SaaS architecture when code is cheap

Multi-tenant SaaS architecture rests on a premise that has held for decades: that the most expensive part of delivering software to many customers is writing the code. Every structural decision follows from it. Write once, share as many layers as possible, express customer difference as configuration, fall back to custom code only when configuration runs out. Because the resulting flexibility is fixed when the product is designed, requests outside it tend to be met under deadline with special cases in shared code, and technical debt accumulates.

This paper takes as its working assumption that the premise no longer holds, and follows the consequences. The question that organises the argument is what the product's core value is, stated in terms of the needs of its domain. The answer decides what stays shared, what can move, and how the product can be priced. When producing code becomes cheap, the layer at which a product draws its line between shared and per-customer can move down. Pressure to move it is arriving from three directions at once. Customers want to start using the product without a long onboarding. They want changes to be easy after go-live. They want it personalised below the tenant, down to the individual user, and they increasingly expect their own agents to operate it.