Build converter software

When your own converter makes sense

A one-off conversion solves yesterday's problem. A converter solves a problem that comes back every month.

It makes sense if one of these applies to you:

• Your upstream system cannot be changed for the foreseeable future but does not produce e-invoices.
• You regularly process invoice data from third parties, such as subcontractors or branches.
• You do not want to buy the conversion externally because the data is sensitive.
• You need traceability towards an audit and want to be able to show at any time how source A became result B.

How such a converter is built

A converter always consists of the same four stages. The difference between a usable and an annoying tool lies in how cleanly those stages are separated.

Reading

The source file is read and transferred into a uniform intermediate structure. This separation matters: if you later add a second source format, only this stage has to be extended, the rest stays untouched.

Mapping

This is where it is defined which source field corresponds to which element of EN 16931. These rules do not belong in the code but in a configuration you can read and change yourself. Typical rules are simple mappings, but also conversions, default values and conditions.

Validating

The file produced is validated against the official rules. On top of that I check it factually: do the totals add up? Does the tax amount match the rate? Is the Leitweg-ID plausible? This second layer catches errors that are formally correct but still wrong.

Output and logging

The finished files land where they are meant to go. In parallel a log is written that records every operation — source file, result, timestamp, any warnings. For questions arising from an audit, that log is often more valuable than the invoices themselves.

Error messages you can read

The official KoSIT validator reports errors in a language made for developers. A typical message says, in effect, that rule BR-DE-15 has been violated. Nobody in your company knows what that means.

I translate those messages. BR-DE-15 then becomes, for example, the note that the Leitweg identification number is missing, which is mandatory on invoices to public sector clients and which your customer has to provide.

That sounds like a detail. In practice it decides whether your finance team can work with the tool or whether every problem lands with IT.

What you get in the end

• the complete source code, which belongs to you
• the mapping rules in readable configuration form
• documentation on how to add new fields or formats
• test cases you can use to check after a change that everything still holds

If you would rather not operate the converter yourself but simply receive the finished conversion as a result, the other route is the cheaper one.