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Software design patterns are an emerging tool for guiding and documenting system design. Patterns usually describe software abstractions used by advanced designers and programmers in their software. Patterns can provide guidance for designing highly scalable distributed systems. Let's see how!

Patterns are in essence solutions to problems. Most of them are expressed in a format called
Alexandrian form which draws on constructs used by Christopher Alexander. There are variants but most look like this:

• The pattern name
• The problem the pattern is trying to solve
• Context
• Solution
• Examples
• Design rationale: This tells where the pattern came from, why it works, and why experts use it

Patterns rarely stand alone. Each pattern works on a context, and transforms the system in that context to produce a new system in a new context. New problems arise in the new system and context, and the next ‘‘layer’’ of patterns can be applied. A pattern language is a structured collection of such patterns that build on each other to transform needs and constraints into an architecture.

The latest POSA book Pattern-Oriented Software Architecture Volume 4: A Pattern Language for Distributed Computing will guide the readers through the best practices and introduce them to key areas of building distributed software systems using patterns.

Consistent hashing is one of those ideas that really puts the science in computer science and reminds us why all those really smart people spend years slaving over algorithms. Consistent hashing is "a scheme that provides hash table functionality in a way that the addition or removal of one slot does not significantly change the mapping of keys to slots" and was originally a way of distributing requests among a changing population of web servers. My first reaction to the idea was "wow, that's really smart" and I sadly realized I would never come up with something so elegant. I then immediately saw applications for it everywhere. And consistent hashing is used everywhere: distributed hash tables, overlay networks, P2P, IM, caching, and CDNs. Here's the abstract from the original paper and after the abstract are some links to a few very good articles with accessible explanations of consistent hashing and its applications in the real world.