Eric Evans on Monolith v. Microservices -- in 2004. #readingToday

The goal of the most ambitious enterprise system is a tightly integrated system spanning the entire business. Yet the entire business model for almost any such organization is too large and complex to manage or even understand as a single unit. The system must be broken into smaller parts, in both concept and implementation. The challenge is to accomplish this modularity without losing the benefits of integration, allowing different parts of the system to interoperate to support the coordination of various business operations. A monolithic, all-encompassing domain model will be unwieldy and loaded with subtle duplications and contradictions. A set of small, distinct subsystems glued together with ad hoc interfaces will lack the power to solve enterprise-wide problems and allows consistency problems to arise at every integration point. The pitfalls of both extremes can be avoided with a systematic, evolving design strategy.

Domain-Driven Design, Eric Evans