When you remove a server from your cluster, should you rehash and move 50% of your data? Or just 5%? Consistent hashing solves the painful problem of data ownership in distributed systems.

The Distributed Storage Scenario

You built a key-value store. Think Redis, but distributed across multiple servers.

Setup:

Proxy (routes requests)
  ↓
Storage Node 0
Storage Node 1  
Storage Node 2

Users send requests: "Store key K1 with value V1" or "Get value for key K3."

Question: Which node should store which key?

Hash-Based Routing: The Simple Approach

Algorithm:

hash(key) % number_of_nodes = node_index

Example:

hash("K1") % 3 = 2 → Store on Node 2
hash("K2") % 3 = 0 → Store on Node 0
hash("K3") % 3 = 1 → Store on Node 1
hash("K4") % 3 = 2 → Store on Node 2
hash("K5") % 3 = 1 → Store on Node 1
hash("K6") % 3 = 0 → Store on Node 0

Distribution:

Node 0: K2, K6
Node 1: K3, K5
Node 2: K1, K4

This works perfectly.

Every key has an owner. All requests for K3 always go to Node 1 because the hash function output never changes.

Why This Matters: Statefulness

Stateless systems (like API servers):

Any server can handle any request. User logs in? Any server works. They hold no data.

If server crashes, send request to another server. No problem.

Stateful systems (like storage nodes):

Data lives on specific servers. K3 exists only on Node 1.

If you send request for K3 to Node 0, it does not have the data. Miss.

Ownership matters.

The Disaster: Removing a Node

Your system is over-provisioned. Traffic dropped. You want to remove Node 2 to save costs.

Before removal:

hash(key) % 3 = node

After removal (now 2 nodes):

hash(key) % 2 = node

The hash function changed.

What Happens to Your Data?

Let us recalculate where every key belongs now:

hash("K1") % 2 = 0 → Now belongs to Node 0 (was Node 2)
hash("K2") % 2 = 0 → Still Node 0 (no change)
hash("K3") % 2 = 1 → Still Node 1 (no change)
hash("K4") % 2 = 0 → Now belongs to Node 0 (was Node 2)
hash("K5") % 2 = 1 → Still Node 1 (no change)
hash("K6") % 2 = 0 → Still Node 0 (no change)

Keys that moved: K1, K4 (2 out of 6 = 33%)

Keys that stayed: K2, K3, K5, K6 (4 out of 6 = 67%)

The Expensive Rebalancing Process

Before you can remove Node 2, you must:

Step 1: Iterate through ALL keys in the system

Step 2: Recalculate ownership with new hash function (% 2 instead of % 3)

Step 3: Move data between nodes:

• Copy K1 from Node 2 → Node 0
• Copy K4 from Node 2 → Node 0

Step 4: Verify all data transferred correctly

Step 5: Only then remove Node 2

The Real-World Impact

Scenario: 1 billion keys, each 1 KB.

Data to move: 333 million keys = 333 GB of data transfer.

Time: At 100 MB/s network speed = 55 minutes of intensive data movement.

During this time: System under heavy load. Risk of failures. Users may experience slowness.

Cost: Network bandwidth, CPU for hashing, disk I/O. Expensive.

The Core Problem

Traditional hashing tightly couples ownership to the number of nodes.

Change node count → Hash function changes → Ownership changes globally → Massive data movement.

For stateless systems (API servers): No problem. They hold no data.

For stateful systems (databases, caches): This is catastrophic.

The Million Dollar Question

Can we change the number of nodes without rehashing everything?

Can we minimize data movement to only the affected keys, not ALL keys?

This is what consistent hashing solves.

Why Companies Need This

Amazon DynamoDB: Billions of keys. Adding/removing nodes daily for auto-scaling.

Netflix CDN: Thousands of edge servers. Servers crash. New servers spin up constantly.

Redis Cluster: Distributed cache. Needs to scale without downtime.

All face the same problem: Traditional hashing forces massive data movement.

Consistent hashing: Moves only a tiny fraction of data when nodes change.

This saves millions in operational costs and prevents service disruptions.

The Big Idea

Instead of hash values mapping directly to node numbers, we place both keys and nodes on the same "ring."

Keys find their owner by moving clockwise on the ring.

Building the Ring: Step-by-Step

Step 1: Define the Hash Space

Pick a hash function. Let us use one that outputs 0 to 15 (simplified from real 0 to 2^128).

This range forms a circle:

        0
   15       1
14             2
13             3
  12       4
     11 5
    10   6
      9 7
        8

Numbers wrap around: After 15 comes 0 again. Hence, "ring."

Step 2: Place Nodes on the Ring

Each storage node gets hashed and placed on the ring.

Node 0: hash(Node_0_IP) = 3 → Place at position 3

Node 1: hash(Node_1_IP) = 12 → Place at position 12

Node 2: hash(Node_2_IP) = 8 → Place at position 8

Ring now looks like:

Positions: 0, 1, 2, [3-Node0], 4, 5, 6, 7, [8-Node2], 9, 10, 11, [12-Node1], 13, 14, 15

Step 3: Place Keys on the Ring

User wants to store key K1.

Hash K1: hash("K1") = 0 → Place at position 0

Who owns K1?

From position 0, move clockwise until you hit a node.

First node clockwise from 0? Node 0 at position 3.

K1 is owned by Node 0.

More Examples

Key K2: hash("K2") = 10 → Position 10

Move clockwise from 10 → First node is Node 1 at position 12

K2 owned by Node 1.

Key K3: hash("K3") = 5 → Position 5

Move clockwise from 5 → First node is Node 2 at position 8

K3 owned by Node 2.

Key K4: hash("K4") = 14 → Position 14

Move clockwise from 14 → Pass 15, wrap to 0, 1, 2, hit Node 0 at position 3

K4 owned by Node 0.

Ownership Summary

Node 0 (position 3): Owns keys in range [13, 14, 15, 0, 1, 2]
Node 2 (position 8): Owns keys in range [4, 5, 6, 7]
Node 1 (position 12): Owns keys in range [9, 10, 11]

Each node owns the keys between itself and the previous node (moving counter-clockwise).

The Critical Insight

Ownership is determined by position on the ring, not by the number of nodes.

When you add or remove a node, only keys near that position are affected.

Most keys stay with their original owners.

What Consistent Hashing Actually Is

Not a service. Not a magical distributed system component.

It is a function: Given a key, return which node owns it.

Implementation: A sorted array of nodes. Binary search to find the owner. That is it.

The proxy (the component routing requests) calls this function:

node = consistent_hash.get_owner("K1")
send_request_to(node)

Simple.

Why the Ring Visualization?

Visually intuitive: Easy to see how keys and nodes relate.

Wrapping behavior: After the last node comes the first node again.

But in code? Just a sorted array and binary search. No linked list. No actual ring structure.

The ring is a mental model, not an implementation detail.

What Consistent Hashing Actually Does

Myth: Consistent hashing is a distributed service that magically handles data movement.

Reality: It is a function. A simple function that answers one question.

Question: "Given key K, which node owns it?"

Answer: "Node N."

That is all it does.

Where the Function Lives

In the proxy/router component.

When a request comes in:

user sends: GET key "K1"
  ↓
proxy calls: node = consistent_hash.get_owner("K1")
  ↓
proxy forwards request to that node
  ↓
node returns value
  ↓
proxy sends response to user

No separate service. No external calls. Just a local function.

The Implementation Truth

Visualized as a ring for understanding.

Implemented as a sorted array.

Example:

nodes = [
  {position: 3, node: Node_0},
  {position: 8, node: Node_2},
  {position: 12, node: Node_1}
]

Finding owner:

key_position = hash("K1") = 0

Binary search in nodes array for first position >= 0
→ Found position 3 → Node_0

Return Node_0

Time complexity: O(log N) where N = number of nodes.

Fast. Simple. No magic.

Amazon DynamoDB: The Real Deal

Problem: Distributed NoSQL database. Petabytes of data. Thousands of nodes.

Solution: Consistent hashing for partitioning.

How it works:

Each table partitioned by hash of partition key.

Partitions distributed across nodes using consistent hashing.

When scaling:

Add node → Only adjacent partition moves.

Remove node → Data migrates to next node.

Result: DynamoDB can scale to millions of requests/second without downtime.

Netflix: Content Delivery at Scale

Problem: 200 million users, thousands of edge servers worldwide.

Solution: Consistent hashing for routing video content.

How it works:

Each video file identified by unique ID.

Edge servers placed on consistent hash ring.

User requests video → System hashes video ID → Routes to nearest edge server on ring.

When server fails:

Hash ring updated → Affected videos reroute to next server.

Only that server range affected. Rest of system unaffected.

Memcached / Redis Cluster: Distributed Caching

Problem: Cache too large for one server. Need to distribute.

Solution: Consistent hashing to partition cache keys.

How it works:

Application hashes cache key → Determines which cache server to query.

Example:

key = "user:12345:profile"
hash(key) = 8
→ Query cache server at position 8 on ring

When cache server added:

Only keys in affected range move to new server.

Cache hit rate barely affected.

Content Delivery Networks (CDNs)

Problem: Billions of files, thousands of servers globally.

Solution: Consistent hashing to distribute content.

How it works:

File hashed → Stored on server owning that hash range.

User requests file → CDN routes to server owning that file.

When server added/removed:

Minimal content redistribution.

Users see no disruption.

The Common Pattern

All these systems:

1. Need to distribute data/requests across many servers
2. Servers frequently added/removed (auto-scaling, failures)
3. Cannot afford massive data movement on every change
4. Use consistent hashing to minimize disruption

What Consistent Hashing Does NOT Do

Does not transfer data for you.

You must implement data movement logic.

Does not detect failures.

You need health checks and monitoring.

Does not balance load perfectly.

Some nodes may get more keys than others (virtual nodes help with this).

Does not store anything.

It is stateless. Just a function call.

What You Must Implement

Data replication: Consistent hashing tells you primary owner. You decide replication strategy.

Failure handling: When node fails, you must trigger data movement to next node.

Monitoring: Track data distribution, detect hot spots.

Load balancing: Use virtual nodes (multiple positions per physical node) for even distribution.

Virtual Nodes: The Load Balancing Trick

Problem: With few nodes, distribution can be uneven.

Example: Node 0 owns range 13-3 (large). Node 1 owns range 9-12 (small).

Solution: Each physical node placed at multiple positions on ring.

Example:

Node 0 at positions: 3, 7, 15
Node 1 at positions: 5, 10, 12
Node 2 at positions: 1, 8, 14

Result: More even distribution of keys across physical nodes.

This is what production systems use.

The Core Takeaway

Consistent hashing is not magic.

It is an elegant algorithm that solves one specific problem: minimizing data movement when cluster size changes.

It is a tool in your distributed systems toolbox.

Use it when you need to distribute data across many nodes and scale frequently.

But remember: You still need to implement the operational aspects—replication, failure handling, monitoring.

Consistent hashing just tells you who owns what. The rest is up to you.

Keys in this range: K1 (position 0), K4 (position 14)

Before: K1 and K4 owned by Node 0

After: K1 and K4 now owned by Node 3

Data Movement Required

Keys to move: K1, K4 (2 out of 6 keys = 33%)

Keys unchanged: K2, K3, K5, K6 (4 out of 6 keys = 67%)

Compare to traditional hashing: Would have moved 50% of keys.

Consistent hashing moved only 33%.

Operational Steps

Step 1: Identify the node to the right of Node 3 → Node 0

Step 2: Take a snapshot of Node 0 (contains K1, K4)

Step 3: Create Node 3 using this snapshot

Step 4: Add Node 3 to the ring (update the sorted array in proxy)

Step 5: Delete keys from Node 3 that no longer belong to it (keys in positions > 1 and < 3)

Step 6: Start serving traffic

No downtime. No complex coordination. Simple copy-paste.

Scaling Down: Removing Node 0

Traffic dropped. Over-provisioned. Remove Node 0.

Step 1: Identify affected range

Node 0 at position 3 is being removed.

Keys it owned: Positions 13, 14, 15, 0, 1, 2

Step 2: Find the next node clockwise

Next node after position 3 → Node 2 at position 8

Step 3: Move data

Copy all keys from Node 0 to Node 2.

Keys moved: K1, K4

Step 4: Remove Node 0 from the ring

Update the sorted array in proxy. Remove Node 0.

Step 5: Shut down Node 0

Data Movement Required

Keys to move: K1, K4 (2 out of 6 keys = 33%)

Keys unchanged: K2, K3, K5, K6 (4 out of 6 keys = 67%)

Traditional hashing: Would have rehashed all 6 keys.

Consistent hashing: Moved only 2 keys.

The Real-World Scale

Amazon DynamoDB: 1 billion keys, remove 1 node out of 100.

Traditional hashing: Rehash 1 billion keys, move ~500 million.

Consistent hashing: Move ~10 million keys (1% of total).

50× reduction in data movement.

Why This Matters

Faster scaling: Minutes instead of hours.

Lower risk: Less data movement = less chance of errors.

No downtime: Gradual rebalancing while system serves traffic.

Cost savings: Less network bandwidth, less CPU, less operational complexity.

The Elegant Property

Adding/removing a node only affects keys in the range between that node and the adjacent node.

All other keys remain untouched.

This locality is what makes consistent hashing revolutionary.