API

What an API is

An API is a defined interface through which two systems talk to each other. It describes which requests are possible and what answer follows, without the calling system needing to know the inside of the other. In a picture: the API is the menu, not the kitchen. Whoever orders does not need to know how the cooking works.

In online retail the API question is almost always a project question. Whether ERP, marketplace, newsletter, shipping or an AI tool is to be connected depends on whether the provider has an API and whether it delivers the fields required.

Which kinds exist

The most widespread is the REST interface. It works with individual addresses that each denote one thing and uses the standard web methods to read, create, change and delete. It is easy to understand and easy to test, which makes it the usual choice.

Alongside it sits GraphQL, where one call delivers several connected things in a single request, which helps with nested data such as product variants. Older are SOAP interfaces, still present in some ERP systems. And with webhooks the direction reverses: instead of asking the partner, the partner reports on an event by itself. For changes that have to arrive immediately, that is the right route.

What matters in the selection

• Completeness: does the interface cover the fields needed. An API for master data without images only helps halfway.
• Call volume: many providers limit how often a request may be made in a period. A sync across a hundred thousand items can fail on that.
• Authentication: how does the system sign in and how are keys renewed. Without clean key management, security gaps appear.
• Error behaviour: is there a comprehensible message, and may a call be retried. Both decide stability.
• Change notices: a provider who changes the interface without lead time breaks every integration.

What goes wrong in operation

The first mistake is the missing reconciliation after the initial import. An integration that pulls data once and then only writes in one direction produces two versions of the truth. Within a short time prices and stock differ, and nobody knows which value applies.

The second mistake is missing logging. Without a log of the calls, a fault cannot be attributed to data delivered wrongly, processed wrongly or never arriving. The third mistake is the missing queue rule: if the partner goes down for hours, the own process has to collect the requests and send them later instead of losing them.

Frequent questions

• Do I need an API for small integrations? Not always. A regular file exchange is enough when delays of hours are unproblematic.
• What is an API key? An identity card for the system. It belongs treated like a password and never in the source code of a page.
• How do I test an interface? With individual calls by hand before the automation is built. That saves most of the troubleshooting.