Modern Data Consistency - Consistent Hashing In Disk Partition & Database

🧾 Goal

• Simulate how keys are mapped to partitions (nodes) using Consistent Hashing.
• Then simulate what happens when you add a new partition.
• Show how only some keys are remapped (not all, unlike naive hash % N).

⚙️ Setup

• Hash Ring Range: 0 - 99 (for simplicity)
  (Real systems may use a space like 0 to 2³² or 2⁶⁴, but 0–99 makes it easier to follow)

🧱 Initial Partitions (Nodes)

Node	Hash Position
A	10
B	40
C	70

🧑 Sample Keys (Users)

Key	Hash(key) (simulated)
alice	15
bob	43
charlie	67
dave	84
eve	5

✅ Step 1: Place Keys on Ring and Assign to Nodes

Rules

• Keys go to the next node clockwise.
• If a key’s hash exceeds the highest node, it wraps around to node at the lowest position.

🔗 Assignment

Key	Hash	Assigned Node	Reason
eve	5	A (10)	5 → next ≥ 10
alice	15	B (40)	15 → next ≥ 40
bob	43	C (70)	43 → next ≥ 70
charlie	67	C (70)	67 → next ≥ 70
dave	84	A (10, wrap)	84 → wrap → next ≥ 10

📊 Initial Mapping Summary

Node	Keys Assigned
A	eve, dave
B	alice
C	bob, charlie

➕ Step 2: Add New Node D at Hash Position 50

🆕 Updated Nodes

• A (10)
• B (40)
• D (50)
• C (70)

🔄 Recalculate Assignments

Key	Hash	New Assigned Node	Previously
eve	5	A (10)	A
alice	15	B (40)	B
bob	43	D (50)	C ✅ moved
charlie	67	C (70)	C
dave	84	A (10, wrap)	A

📉 Impact of Adding Node D

Key	Moved?
bob	✅ Yes (C → D)
Others	❌ No

Only one key (bob) was remapped — this demonstrates the efficiency of consistent hashing.

📌 Visual Ring Representation (Simplified)

  0           25           50           75          99
   +-----------+------------+------------+-----------+
   |           |            |            |           |
 [A-10]     [B-40]      [D-50]       [C-70]        wrap
   ^           ^            ^            ^
   |           |            |            |
 eve,dave    alice         bob        charlie

🧠 Why Consistent Hashing is Great

Feature	Benefit
Key hash independent of N	Stable unless key changes
Only a subset of keys move	Scales easily
Balanced load (with vnodes)	More even distribution

🧮 How Do We Get Node D at Hash Position 50?

✅ Short Answer

The node’s hash position is calculated using the same hash function used for keys, applied to the node’s unique identifier (e.g., "NodeD").

🔍 Step-by-Step

1. Choose an Identifier

• "NodeD"
• Or something like "192.168.0.4:8080" or "server-004"

2. Apply the Hash Function

# Simplified example
hash("NodeD") % 100  → 50

This gives Node D a position of 50 on the ring.

3. Use Consistent Hash Function

import hashlib

def get_ring_position(node_id, ring_size=100):
    hash_bytes = hashlib.md5(node_id.encode()).digest()
    hash_int = int.from_bytes(hash_bytes, byteorder='big')
    return hash_int % ring_size

get_ring_position("NodeD")  # → e.g., 50

🎯 Bonus: Why Use Virtual Nodes?

Node	Virtual Node Positions
Node D	50, 73, 91

• Using virtual nodes (multiple positions per node) helps avoid uneven load.
• This is especially useful if nodes end up clustered in the hash space.

🌍 Real-World Uses of Consistent Hashing

Consistent hashing is widely used in modern distributed systems for scalability, fault tolerance, and load balancing.

✅ Distributed Databases & Caches

🔹 Amazon DynamoDB

• Uses consistent hashing to partition data and support horizontal scaling.

🔹 Apache Cassandra

• Each node owns a range on the ring.
• New nodes join by taking part of the range, minimal key movement.

🔹 Memcached

• In multi-node deployments, consistent hashing ensures that only a small subset of keys are remapped when a node is added/removed.

🌐 Content Delivery Networks (CDNs)

🔹 Akamai, Cloudflare

• Use hashing to assign content or requests to edge servers.
• Minimal rebalance on server change.

⚖️ Load Balancers

🔹 NGINX, Envoy

• Hash-based load balancing (e.g., hash by IP) supports sticky sessions.
• Adding/removing backends only affects affected client hashes.

🗄️ Object Storage Systems

🔹 Ceph, OpenStack Swift

• Consistent hashing determines where objects are stored across many disks/nodes.

🔁 Service Mesh / Routing

🔹 Istio, Linkerd

• Traffic routing based on consistent hashing of keys (like user IDs) to maintain session stickiness or shard requests.

📬 Pub/Sub and Messaging Systems

🔹 Apache Kafka

• Partitions are often chosen by key-based hashing.
• Consumers scale predictably with minimal rebalance.

🧠 DNS Load Balancing

🔹 Netflix, Google

• Use hash-based routing to send users to the same edge nodes or POPs based on IP or region.

---

🧠 Why It’s So Popular

Feature	Benefit
Minimal remapping	Only affected keys move
Scalable	Add/remove nodes easily
Predictable	Same key → same node
Fast	Hashing is computationally cheap
Stateless	No central coordinator needed

✅ Final Summary

• Each key is hashed into a number (0–99).
• It’s then assigned to the next node clockwise.
• When you add a new node, only some keys (between previous and new node) are remapped.