_interceptor.exceptions import NotFound

class MyService(my_pb2_grpc.MyServiceServicer):
    def MyRpcMethod(
        self, request: MyRequest, context: grpc.ServicerContext
    ) -> MyResponse:
        thing = lookup_thing()
        if not thing:
            raise NotFound("Sorry, your thing is missing")
        ...

This results in the gRPC status status code being set to NOT_FOUND, and the details "Sorry, your thing is missing". This saves you the hassle of catching exceptions in your service handler, or passing the context down into helper functions so they can call context.abort or context.set_code. It allows the more Pythonic approach of just raising an exception from anywhere in the code, and having it be handled automatically.

Client Interceptor

We will use an invocation metadata injecting interceptor as an example of defining a client interceptor:

from grpc_interceptor import ClientCallDetails, ClientInterceptor

class MetadataClientInterceptor(ClientInterceptor):

    def intercept(
        self,
        method: Callable,
        request_or_iterator: Any,
        call_details: grpc.ClientCallDetails,
    ):
        """Override this method to implement a custom interceptor.

        This method is called for all unary and streaming RPCs. The interceptor
        implementation should call `method` using a `grpc.ClientCallDetails` and the
        `request_or_iterator` object as parameters. The `request_or_iterator`
        parameter may be type checked to determine if this is a singluar request
        for unary RPCs or an iterator for client-streaming or client-server streaming
        RPCs.

        Args:
            method: A function that proceeds with the invocation by executing the next
                interceptor in the chain or invoking the actual RPC on the underlying
                channel.
            request_or_iterator: RPC request message or iterator of request messages
                for streaming requests.
            call_details: Describes an RPC to be invoked.

        Returns:
            The type of the return should match the type of the return value received
            by calling `method`. This is an object that is both a
            `Call <https://grpc.github.io/grpc/python/grpc.html#grpc.Call>`_ for the
            RPC and a `Future <https://grpc.github.io/grpc/python/grpc.html#grpc.Future>`_.

            The actual result from the RPC can be got by calling `.result()` on the
            value returned from `method`.
        """
        new_details = ClientCallDetails(
            call_details.method,
            call_details.timeout,
            [("authorization", "Bearer mysecrettoken")],
            call_details.credentials,
            call_details.wait_for_ready,
            call_details.compression,
        )

        return method(request_or_iterator, new_details)

Now inject your interceptor when you create the grpc channel:

interceptors = [MetadataClientInterceptor()]
with grpc.insecure_channel("grpc-server:50051") as channel:
    channel = grpc.intercept_channel(channel, *interceptors)
    ...

Client interceptors can also be used to retry RPCs that fail due to specific errors, or a host of other use cases. There are some basic approaches in the tests to get you started.

Note: The method in a client interceptor is a continuation as described in the client interceptor section of the gRPC docs. When you invoke the continuation, you get a future back, which resolves to either the result, or exception. This is different than invoking a client stub, which returns the result directly. If the interceptor needs the value returned by the call, or to catch exceptions, then you'll need to do future = method(request_or_iterator, call_details), followed by future.result(). Check out the tests for examples.

Documentation

The examples above showed usage for simple unary-unary RPC calls. For examples of streaming and asyncio RPCs, read the complete documentation here.