Composability

Composability is the property of a system whose components can be combined to produce new functions that are predictable from the properties of the components and the rules of combination. It is the operational form of modularity: where modularity describes structure, composability describes behavior.

Composability is rare. Most systems are modular in structure but not composable in practice, because the interactions between components produce emergence that violates the composition rule. The financial system is modular in its instruments and institutions, but the 2008 crisis demonstrated that the system was not composable: the risk of a portfolio of mortgage-backed securities could not be computed from the risks of the individual securities.

The design challenge is to build systems that are both modular and composable — systems in which composition preserves safety properties, performance guarantees, and semantic coherence. This is the goal of formal verification in software, of standardization in engineering, and of protocol design in distributed systems.