The SOA/web services were not ideal for exposing business functionality as APIs. This was due to the complex nature of web service-related technologies in which SOAP is used as a message format for service-to-service communication. SOAP was also used for securing web services and service-to-service communication, as well as for defining service discovery metadata. SOAP lacked a self-service model, which hindered the development of an ecosystem around it.

We use application programming interface (API), as a term, to expose a service over REST (HTTP/JSON) or a web service (SOAP/HTTP). An API gateway was typically built on top of existing SOA/ESB implementations for APIs that could be used to expose business functionality securely as a managed service. This can be seen in the following diagram:

In the preceding diagram, the API gateway is used to expose the three-tier and SOA/ESB-based services in which the business logic contained in the ESB still hinders the development of the independent services. 

With containerization availability, the new paradigm of microservices started to evolve from the SOA/ESB architecture in 2012 and seriously took off in 2015.