Software Growth

API (Application programming interface)

An API is a defined way for one piece of software to ask another for data or actions, so products can connect without exposing their internals.

API stands for application programming interface. Red Hat describes it as a set of definitions and protocols for building and integrating application software. In plain terms, it is a contract: send a request in an agreed format and you get a predictable response, without knowing how the other system works inside.

How an API works in a SaaS product

Your app is a server that holds data (customers, invoices, projects). Your own web app already talks to it through an internal API. A public API simply lets your customers, or their other tools, do the same thing with code.

Most SaaS APIs today are REST over HTTPS and return JSON. A request looks like "give me invoice 1042" and the response is a JSON document. Authentication is usually an API key or an OAuth token, so you know who is asking and what they may see.

Why customers ask for one

  • Integrations. They want your data in their CRM, warehouse or spreadsheet.
  • Automation. They want to create records from their own systems instead of clicking through forms.
  • Exit and safety. A good API is a way to get data out, which lowers fear of vendor lock-in.
  • Procurement. Larger buyers often put "has an API" on the checklist.

API vs webhook vs SDK

These three are often confused. With an API, the customer's code asks you for something. With a webhook, you call the customer's server when something happens, so they do not have to keep asking. An SDK is a library that wraps your API for one programming language so developers write less boilerplate. Red Hat also notes that webhooks enable event-driven communication between systems, as an alternative to polling an API.

What it costs you

The first version is cheap if your app already has a clean back end. The expensive part is the promise. Once a customer builds on an endpoint, you cannot rename a field or change a response without breaking them. That means versioning, documentation, rate limits, error messages that make sense, and support for developers who are not your usual users.

Stripe is the standard example: its docs and stable behavior are part of the product. You do not need that level on day one, but you do need to decide what you will keep stable.

When a small SaaS should care

Build a public API when three or more customers ask for the same integration, or when your product only becomes valuable once it connects to other tools. Before that, a Zapier integration or CSV export often covers the need at a fraction of the cost. If integrations are central to how you win, consider designing API-first. Keep a sandbox in mind for customers who need to test safely.

Related terms

Sources

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