API

What it means

An API is an agreed interface through which two programs exchange data without a person retyping it. The webshop pushes an order into the accounting system, the bank hands over account movements, the warehouse reports a dispatch - always in a structure the other side can read.

It differs from a webhook in direction: we ask an API for something ("give me March invoices"), while a webhook tells us by itself that something happened. A real integration usually uses both.

An API is often confused with RPA. A software robot imitates a person - it opens screens, clicks and copies fields - so even a small change to the application’s layout can stop it. An API reaches the data directly through a documented interface that the vendor keeps stable through a redesign. A robot is the fallback where no API exists.

How it works in practice

For bookkeeping an API is the difference between retyping hundreds of documents and transferring them overnight. Connecting an online store, a payment gateway or an ERP to the accounting software normally pays for itself at the volume where manual handling would cost a day a month.

Three questions settle a connection: which fields travel (date, amount, currency, VAT rate, variable symbol), what happens when a transfer fails, and who holds the keys. The third one is a security question and belongs to access control.

In a small company the first connection is usually the bank, not the webshop. Thanks to PSD2, the accounting software can, with the client’s consent, read account movements straight from the bank and match them to invoices by variable symbol. Statements no longer travel by e-mail, and we see unpaid invoices the same day rather than at the monthly close.

What to watch out for

Data arriving over an API is not thereby correct. If the webshop sends the wrong VAT rate or omits the customer’s VAT ID, the mistake simply multiplies faster. A check on the accounting side has to stay in place.

A connection can fail silently. An API key or a bank consent expires, the vendor moves to a new version of the interface, and documents stop arriving without any error message. If nobody notices, a whole month of invoices is missing from the VAT return. Every connection therefore needs a check that raises an alert when the data stops flowing.