Ka Lun Chan
Section303

Case study

WhereYippify client engagement
RoleArchitecture & delivery lead

Modernizing a public-sector service without a big-bang rewrite

A legacy public service moved onto a modern, accessible platform while it stayed in service.

Government technology / Software architecture / Cloud & infrastructure

Problem

A government service ran on an ageing system that was hard to change, hard to secure and hard to use on a phone. Residents still depended on it every day, so switching it off to rebuild was not an option.

Constraints

  • Continuous public availability
  • Accessibility (WCAG) and security review as release gates
  • Procurement and hand-over: the agency team must own the result
  • Legacy data that could not be migrated in one step

Architecture & approach

Put an API facade in front of the legacy system, then replace one journey at a time behind it. New services run as containers on AWS, provisioned with Terraform, and ship through a pipeline with automated accessibility and security checks.

Fig. 303-2Strangler migration behind an API facade Critical path

Key decisions

How do we replace a system the public relies on daily? 303-3.1

  • RejectedFull rewrite, then switch overYears without user value, plus one very risky launch day.
  • RejectedLift-and-shift to the cloudNew hosting, same problems.
  • ChosenStrangler pattern behind an API facadeShip one journey at a time. The legacy system shrinks with every release.

What stack will the agency own after we leave? 303-3.2

  • RejectedBespoke microservice meshPowerful, but needs a platform team the agency does not have.
  • ChosenDjango + React on managed AWS, all in TerraformConventional, well documented, and easy to hire for.

Outcome

  1. Resident journeys moved to the new platform one at a time, with the service up throughout
  2. Accessibility and security checks built into every release
  3. Infrastructure and runbooks handed over to the agency team

Technology

Python / Django, React, AWS ECS, RDS PostgreSQL, Terraform, CI/CD, WCAG accessibility

Lessons & tradeoffs

  • In government, hand-over is the product. Design for the team that inherits it.
  • A facade turns one terrifying migration into a series of boring releases.