Interface and API development
When systems talk to each other
Most businesses run several systems side by side: an inventory management or ERP system, an online shop, a CRM, accounting and often their own applications. Without a connection, people take on the role of the interface – they retype orders, export lists and reconcile data by hand. That takes time and causes errors that are often only noticed weeks later.
We develop interfaces that close these gaps: from a simple transfer between ERP and online shop to middleware that connects several systems and external partners in near real time. We also develop APIs for your own software, through which apps, web interfaces and partners securely access the same data.
What we connect for you
ERP and inventory management. Exchange orders, items, customers and documents between your ERP and your other applications – even if the ERP has no modern interface and the integration has to run right next to its database.
Online shop, CRM and accounting. Book orders automatically as invoices, create customers in the CRM, reconcile stock levels – entered once, up to date everywhere.
APIs for your own software. Application programming interfaces through which your apps, web applications and partners read and write data – secured and with clear access rights.
Middleware and data sync. A central hub that translates, validates and logs data between several systems and external partner APIs – with clear error handling when a system is unavailable.
Robust, not just connected
An interface does not prove its quality on day one, but when something goes wrong: when a system is unavailable, a record arrives incomplete or a vendor changes its API. That is why we build in retries, logging and notifications from the start, and agree with you which system is the leading source for which data.
Automated tests ensure that both sides understand the same data. In our product Stundenplan-Digital, for example, contract tests check that browser and server deliver identical calendar data. For standard processes without much custom logic, we also use a workflow automation platform such as n8n – this is often quicker to implement and easier to adapt than custom code.
From our projects: integration with the GDI ERP system
For Stenger, we connected a new service app to the GDI ERP system without replacing the proven system. Every night, the back end picks up new jobs from GDI and assigns them to the responsible technician. Every morning at 5 a.m., the completed jobs go back – with the report as a PDF, photos, spare parts and notes. To make this work, a lean connector runs at Stenger right next to the GDI database. Find out more in the Stenger case study.
How much does an interface cost?
Based on our cost model, an interface between ERP and online shop costs about €4,500–8,500, connecting ERP, online shop and CRM in near real time €9,500–17,500 and middleware with a partner API integration €39,000–70,000 – all net. The biggest difference is whether the systems involved come with a documented interface.
The project cost calculator estimates your integration in just a few minutes. Why interfaces still need care after go-live is explained in our guide Ongoing software costs.
How your interface comes to life
- Assessment: clarifying systems, data flows and leading sources
- Defining data mapping and behaviour in case of errors
- Development and testing with realistic sample data
- Go-live with monitoring and ongoing support
- Assessment: clarifying systems, data flows and leading sources
- Defining data mapping and behaviour in case of errors
- Development and testing with realistic sample data
- Go-live with monitoring and ongoing support
Frequently asked questions about interfaces and APIs
Our ERP has no modern interface – can it still be done?
Often, yes. Depending on the system, data can be exchanged via the database, via file exports or via a small connector that runs alongside the ERP. That is how we connected the GDI ERP system at Stenger. Beforehand, we check what the vendor permits and which approach is the most robust in day-to-day operation.
What is the difference between an API and an interface?
Interface is the umbrella term for any connection between two systems – from regular file exchange to direct database access. An API is an application programming interface that a system deliberately provides for other programs: with defined queries, formats and access rights. Modern software usually offers an API; older systems often require other approaches.
Do we need real-time synchronisation?
That depends on the process. Stock levels in an online shop should be accurate in near real time so that nothing is sold that is no longer available. For passing completed orders on to accounting, on the other hand, a nightly sync is often enough. It is simpler to implement, easier to monitor and usually more affordable.
Custom interface or workflow automation?
A workflow automation platform is well suited to processes between widely used systems for which ready-made connectors exist, and to manageable data volumes. Custom code pays off for complex logic, large data volumes or systems without a ready-made connector. The two can be combined. Find out more on our page about workflow automation and integrations.
What happens if a vendor changes its API?
Interfaces need care because the connected systems keep evolving. Good logging shows early on when data stops arriving. If you wish, we take over monitoring as part of maintenance and support and adapt the interface when a connected system changes.
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Tell us which systems need to talk to each other – and we will show you how we would connect them.