API

What is an API?

API (Application Programming Interface) is an agreed interface through which two software systems can communicate and exchange data — without needing to know the internal implementation of the other side. In the modern cloud B2B world, APIs are typically implemented via REST (Representational State Transfer) or GraphQL over HTTPS, with data in JSON format.

A typical anatomy of a REST API call:

• Method — GET (read), POST (create), PUT / PATCH (update), DELETE (delete)
• Endpoint — URL, e.g. https://api.modulario.eu/v1/invoices/12345
• Authentication — typically Bearer token (OAuth 2.0) or API key in the header
• Body — JSON payload for POST/PUT
• Response — HTTP status code + JSON response

A well-designed API has:

• Versioning (/v1/, /v2/) — for backward compatibility
• Rate limiting — restricting the number of calls per minute/hour
• Idempotency — repeating a call with the same ID does not create a duplicate
• Error handling — predictable error structures
• Pagination — for large datasets

In B2B context, an API allows, for example, connecting an online store (WooCommerce, Shopify) to an ERP — an order from the store is automatically created in the ERP via API, and stock levels are synchronised back to the web via API.

When it is used

APIs are a necessity today — without an open API, modern ERP/CRM software is difficult to sell because it cannot participate in the integration ecosystem.

See the API page, the Workflows module, and integrations.

Related terms

• Webhook — push notification from an API. See /en/glossary/webhook.
• SaaS — API is the standard interface of SaaS. See /en/glossary/saas.
• MCP — an AI-friendly layer on top of an API. See /en/glossary/mcp.
• Workflow automation — uses APIs to connect systems. See /en/glossary/workflow-automation.

In Modulario

Modulario provides a complete REST API for every module — authentication via OAuth 2.0 and API keys, rate limiting, idempotency. Documentation, examples, and Postman collections at /api.

The Modulario API has a public OpenAPI 3.1 specification, auto-generated clients for Python, TypeScript, PHP, and Java, a sandbox environment for testing, and detailed examples for typical scenarios (creating an invoice, synchronising customers, downloading a report).