Hashing §

Hashing is often used to help balance traffic loads. This can be done by computing a hash of:

• The client IP
• The user ID
• Or any other thing which makes sense for the problem you’re solving

Hash functions could naively work like the hash functions used in storing/retrieving items in Hash Table data structures.

input -> number % server count -> server index

However this would mean that spinning up new servers due to traffic, or a server going down, would result in a complete re-compute of all indexes, resulting in client traffic hitting entirely new servers and a tonne of cache misses.

Consistent Hashing §

This is a type of hashing that minimises the number of remapped keys when a server is added or removed. This therefore minimises the number of requests which end up hitting new servers.

This is implemented in a closest-neighbour way - it can be thought of as distributing traffic across a circle or a hash ring.

Rendezvous Hashing §

This type of hasing is also known as “highest random weight” hashing.

1. Calculate a score for each of your servers
2. Pick the highest one
   1. If your server goes down, pick the second highest
   2. If a new server is added, it doesn’t affect you