UML: Data Model of Service
Before implementing a service, it is important to construct concrete a data model, definitions and relationships between entities. Without a concrete data model, design and implementation of service are highly likely to be changed during development, resulting in an unstable product and risk of missing feature.
To design a class diagram, you can use one of software (or others):
OpenAPI: Protocol between Backend and Frontend
After designing data model, a developer needs to implement backend service provider and frontend service request. OpenAPI enables a developer to define a protocol: which paths, parameters and body contents to use, how responses will be.
To design OpenAPI, you can use one of software (or others):