Project 9: Record Locking Database

A simple key-value store where concurrent processes can read and write records using fcntl byte-range locks.

Quick Reference

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

What You Will Build

A simple key-value store where concurrent processes can read and write records using fcntl byte-range locks.

Why It Matters

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

Core Challenges

  • Deadlock detection → fcntl with F_SETLKW can deadlock
  • Lock inheritance → What happens across fork()?
  • Advisory nature → Uncooperative processes can ignore locks

Key Concepts

  • Map the project to core concepts before you code.

Real-World Outcome

# Start multiple concurrent clients
$ ./kvstore set key1 value1 &
$ ./kvstore set key2 value2 &
$ ./kvstore get key1 &
$ ./kvstore set key1 updated &

# All complete without corruption
$ ./kvstore get key1
updated
$ ./kvstore get key2
value2

# Verify locking works
$ ./kvstore lock-test key1
Process 1: Acquired write lock on key1
Process 2: Waiting for lock...
(Process 1 holds lock for 2 seconds)
Process 1: Released lock
Process 2: Acquired write lock on key1

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