Project 10: POSIX Semaphore Connection Pool

A database connection pool manager using POSIX counting semaphores to limit concurrent connections.

Quick Reference

Attribute Value
Primary Language C
Alternative Languages N/A
Difficulty Level 3 (Advanced)
Time Estimate See main guide
Knowledge Area Resource Management, Concurrency
Tooling See main guide
Prerequisites See main guide

What You Will Build

A database connection pool manager using POSIX counting semaphores to limit concurrent connections.

Why It Matters

This project builds core skills that appear repeatedly in real-world systems and tooling.

Core Challenges

  • sem_timedwait → Handling connection timeout
  • Process-shared semaphores → Allocating in shared memory
  • Cleanup on crash → Returning connections when process dies

Key Concepts

  • Map the project to core concepts before you code.

Real-World Outcome

$ ./pool_manager --max-connections=5 &
Pool manager started (5 connections available)

# Spawn 10 workers, each needs a connection
$ for i in {1..10}; do ./worker $i & done

Worker 1: Acquired connection (4 remaining)
Worker 2: Acquired connection (3 remaining)
Worker 3: Acquired connection (2 remaining)
Worker 4: Acquired connection (1 remaining)
Worker 5: Acquired connection (0 remaining)
Worker 6: Waiting for connection...
Worker 7: Waiting for connection...
Worker 1: Released connection (1 remaining)
Worker 6: Acquired connection (0 remaining)
...

All workers completed successfully

Implementation Guide

  1. Reproduce the simplest happy-path scenario.
  2. Build the smallest working version of the core feature.
  3. Add input validation and error handling.
  4. Add instrumentation/logging to confirm behavior.
  5. Refactor into clean modules with tests.

Milestones

  • Milestone 1: Minimal working program that runs end-to-end.
  • Milestone 2: Correct outputs for typical inputs.
  • Milestone 3: Robust handling of edge cases.
  • Milestone 4: Clean structure and documented usage.

Validation Checklist

  • Output matches the real-world outcome example
  • Handles invalid inputs safely
  • Provides clear errors and exit codes
  • Repeatable results across runs

References

  • Main guide: UNIX_IPC_STEVENS_VOL2_MASTERY.md
  • Primary references are listed in the main guide