Legacy System Modernisation Decision Log
This is a practical guide to building a decision log for a legacy system modernisation project — a record of the key decisions made, by whom and why, adapted to the realities of modernising or replacing ageing core systems.
What a Decision Log is
A decision log is a record of the key decisions made, by whom and why. For the full concept and how it works in general, see Decision Log. On a legacy system modernisation project it plays the same role, tuned to this kind of work.
Why it matters for a Legacy System Modernisation project
Legacy System Modernisation projects live or die on modernising or replacing ageing core systems. A well-built decision log gives the team a shared, explicit reference for exactly that — reducing ambiguity, aligning stakeholders, and making problems visible early enough to act. Skipping it, or doing it generically, is how legacy system modernisation projects drift into avoidable delay and cost.
What to include
- Decision
- Date and owner
- Rationale
- Alternatives considered
- Impact
Legacy System Modernisation-specific considerations
Tailor the decision log to the risks that most often derail legacy system modernisation projects:
- Undocumented legacy behaviour
- Phased cutover risk
- Knowledge loss
Example
On a real legacy system modernisation project, the decision log would be shaped by modernising or replacing ageing core systems. In particular, it should explicitly account for the project’s biggest risks — undocumented legacy behaviour, phased cutover risk, knowledge loss — rather than treating them as afterthoughts.