Replacing legacy AS400 templates with a modern templating platform

The Anvil

Business-critical templates — the documents and data feeds distribution centers and partners depend on — lived inside decades-old AS400 systems. Changing them meant editing logic buried in legacy infrastructure, with no UI for anyone outside engineering, and real risk every time a change rippled upstream to a partner.

The Strike

I tech-led the build of the replacement platform and the migration off AS400. The pieces:

• Templating platform — a shared Angular library plus a .NET 10 / FastEndpoints microservice (EF Core on Postgres), deployed as an AWS Lambda via Serverless Framework. Gives the business a UI to author and publish templates directly, instead of editing AS400 logic.
• Authentication — migrated off an internal dev-token flow onto real OIDC login.
• Multi-tenant safety — an IUserContext abstraction, an EF Core audit-stamping interceptor, and a guard making tenant assignment immutable after creation, all backed by an ADR so the reasoning survives past the initial build.
• Migration strategy — sequenced the cutover by distribution center and partner grouping, ordered by upstream impact, so the riskiest dependencies move last.

The Edge

The platform is live and handling production template rendering and publishing for every group already cut over. Migration continues system by system, template by template — a deliberately paced rollout rather than a big-bang switch, so a mistake in one group never cascades to the next.