SOA is a predecessor to microservices. It is an architectural style that treats services as independent, heterogeneous providers of business capabilities.

“Heterogeneity” here acknowledges that services may:

• belong to different vendors
• run on different platforms
• be written in different languages
• communicate over different protocols
• be developed by different teams

SOA emphasizes:

• well-defined, explicit service interfaces
• stability and genericity of contracts to serve multiple consumers over long periods of time
• service discovery via service registries
• centralized service administration with approval of contracts, schemas, and compatibility guarantees

The focus on central governance and long-lived, broadly reusable contracts is the key distinction between SOA and modern microservices. It is also its key limitation factor: services cannot evolve quickly because of dependence on central governance and strictness of agreed-upon interfaces.