An API (Application Programming Interface) is a programming interface through which software systems exchange functions and data. REST (Representational State Transfer) is the most widely used architectural style for this on the web: resources are addressed via URLs and manipulated with HTTP methods such as GET, POST, PUT and DELETE, with responses usually delivered in JSON format.
A REST API works like a well-organised service desk: one program submits a clearly phrased request – for example, for all of today's orders – and receives a structured response in return. This lets shop, merchandise management and apps work together without knowing each other's internals.
Why do I need a REST API?
As soon as two systems need to exchange data, a defined interface is required. REST APIs are the de facto standard for this on the web: payment providers, shipping carriers, marketplaces and ERP systems typically expose their functionality via REST interfaces. Modern shop systems such as Shopware 6 even ship with several REST APIs of their own – an Admin API for management tasks and a Store API for the frontend, for example.
The advantage over older approaches such as SOAP lies in simplicity: REST uses the web's existing infrastructure – HTTP, URLs, status codes – and delivers compact JSON responses that can be processed in practically any programming language. Headless architectures, where the frontend is decoupled from the shop system, are also built entirely on APIs.
Practical relevance for shop and website owners
As an operator you rarely program against an API yourself – but almost every extension of your shop uses one: the ERP connection as well as synchronisation with marketplaces or the transfer of product data to Google Merchant Center. Whether a desired integration is feasible therefore usually depends on whether the target system offers a documented API – an important criterion in any software selection. For custom integrations, we build tailored solutions as part of our programming and development services.
Typical mistakes when working with REST APIs
In our experience, these problems occur particularly often in API projects:
- API keys in the frontend: Credentials that end up in browser code are publicly visible – a serious security risk.
- Rate limits ignored: Many providers cap the number of requests per time window; without throttling and queueing, synchronisations break down.
- Polling instead of webhooks: Asking for news every minute creates unnecessary load – webhooks actively notify you of changes instead.
- Missing error handling: Without retry logic and logging, data is silently lost during temporary outages.
- API versions overlooked: Providers eventually retire old versions; without an update strategy, integrations suddenly stop working.
What to look out for
When selecting software, look for complete, up-to-date API documentation and ideally a sandbox environment for safe testing. Authentication should use established methods such as OAuth 2.0 or access tokens with the narrowest possible permissions. With many parallel integrations, a middleware can make sense as a central hub; in traditional B2B environments, some business partners still require EDI instead. The switch to the Google Merchant API shows how important a timely migration strategy is.
Alongside REST, GraphQL has established itself as a query language in which the client itself specifies which fields it wants to receive. For most shop integrations, however, REST remains the most common approach.