Implement a modern RPC interface for rTorrent

Introduction

rTorrent currently uses an antique XMLRPC interface, which is limited in capability, not scalable, hard to use, insecure and low-performance. Community needs a modern RPC interface with full bidirectional, stream processing, incremental data loading and high concurrency capabilities, which allows real-time events for RPC users, reduced serialization/transfer overheads, potentially better security (with authentication and/or TLS), etc.

jesec implemented feature-complete JSON-RPC over SCGI support for rTorrent [1]. Preliminary tests showed that, when compared with XMLRPC, JSON-RPC yields 2x performance, 15% lower total CPU time in rTorrent process, 33% lower total CPU time in RPC user’s process. Flood supports it from v4.5, which allows it to halve the response time of 28890-torrent list from 2.5 seconds to 1.2 seconds [2].

Your job

The goal here is to implement a modern RPC interface that allows the users to receive events like download.completed.

The logical step is to implement JSON-RPC over WebSockets support on top of the existing JSON-RPC over SCGI (raw socket) implementation, hook event emitting function and send event to WebSocket subscribers.

Other modern RPC protocols such as gRPC are also accepted. However, note that it is probably more difficult to start from scratch.

The interface should support at least two simultaneous subscribers.

Some C++ libraries: