Interface and API development: connecting your systems cleanly

Media breaks are among the biggest time wasters in everyday work. An order arrives in the online shop, is transferred by hand into the inventory system, the invoice is created in a third program, and the customer receives her confirmation by a manually written email. Each of these steps costs time and carries the risk of typing errors.

A well-built interface transfers this information automatically as soon as an event occurs. Data is captured once and is then available in all systems involved. Your team can concentrate on tasks where it is really needed.

At the same time, data quality increases. When information is no longer copied by hand, there are fewer discrepancies between systems, and reports are based on a consistent data set.

We develop our own REST APIs through which your applications provide or receive data, for example for partners, mobile applications or other internal systems. This includes a clear endpoint structure, clean authentication, understandable error messages and documentation that other developers can work with.

We also connect external services: payment providers such as Stripe, communication channels such as the WhatsApp Business API, email delivery, Google services such as the Search Console API or AI interfaces. Via webhooks your software reacts to events in real time, for example a successful payment.

Where no modern API exists, structured data imports and exports help, for example from CSV or Excel files. For parking operators, for example, we implemented imports of bookings from broker portals such as Parkos or ich-parke-billiger.

Our own booking platforms Friend-Park and the platform for Düsseldorf Airport live on interfaces. There we connect the booking portal, partner dashboards, online payment, email and WhatsApp notifications and reports into one continuous process.

Along the way we have learned what matters in live operation: payment confirmations must not be processed twice, failed external services must not paralyse the entire system, and every transfer must be traceable in case of an error.

We bring these experiences into customer projects. We plan interfaces not only for the normal case but also for timeouts, retries, incomplete data and changes on the other side.

Network connections are never one hundred percent reliable. A robust interface expects a request to fail or a response to be missing. That is why we build in controlled retries with waiting times instead of resending endlessly or silently swallowing errors.

Idempotency is also important: if the same message is transmitted twice, for example because a confirmation was lost, this must not result in a duplicate booking or duplicate payment. With unique identifiers and checks, we ensure that every action takes effect exactly once.

Monitoring with meaningful logs makes it visible when an interface is not working as expected. This way problems can be detected and fixed before customers notice them.

Interfaces open your systems to the outside world. Access must be protected accordingly. We rely on encrypted connections, authentication via tokens or keys, finely graded permissions and validation of incoming data before it is processed.

Signed webhooks ensure that a message really comes from the expected sender. Credentials and API keys are kept in a protected configuration and not in the source code. Services only receive the rights they really need for their task.

For data protection, the principle of data minimisation applies: only the data the recipient actually needs is transferred via an interface. Personal information that is not required for a process stays where it belongs.

Technically we rely on Microsoft .NET and ASP.NET Core with C#, SQL Server and Entity Framework and, if required, Microsoft Azure. This platform is ideal for high-performance, easily testable APIs and background services that process data on a schedule or event-driven basis.

At the start we clarify which systems are to be connected, which data flows in which direction and which interfaces the providers involved offer. This results in a clear concept with data model, error handling and test cases.

Every interface is secured with automated tests that also cover error cases such as timeouts or invalid responses. This keeps the connection stable even if details change on one side later.

Interfaces are worthwhile wherever data is regularly transferred between systems: in e-commerce between shop, payment provider and inventory management, for service providers between booking system, calendar and customer communication, or in administration between specialist application and accounting.

A dedicated API also makes sense for companies that want to connect partners or customers directly. Instead of exchanging lists by email, partners can retrieve or submit data automatically – with clear rules on who may see which information.

If you do not yet know whether an interface is feasible in your case, we are happy to check this in the initial consultation. We look at the documentation of the systems involved and tell you honestly which approach makes technical and economic sense.

Not every project starts from scratch. Often there are already integrations that have grown over the years, are poorly documented or keep failing. We analyse such existing connections, identify weaknesses and propose a step-by-step improvement.

Common starting points are missing error handling, credentials hard-coded in the source, outdated libraries or dependencies on interface versions that the provider will soon switch off. Such risks can be eliminated without rebuilding the whole system.

After the revision you receive clear documentation so that future changes remain traceable – regardless of whether we or another team take over further development.

A classic scenario is connecting a booking or ordering system with a payment provider. The customer pays online, the payment provider reports success via webhook, and your software automatically marks the booking as paid, sends the confirmation and creates the receipt – without anyone having to intervene.

A second example is automated customer communication. When the status of an order changes, the customer receives a message by email or via the WhatsApp Business API. Your team does not have to write every notification individually, and the information is always up to date.

Thirdly, we connect data sources for reporting. A background service retrieves key figures daily from external services such as Google Search Console, stores them in a structured way and displays them in a dashboard. This produces reports that previously had to be compiled manually.