Interview Use
Use managed systems
Most modern distributed systems handle sharding and data distribution for you.
Go deep for infrastructure
Distributed databases, caches, and message brokers are the common from-scratch prompts.
Avoid custom implementation
Most interviews only need to acknowledge that existing solutions handle this complexity.
Distribution Choice
- Modulo Hashing: Use hash(key) % node count; adding or removing a node redistributes most data.
- Consistent Hashing: Hash keys and nodes onto a ring; walk clockwise to the next node to minimize redistribution.
Hash Ring
Lookup Flow
1
Create the hash ring
Use a fixed circular hash space for both data and servers.
2
Place server nodes
Map each database, cache, broker, or app server to positions on the ring.
3
Route each key
Hash the key, find its point, then move clockwise until a node is found.
Membership Changes
- Adding a Node: Only the new node's claimed range moves; all other keys stay put.
- Removing a Node: Only keys mapped to the removed node move; they remap to the next clockwise node.
Load Balance
Virtual Node Effects
- Virtual Nodes: Hash multiple variations of each database name to place it at many ring positions.
- Failure Spread: A failed node's ranges redistribute across multiple remaining databases, not one neighbor.
- New Node Spread: A new database absorbs small chunks from multiple existing nodes from the start.
Hot Spot Fixes
- Read Replicas: Most common approach; replicate hot keys and load-balance reads across nodes.
- Key-Space Salting: Append random suffixes to hot keys so reads scatter across nodes and aggregate.
- Adaptive Rebalancing: Operationally complex; monitor traffic and move specific key ranges off overloaded nodes.
Real World
- Apache Cassandra: Uses consistent hashing to distribute data across the ring.
- Amazon DynamoDB: Uses consistent hashing under the hood for partition placement.
- Content Delivery Networks (CDNs): Use consistent hashing to choose which edge server should cache content.
Your account is free and you can post anonymously if you choose.