The Anna Karenina Principle says “All happy families are alike; each unhappy family is unhappy in its own way.”  The same is true for software scalability.  All scalable systems are alike; each unscalable system is unscalable in its own way.

Scalable Systems Scale Linearly

Scaling software systems means doing more work with more resources.  As software scales various state management issues will require ever more resources.  In the beginning processing 2x more requests will require less than 2x more resources.  Over time, the ratio will become 1-to-1 and continue to slide.  2x more requests will require 4x more resources.

So long as the ratio is linear, the system is scalable.  When the ratio becomes exponential, for example 2x more requests require x^2 more resources, the system is no longer scalable.

Scalable Systems Are Fault Tolerant

A system with 5 nines will process 99.999% of all events correctly.  Failures are 1 in a million.  A system doing 1 million events per second will have a failure every second.

Scalable systems have defined ways of handling failure – they will return an error and make the caller handle it, they will do retries, they may even fail silently.  What they won’t do is crash or error out.

Scalable Systems Are Idempotent

Exactly once delivery is nearly impossible.  As a system scales, it becomes inevitable that some events will be processed multiple times.

Scalable systems are idempotent and indifferent to how many times an event is processed.  It doesn’t matter how many times an event enters the system, once, twice, 100 times, it is all the same.

Non-idempotent systems are much easier to build, but they will fail in all kinds of different ways as customers and networks send in duplicate events.

Scalable Systems Have Minimal Human Interaction

Humans are expensive and do not scale.  The more human intervention a system requires, the less scalable it is.

Scalable systems do not require humans to deploy code, scale up or down, or watch the logs.

Conclusion

Remember, all scalable systems are alike:

1. Linear Scaling
2. Fault Tolerant
3. Idempotent
4. Require Minimal Human Interaction

Systems that are not scalable are not scalable in their own unique way.

If you don’t know why your system isn’t scalable, these 4 points are a great place to start looking.