Inheriting a reporting service and making it last

The problem

The service produces the documents the business runs on: invoices, delivery and gate documents, purchase offers and inspection reports. It had been in production for years on an older .NET version and a single commercial reporting engine, with no automated tests. People depended on its output every day, so a rewrite was never on the table.

What I did

• Upgraded the runtime twice and the reporting engine once. Moved to central package management and a build pipeline. Replaced raw HTTP calls to around a dozen downstream services with typed clients that support cancellation.
• Built the inspection report end to end. It pulls evaluation data, photos, test and diagnostic results, and a plain-language summary written by an internal AI service.
• Introduced a second rendering engine alongside the existing one: HTML templates rendered to PDF in headless Chromium, with templates editable at runtime. It touches none of the old engine’s code. Colleagues have since moved other documents onto it.
• Diagnosed font and template crashes that only appeared in Linux containers.
• Hardened rendering: an isolated browser context per document, a resource allow-list, a concurrency limit, and automatic browser relaunch after a crash.

Decisions and tradeoffs

• Additive, not a rewrite. Both engines run side by side. Documents move across one at a time, and nothing that works today has to change on the same day as everything else.
• Golden tests as the safety net. Every report has snapshot tests that lock its data model, its HTML and its PDF: page text plus an image of each page. A test fails if a report has no golden case at all. Baselines are kept per operating system, because fonts render differently.
• Boring mapping over clever mapping. I replaced automatic object mapping with explicit code, so a renamed field breaks the build instead of a PDF.

Where it landed

The test suite went from nothing to around 117 tests, most of them golden cases. The new engine is how new documents get built now.