System Design FAQ: Top Questions

A Distributed Locking System allows multiple nodes to coordinate safely over shared resources without race conditions — critical for leader election, job scheduling, and safe updates in distributed systems.

📋 Functional Requirements

• Acquire and release lock atomically
• Handle lock expiry (TTL)
• Ensure only one owner per lock at any time
• Optional fencing token to prevent stale lock usage

📦 Non-Functional Requirements

• Low-latency acquire/release (<10ms)
• Highly available and fault-tolerant
• Idempotent unlock operations

🏗️ Core Components

• Lock Store: Redis, ZooKeeper, or etcd
• Lock Client: Sends acquire/release requests
• Leader Election: Optional heartbeat-based monitor

🔐 Redis Locking via Redlock (Python)

import redis
import uuid
import time

client = redis.StrictRedis()
lock_key = "lock:job42"
lock_value = str(uuid.uuid4())
ttl = 5000  # in ms

# Acquire lock
if client.set(lock_key, lock_value, nx=True, px=ttl):
    print("Lock acquired!")

# Release lock
stored = client.get(lock_key)
if stored and stored.decode() == lock_value:
    client.delete(lock_key)
        

🧪 Etcd Locking (gRPC)

etcdctl lock job42
# blocks until lock is acquired; stores lease ID for release
        

🧩 Fencing Token Concept

• Each lock acquisition increases a version counter
• All operations receive a fencing token (monotonic ID)
• Storage or service checks fencing ID for freshness

📄 Lock Table (PostgreSQL Example)

CREATE TABLE locks (
  resource TEXT PRIMARY KEY,
  owner_id TEXT,
  fencing_token BIGINT,
  updated_at TIMESTAMP DEFAULT now()
);
        

📈 Observability

• Lock acquisition wait time
• Abandoned locks cleanup frequency
• Failed release attempts

🧰 Tools/Infra Used

• Redis: Fast atomic SET/DEL support
• etcd: Consensus-based distributed coordination
• ZooKeeper: Hierarchical ephemeral nodes

📌 Final Insight

Distributed locks are tricky — they require TTL, fencing, and consensus strategies for robustness. Prefer libraries like Redlock or use purpose-built coordination stores like etcd when correctness is critical.