What Is an API Schema? The RESTful API design pattern is in use by an overwhelming majority of enterprise software projects that rely on machine-to-machine communication. The OpenAPI Standard, now in its third revision–OAS3–defines the functionality built into RESTful APIs, allowing automatic documentation, creation of test cases, and general “discoverability” of API resource specifications and service definitions. Colloquially, this OpenAPI Standard definition is called an API schema. API schemas are critical in microservice mesh architecture, in client-server architecture, and in a great deal of automated tooling and build system automation. They’re also quite useful for adding another layer of security to your APIs. API schemas give you the ability to define the expected usage of API endpoints, and then let you compare that definition to how they are actually being executed. With this information, you can then find the places where expectation and reality don’t match up and make informed decisions on how to respond.