How to define a global error handler in gRPC python

Question

Im trying to catch any exception that is raised in any servicer so I can make sure that I only propagate known exceptions and not unexpected ones like ValueError, TypeError etc.

Answer 1

Maybe this will help you :)

def _wrap_rpc_behavior(handler, fn):
    if handler is None:
        return None

    if handler.request_streaming and handler.response_streaming:
        behavior_fn = handler.stream_stream
        handler_factory = grpc.stream_stream_rpc_method_handler
    elif handler.request_streaming and not handler.response_streaming:
        behavior_fn = handler.stream_unary
        handler_factory = grpc.stream_unary_rpc_method_handler
    elif not handler.request_streaming and handler.response_streaming:
        behavior_fn = handler.unary_stream
        handler_factory = grpc.unary_stream_rpc_method_handler
    else:
        behavior_fn = handler.unary_unary
        handler_factory = grpc.unary_unary_rpc_method_handler

    return handler_factory(fn(behavior_fn,
                              handler.request_streaming,
                              handler.response_streaming),
                           request_deserializer=handler.request_deserializer,
                           response_serializer=handler.response_serializer)


class TracebackLoggerInterceptor(grpc.ServerInterceptor):

    def intercept_service(self, continuation, handler_call_details):
        def latency_wrapper(behavior, request_streaming, response_streaming):

            def new_behavior(request_or_iterator, servicer_context):
                try:
                    return behavior(request_or_iterator, servicer_context)
                except Exception as err:
                    logger.exception(err, exc_info=True)
            return new_behavior

        return _wrap_rpc_behavior(continuation(handler_call_details),    latency_wrapper)

Answer 2

As some of the previous comments suggested, I tried the meta-class approach which works quite well.

Attached is a simple example to demonstrate how to intercept the grpc calls. You could extend this by providing the metaclass a list of decorators which you could apply on each function.

Also, it would be wise to be more selective regarding the methods you apply the wrapper to. A good option would be to list the methods of the autogenerated base class and only wrap those.

from types import FunctionType
from functools import wraps


def wrapper(method):
    @wraps(method)
    def wrapped(*args, **kwargs):
        # do stuff here
        return method(*args, **kwargs)

    return wrapped


class ServicerMiddlewareClass(type):
    def __new__(meta, classname, bases, class_dict):
        new_class_dict = {}

        for attribute_name, attribute in class_dict.items():
            if isinstance(attribute, FunctionType):
                # replace it with a wrapped version
                attribute = wrapper(attribute)

            new_class_dict[attribute_name] = attribute

        return type.__new__(meta, classname, bases, new_class_dict)


# In order to use
class MyGrpcService(grpc.MyGrpcServicer, metaclass=ServicerMiddlewareClass):
   ...

Answer 3

gRPC Python currently don't support server-side global error handler. The interceptor won't execute the server handler inside the intercept_service function, so there is no way to try/except.

Also, I found the gRPC Python server interceptor implementation is different from what they proposed original at L13-Python-Interceptors.md#server-interceptors. If the implementation stick to the original design, we can use interceptor as global error handler easily with handler and request/request_iterator.

# Current Implementation
intercept_service(self, continuation, handler_call_details)

# Original Design
intercept_unary_unary_handler(self, handler, method, request, servicer_context)
intercept_unary_stream_handler(self, handler, method, request, servicer_context)
intercept_stream_unary_handler(self, handler, method, request_iterator, servicer_context)
intercept_stream_stream_handler(self, handler, method, request_iterator, servicer_context)

Please submit a feature request issue to https://github.com/grpc/grpc/issues.