Almost every business works with several programs: an online shop, an inventory system, a shipping tool, an accounting package. Without a connection between them, someone retypes data from one program into the next. An interface takes that job off their hands.

An interface in one sentence

An interface is an agreed point at which one program hands data to another or collects it - in a form both understand.

An everyday example

A customer orders in the online shop. If the systems are connected, this is what happens:

  1. The shop passes the order to the inventory system.
  2. The inventory system checks stock and creates the order.
  3. The shipping tool receives the delivery address and creates the label, for example for DHL or DPD.
  4. The tracking number goes back to the shop, and the customer gets the shipping email.
  5. The invoice lands in accounting, for instance in DATEV.

Without interfaces, these are five places where someone copies data by hand - and five places where mistakes happen.

The common kinds

Programming interface (API). The program offers fixed addresses through which another program can request or send data. Most newer systems have one, for example Shopify, Shopware or the major ERP systems. This is the usual way today.

File exchange. One program drops a file, usually CSV or XML, and the other collects it. Simple and robust, but not instant, and someone has to make sure no file gets stuck.

EDI. The electronic exchange of business documents between companies. More on that below.

Direct database access. One program reads or writes straight into the other’s database. It works, but it is fragile: if the manufacturer changes the database, the connection breaks.

The last resort: operating the screen. If an older system has no interface at all, a program can imitate a user and enter data just as a person would. This is the fallback when nothing else works.

What is EDI?

EDI stands for electronic data interchange, the electronic exchange of business documents between companies. Orders, dispatch advices and invoices no longer travel as PDFs by email, but as structured data directly from system to system.

What matters are fixed formats that all parties agree on:

  • EDIFACT is the international standard and widespread in retail. Many large retail chains require their suppliers to exchange orders and invoices this way.
  • VDA is the standard of the German automotive industry.
  • XRechnung and ZUGFeRD are the formats for electronic invoices in Germany. Since 2025, businesses there must be able to receive such e-invoices, and sending them is becoming mandatory in stages. Your tax adviser can tell you what applies to you and when.

EDI often runs through a service provider that translates the formats and maintains the connections to trading partners. Your inventory system then only has to deliver the data in a form that provider understands.

How to recognise a good interface

Getting data to flow is only half the job. A good interface also answers the questions that only come up in daily use:

  • What happens when something fails? If a system is briefly unreachable, no order may be lost. The interface tries again later.
  • Is anything created twice? If the same order arrives twice, there must still be only one order.
  • Does anyone notice when it stalls? An interface that silently stops is otherwise only noticed when customers complain.
  • Can you trace what was transferred? A log shows which data went where and when.
  • Is it documented? Whoever has to touch the interface later needs a description.

And if a system has no interface?

Older systems in particular often have no interface or only a very limited one. There is usually still a way: an export the system can already do, access to the database or, if need be, the last resort via the screen. Which route works depends on the system. That is why we connect existing systems where possible and say up front where the limits are.

Before the first conversation

If you are thinking about connecting your systems, these questions help:

  • Which programs are involved, and which data should flow?
  • In which direction, and how quickly?
  • Which system wins when the same information exists in two places?
  • Who should be notified when something goes wrong?