Banking APIs in practice. Connecting systems and partners.

An API is an agreed way to communicate

Imagine an application displaying a user’s account history. It needs information from the bank without knowing the internal structure of its core system or databases. The API defines what can be requested, in which format and what response to expect.

Both sides share a technical contract: operations, data fields, authentication rules and error behaviour. Teams can develop their systems independently while preserving the agreed interface.

Who takes part?

A simplified Open Banking scenario includes a user, a bank and an authorised partner. The user uses a service and grants the appropriate permissions. The bank holds the account and exposes defined operations. The partner builds a service using the bank’s interface.

The connection also needs an API layer, authentication mechanisms and integration with the bank’s systems. Documentation and a test environment matter to partner teams just as much.

A good API helps both sides understand what should happen, including when things go wrong.

Account data and payments are different scenarios

AIS, the account information service, concerns access to defined information, for example to display a user’s finances. PIS, the payment initiation service, starts a payment order at the user’s request.

Their purposes and flows differ. Permission to read information must not be confused with permission to initiate payments. The provider’s role and access scope must match the service.

Why have a common standard?

If each bank describes a similar operation differently, partners need to handle many variants. A standard such as PolishAPI helps establish a common language and a point of reference.

It does not remove all integration work. Teams still need to understand interface versions, supported scenarios, data scope and the behaviour of a specific implementation. Test error cases as carefully as successful flows.

What makes integration work well?

• Documentation that matches the implementation and remains current.
• A sandbox with known scope and limitations.
• Errors that a partner can interpret and handle.
• Permissions enforced consistently throughout the flow.
• Monitoring and communication that locate issues and establish responsibility.

Start an API review with actual requests, responses, timings and incident context. Concrete evidence is a better basis for diagnosis than a general statement that an integration does not work.