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Project Overview

What is PCS

PCS (Pacemaker/Corosync Configuration System) is a tool for configuring and managing HA (High Availability) clusters based on Pacemaker and Corosync. See README.md for technical details.

Context

PCS operates in the HA cluster ecosystem:

  • Pacemaker — cluster resource manager
  • Corosync — cluster communication layer
  • PCS — configuration and management interface (CLI + daemon)

Users interact with PCS to configure cluster nodes, resources, constraints, and other aspects rather than editing raw XML and other configuration files.

Evolution

The project has a long history. Requirements, understanding, and vision have evolved over time. To maintain stability and continuity with limited resources, we've had to accept legacy code alongside new approaches.

Ruby to Python migration

Ruby was used early in the project. Maintaining two language ecosystems is costly, so we're gradually migrating to Python-only.

Current state:

  • Main codebase is Python
  • Legacy Ruby daemon still runs alongside the Tornado daemon (see architecture/daemon.md)
  • Ruby code is being phased out, not expanded

Ruby code as a knowledge source: While Ruby code should not be used as a template for new Python code (different idioms, different architecture), it often contains valuable domain knowledge accumulated over years of development and user feedback — edge case handling, retry logic, authentication details, error conditions. When implementing Python replacements for Ruby functionality, review the corresponding Ruby code for what concerns it addresses, not for how it implements them.

APIv2 migration

APIv2 is replacing the older synchronous API layers. See architecture/daemon.md for details on API layers and the APIv2 architecture.

Why:

  • Enables async request processing
  • Modern HTTP API patterns

Current state:

  • APIv2 implementation exists and is being expanded
  • Old synchronous APIs still present
  • Migration ongoing, not complete

Git Branches

  • The project has multiple production branches: 'main' branch and several 'pcs-$version' (e.g. pcs-0.10, pcs-0.11, pcs-1.x) branches.
  • A branch which is supposed to be merged to a 'pcs-$version' branch is supposed to be prefixed with that branch name, e.g. pcs-0.11_new-feature. A branch which is supposed to be merged to the main branch is not prefixed like this.

Development philosophy

See development-principles.md for detailed guidelines.

Key points:

  • Stability first — breaking changes are avoided when possible
  • Compatibility matters — we depend on Linux, Pacemaker, Corosync, Python, and (temporarily) Ruby
  • Pragmatic evolution — we improve architecture while keeping the app functional, so not all code reflects the target vision