nameko-grpc 1.4.0rc3
Nameko gRPC extensions
License: Apache License, Version 2.0

# nameko-grpc

This is a prototype implementation of a gRPC server and client for use in [nameko](https://nameko.io) microservices.

All four of the request-response patterns are implemented and tested:

1. unary-unary
2. unary-stream
3. stream-unary
4. stream-stream

Asynchronous calls are also supported for every pattern.

Python 3.4+ is supported.

## Installation

```
$ pip install nameko-grpc
```

## Example

### Server

Example Nameko service that can respond to gRPC requests:

``` python
from example_pb2 import ExampleReply
from example_pb2_grpc import exampleStub

from nameko_grpc.entrypoint import Grpc

grpc = Grpc.implementing(exampleStub)


class ExampleService:
    name = "example"

    @grpc
    def unary_unary(self, request, context):
        message = request.value * (request.multiplier or 1)
        return ExampleReply(message=message)

    @grpc
    def unary_stream(self, request, context):
        message = request.value * (request.multiplier or 1)
        yield ExampleReply(message=message, seqno=1)
        yield ExampleReply(message=message, seqno=2)

    @grpc
    def stream_unary(self, request, context):
        messages = []
        for req in request:
            message = req.value * (req.multiplier or 1)
            messages.append(message)

        return ExampleReply(message=",".join(messages))

    @grpc
    def stream_stream(self, request, context):
        for index, req in enumerate(request):
            message = req.value * (req.multiplier or 1)
            yield ExampleReply(message=message, seqno=index + 1)

```

### Client

Example Nameko service that can make gRPC requests:

``` python
from example_pb2 import ExampleReply
from example_pb2_grpc import exampleStub

from nameko.rpc import rpc

from nameko_grpc.dependency_provider import GrpcProxy


class ClientService:
    name = "client"

    example_grpc = GrpcProxy("//127.0.0.1", exampleStub)

    @rpc
    def method(self):
        responses = self.example_grpc.unary_stream(ExampleRequest(value="A"))
        for response in responses:
            print(response.message)

```

Example standalone client, can be used with or without Eventlet:

``` python
from example_pb2 import ExampleReply
from example_pb2_grpc import exampleStub

from nameko_grpc.client import Client

with Client("//127.0.0.1", exampleStub) as client:
    responses = client.unary_stream(ExampleRequest(value="A"))
    for response in responses:
        print(response.message)

```

### Protobuf

The protobuf for the above examples is:

```
syntax = "proto3";

package nameko;

service example {
  rpc unary_unary (ExampleRequest) returns (ExampleReply) {}
  rpc unary_stream (ExampleRequest) returns (stream ExampleReply) {}
  rpc stream_unary (stream ExampleRequest) returns (ExampleReply) {}
  rpc stream_stream (stream ExampleRequest) returns (stream ExampleReply) {}
}

message ExampleRequest {
  string value = 1;
  int32 multiplier = 2;
}


message ExampleReply {
  string message = 1;
  int32 seqno = 2;
}

```

## Style

The example protobufs in this repo use `snake_case` for method names as per the Nameko conventions rather than `CamelCase` as per gRPC. This is not mandatory -- decorated method names simply match to the methods defined in the protobufs; similarly for service names.

## Context and Metadata

Insofar as it is implemented, the `context` argument to service methods has the same API as the standard Python implementation:

* `context.invocation_metadata()` returns any metadata provided by the calling client.
* `context.send_initial_metadata()` can be used to add metadata to the response headers.
* `context.set_trailing_metadata()` can be used to add metadata to the response trailers.

The standalone Client and DependencyProvider both allow metadata to be provided using the `metadata` keyword argument. They accept a list of `(name, value)` tuples, just as the standard Python client does. Binary values must be base64 encoded and use a header name postfixed with "-bin", as in the standard Python client.

gRPC request metadata is added to the "context data" of the Nameko worker context, so is availble to other Nameko extensions.

The DependencyProvider client adds Nameko worker context data as metadata to all gRPC requests. This allows the Nameko call id stack to be populated and propagate, along with any other context data.

## Compression

Compression is supported in both the server and the client. The `deflate` and `gzip` algorithms are available by default and will be included in the `grpc-accept-encoding` headers on requests from the client and responses from the server.

The server honours any acceptable compression algorithm that it is able to, preferring to encode the response with the same algorithm as the request.

A default compression algorithm is specified when creating the client, and/or can specified per-call using the `compression` keyword argument:

``` python
client = Client(default_compression="deflate", ...)
client.unary_unary(ExampleRequest(value="foo"), compression="gzip")  # use gzip instead
```

Compression levels are not supported.

The gRPC spec allows for the server to respond using a different algorithm from the request, or not compressing at all. This is not currently supported in the standard Python gRPC implementation nor nameko-grpc.


## Errors

### Client side

gRPC errors are raised by the client as instances of the `GrpcError` exception class. A `GrpcError` encapsulates the status [`code`](https://github.com/grpc/grpc/blob/master/doc/statuscodes.md) and a `message` string describing the error. These are transmitted as the `grpc-status` and `grpc-message` headers defined by the gRPC spec.

Additionally, `GrpcError` has a `status` attribute that can hold a `google.rpc.status.Status` protobuf message for holding additional information about the error. This is similar to the [`grpc_status` package](https://grpc.github.io/grpc/python/grpc_status.html) that is part of the official Python gRPC library, and indeed that package is compatible with the nameko-grpc client. (TODO test)

The `google.rpc.status.Status` message received in the `grpc-status-details-bin` trailing header.

### Server side

If a service method raises an exception, the error that is generated has the following attributes by default:

    * `code`: grpc.StatusCode.UNKNOWN
    * `message`: "Exception calling application: <stringified exception>"
    * `status`: `google.rpc.Status` protobuf message

The `google.rpc.Status` message encapsulates the `code` and `message` again, along with a `details` attribute containing the exception traceback as a `google.rpc.error_details.DebugInfo` message.

You can customise the errors returned by the server in two ways:

1. Explictly set the code, message, and trailing headers using the `context` object. This is essentially how the official Python gRPC library does it:


``` python
class Service:

    ...

    @grpc
    def stream_error_via_context(self, request, context):
        for index, item in enumerate(...):
            if index > MAX_TOKENS:
                context.set_code(StatusCode.RESOURCE_EXHAUSTED)
                context.set_message("Out of tokens!")
                context.set_trailing_metadata([
                    ("grpc-status-details-bin", make_grpc_status(...))
                ])
                break
            yield Reply(...)
```

2. Return a `GrpcError` directly:

``` python
from nameko_grpc.errors import GrpcError, StatusCode


class Service:

    ...

    @grpc
    def stream_grpc_error(self, request, context):
        for index, item in enumerate(...):
            if index > MAX_TOKENS:
                raise GrpcError(
                    code=StatusCode.RESOURCE_EXHAUSTED,
                    message="Out of tokens!",
                    status=make_grpc_status(...)
                )
            yield Reply(...)
```

3. Register an error handler mapping a given exception type to a function that generates a `GrpcError` instance from the raised exception:

``` python
from nameko_grpc.errors import register_handler, GrpcError, StatusCode


class NoMoreTokens(Exception):
    pass


def handle_no_more_tokens(exc, code=None, message=None):
    return GrpcError(
        code=StatusCode.RESOURCE_EXHAUSTED,
        message=str(exc),
        details=make_grpc_status(...)
    )


register_handler(NoMoreTokens, handle_no_more_tokens)

class Service:

    ...

    @grpc
    def grpc_error_from_exception(self, request, context):
        for index, item in enumerate(...):
            if index > MAX_TOKENS:
                raise NoMoreTokens("Out of tokens!")
            yield Reply(...)
```

The final approach is useful when you want to map an exception without wrapping it in a try/except in the service method, or when there is no opportunity to do so -- for example when an exception is raised by a decorator on the service method.


## Timeouts

The client and server both support timeouts, and will raise `DEADLINE_EXCEEDED` if an RPC has not completed within the requested time. The clock starts ticking on the client when the request is initiated, and on the server when it is received.

The deadline is calculated as the current time plus the timeout value.

On the client, the timeout value is specified in seconds by using the `timeout` keyword argument when invoking a method:

``` python
client = Client(...)
client.unary_unary(ExampleRequest(value="foo"), timeout=0.1)  # 100 ms timeout
```

There is no default because there's no sensible value applicable to all use-cases, but it is [recommended](https://grpc.io/blog/deadlines) to always set a deadline.

## Tests

Most tests are run against every permutation of gRPC server/client to Nameko server/client. This roughly demonstrates equivalence between the two implementations. These tests are marked with the "equivalence" pytest marker.

Additionally, we run the interop tests from the official gRPC repo, which are used to verify compatibility between language implementations. The Nameko gRPC implementation supports every feature that the official Python gRPC implementation does. These tests are marked with the "interop" pytest marker.

The `test/spec` directory contains the protobufs and server implementations used in the various tests.

### Running the tests

Clone or download the repository, and ensure the development dependencies are installed:

```
$ pip install nameko-grpc[dev]
```

Then run the tests:

```
$ pytest test
```

The interop tests require docker. They use the image at https://hub.docker.com/r/nameko/nameko-grpc-interop which contains the pre-built C++ interop client. To run all tests excluding the interop tests:

```
$ pytest test -m "not interop"
```


## Implementation Notes

gRPC is built on HTTP2, so nameko-grpc relies heavily on the [hyper-h2](https://python-hyper.org/projects/h2/en/stable/) library. H2 is a finite state-machine implementation of the HTTP2 protocol, and its documentation is very good. The code in nameko-grpc is much more understandable when you're familiar with h2.

Much of the heavy-lifting in nameko-grpc is done by either the server or client subclasses of `ConnectionManager`. A `ConnectionManager` handles a single HTTP2 connection, and implements the handlers for each HTTP2 event on that connection (e.g. `request_received` or `stream_ended`). See:

* `nameko_grpc/client.py::ClientConnectionManager`
* `nameko_grpc/entrypoint.py::ServerConnectionManager`
* `nameko_grpc/connection.py::ConnectionManager`

The next most significant module is `nameko_grpc/streams.py`. This module contains the `SendStream` and `ReceiveStream` classes, which represent an HTTP2 stream that is being sent or received, respectively. A `ReceiveStream` receives data as bytes from a `ConnectionManager`, and parses them into a stream of gRPC messages. A `SendStream` does the opposite, encoding gRPC messages into bytes that can be sent across an HTTP2 connection.

The `@grpc` Entrypoint is a normal Nameko entrypoint that executes a service method when an appropriate request is made. The entrypoint deals with a `ReceiveStream` object encapsulating the request, and a `SendStream` object that accepts the response. The streams are managed by a shared `GrpcServer`, which accepts incoming connections and wraps each in a `ServerConnectionManager`.

The standalone Client is a small wrapper around a `ClientConnectionManager`. The Client simply creates a socket connection and then hands it to the connection manager. When a method is invoked on the client, the connection manager initiates an appropriate request. The headers for that request describe the method being invoked, encodings, message types etc. This logic is all encapsulated into the `Method` class.

The gRPC DependencyProvider is a normal Nameko DependencyProvider, which is also just a small wrapper around a `ClientConnectionManager`. It functions in exactly the same manner as the standalone Client.


## Equivalence tests notes

To demonstrate equivalence between the nameko-grpc implementations and the standard gRPC implementations, all tests marked with the `equivalence` marker run against every permutation of:

* gRPC standard server (Python implementation) or
* Nameko server

and

* gRPC standard client (Python implementation) or
* Nameko standalone client or
* Nameko DependencyProvider client

Nameko uses Eventlet for concurrency, which is incompatible with the standard gRPC server and client. Consequently, these must be run in a separate process and somehow communicated with in order to make assertions about the behaviour of the standard implementation.

The scripts which run the out-of-process client and server can be found in `test/grpc_indirect_client.py` and `test/grpc_indirect_server.py`

The communication is done with ZeroMQ. The logic for this is contained within the  `RemoteClientTransport` and `Command` classes within `test/helpers.py`, and the `start_grpc_client` and `start_grpc_server` fixtures in `test/conftest.py`.

In the future this arrangement would allow us to run equivalence tests against a different (more feature-complete) standard gRPC implementation.