What are Internal APIs?

What are internal APIs?

An internal API, also called a private API, is an interface that only a company's own systems, teams and applications are meant to call. It is not published for outside developers. It lets the billing service talk to the user service, or a mobile app talk to its backend, through a documented contract instead of shared databases or one-off code.

How internal APIs work

An internal API is an ordinary API: usually HTTP with JSON (REST) or gRPC, with endpoints, authentication and a versioned contract. What makes it internal is who may call it, not the technology. Access is limited by network placement (a private network or VPN), by an API gateway, and by credentials issued to services and teams. Good setups use all three. Internal APIs are the glue of microservices: each service owns its data and shares it with others only through its API.

Three audiences are usually distinguished: internal APIs for your own teams, partner APIs for named business partners under an agreement, and public APIs for anyone who signs up.

Example: checkout in an online shop

A shop runs separate order, inventory and payment services. When a customer checks out, the order service calls GET /inventory/items/42 on the inventory service and POST /payments on the payment service. Neither is exposed to the internet, and the checkout page only talks to the order service. When another team builds a returns service, it reads the same contracts instead of asking for a database export.

Common mistakes

• Treating internal as safe. Network placement is not authorization. If one service is compromised, every internal API without its own checks is open. The OWASP API Security Top 10 (2023) lists Broken Object Level Authorization and Improper Inventory Management, and both apply inside the perimeter. See API security.
• No inventory. Old versions, test endpoints and undocumented routes pile up. That is API sprawl, and API discovery is how you find what already exists.
• No contract or versioning. A field renamed by one team breaks another team's service without any warning.
• No limits. One noisy internal client can exhaust a shared service. Set rate limits and timeouts between services too.
• Testing only the front door. Public pages get tested, the calls behind them do not.

Testing and monitoring internal APIs

Internal APIs fail in chains: a slow inventory call makes checkout slow. Check each endpoint's status code, response body and response time, and test it under concurrent load. LoadFocus API monitoring runs such checks on a schedule (every 5 minutes on the free plan) and alerts you on failures, and load testing puts concurrent users on an endpoint. An endpoint that is only reachable inside a private network cannot be checked from the public internet, so point checks at a staging copy or a protected route that is reachable. For authenticated endpoints, see the guide to API performance testing with OAuth2.

FAQ: Internal APIs

Are internal APIs the same as private APIs?

In most usage, yes. Both terms mean an API meant only for the organization's own systems and teams.

Do internal APIs need authentication?

Yes. Being on a private network is not enough. Services should prove who they are, and each call should be checked against what that caller may do.

What is the difference between internal, partner and public APIs?

The audience. Internal APIs serve your own teams, partner APIs serve named partners under an agreement, and public APIs are open to any developer who registers.

Can an internal API become public?

Yes, but it needs more than a new URL: documentation, versioning promises, rate limits, stricter authorization and support.

Should internal APIs be load tested?

Yes. Shared services take traffic from many callers, so a limit you never tested tends to show up as a slowdown in another team's product.