What Is API (Application Programming Interface)?

API (Application Programming Interface) Explained

An API is fundamentally an agreement about boundaries. It says: here is what you may ask me to do, here is the shape of the request, here is the shape of the answer, and here is what happens when something goes wrong. Everything behind that boundary - the database schema, the language, the deployment - can change without breaking callers, provided the contract holds. That decoupling is why APIs replaced direct database access as the sanctioned way to integrate with ERP and other systems of record.

A usable API specifies several things beyond operations. Authentication establishes who is calling, typically through OAuth 2.0 flows, API keys, or mutual TLS in higher-assurance environments. Authorization determines what that caller may see, which in ERP contexts must respect the same site, entity, and field-level restrictions a user would face. Rate limits protect the backing system from being overwhelmed. Pagination governs how large result sets are traversed. Error semantics define which failures are retryable and which are permanent.

In the ERP world, APIs sit on top of decades of existing application logic rather than replacing it. Infor SyteLine exposes its business logic through IDOs, the Intelligent Data Objects that encapsulate data access and business rules, callable through web service interfaces; Infor OS provides an API gateway that fronts those and other services with consistent security. Infor LN and Baan expose business object interfaces and generated service definitions. Salesforce and ServiceMax expose both REST and SOAP interfaces with governor limits that shape integration design.

Calling an API is not the same as integrating correctly. ERP APIs enforce business rules, and an integration that bypasses them by writing directly to tables will eventually corrupt data in ways that surface as inventory variances or unbalanced ledgers months later. Equally, an API that is called in a tight loop per record, when a bulk operation exists, will hit rate limits and lengthen batch windows. Good ERP integration design chooses the right granularity, respects transaction boundaries, and handles partial failure explicitly.

Versioning and lifecycle discipline determine long-term cost. Additive changes such as new optional fields are usually safe; removing a field, changing a data type, or tightening validation is breaking. Mature programs publish versioned endpoints, maintain deprecation timelines, and document changes in a specification such as OpenAPI so consumers can generate clients and detect drift. In regulated environments, the API contract and its change history also become audit evidence for how data moved between systems.