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ARPC - An RPC framework that supports file descriptor passing

Introduction

ARPC is an RPC framework that is heavily inspired by Google's Protobuf and GRPC and aims to be compatible with its core C++ API.

Similar to GRPC/Protobuf, ARPC ships with a script called aprotoc that can convert .proto files to C++ header files containing message and service bindings. Where ARPC differs from GRPC/Protobuf is that it provides support for attaching file descriptors to messages and sending them to other processes on the system. It accomplishes this by making use of file descriptor passing, a feature that is available when making use of AF_UNIX sockets.

ARPC can be very useful for adding privilege separation to your software. For example, a mail server could run with almost no privileges, but still be able to deliver mail to user's mailboxes by making use of an auxiliary process that hands out file descriptors to mail spools stored on disk.

ARPC does not support any authentication and authorization, for the reason that it is mainly intended to be used across UNIX sockets. It also does not provide any support for concurrency and GRPC's asynchronous API. Concurrency can be introduced by opening multiple channels across separate UNIX sockets.

ARPC has been built on top of a serialization library called Argdata, which in its turn has been developed as hierarchical configuration file format for the CloudABI sandboxing framework.

Building ARPC

ARPC can be built natively using:

mkdir build
cd build
cmake -DCMAKE_BUILD_TYPE=Release ..
make

It can be compiled for CloudABI for i686 as follows:

mkdir build
cd build
cmake -DCMAKE_TOOLCHAIN_FILE=../cmake/Toolchain-i686-cloudabi.cmake ..
make

To install in a specific location, give -DCMAKE_INSTALL_PREFIX to CMake. 'make install' will then install the headers, library and aprotoc there.

Using ARPC

ARPC should be easy to use if you already have some experience using GRPC. Be sure to check out the GRPC C++ tutorial to become familiar with the basics. Differences between GRPC and ARPC worth mentioning:

• All of ARPC's definitions are stored in a single header file, <arpc++/arpc++.h> and are part of namespace arpc.
• In addition to the commonly used Protobuf datatypes (e.g., int32, string, bool), ARPC's aprotoc allows you to declare fields of type fd, which adds a field to the message of type std::shared_ptr<FileDescriptor>.
• ARPC servers and channels do not create UNIX sockets themselves. File descriptors of connected AF_UNIX, SOCK_STREAM sockets must be provided to arpc::CreateChannel() and arpc::ServerBuilder.
• The unit tests also contain some examples of how to use ARPC.