Network Protocol

Requirements

Client to Daemon (request)

• Job identifier (unique)
• Name of Docker image
• Name of the Makefile target to build
• Contents of Makefile
• Input files needed for the target
• Human-readable metadata
  • Name of the job

Daemon to Client (replies)

• Job identifier (echoed)
• Current job status
  • In progress (heartbeat)
  • Failed to execute, with cause
  • Finished with return code
• Output files produced by the target
• stdout and stdin

Implementation

SSH

ssh2 is a high-quality Node.js implementation of SSH. Both the client and the daemon would use this library, not OpenSSH. We are using SSH somewhat creatively as a platform on which to build our protocol, so quell any connotations regarding command execution as literal exec() on the server, or SFTP as direct filesystem access.

• Detect unavailability: SSH has built-in heartbeat.
• Transfer input files: client opens SFTP channel and writes files.
• Transfer output files: client opens SFTP channel and reads files.
• Trigger a job: command execution is essentially a remote procedure call (RPC).
  • Send job information as JSON over stdin.
  • Receive stdout and stderr directly from target execution.
• Authentication of both daemon and client.
  • Opt-in: we can ignore this feature initially, but it's there when we need it.
• Encryption and compression for free!

A few other advantages, which haven't been fully explored:

• Easy debugging: we can use command-line ssh and sftp sharing a Unix socket as a mock client.
• Extensible: we can open full-duplex streams for arbitrary data, if SFTP and command execution is not enough. This is essentially using SSH as SSL.