Migration strategy.
WHAT NOT TO DO
Rewrite everything and switch over in one step.
WHY
It takes longer than estimated, and the old system continues changing meanwhile.
WHAT TO DO INSTEAD
Migrate incrementally, one area at a time.
HOW THAT WORKS
Place the new application alongside Route specific paths to it Move areas across progressively
WHAT TO SHARE DURING TRANSITION
The database, usually.
WHAT TO BE CAREFUL WITH
Sessions, which differ between systems Authentication, which must work across both Duplicated logic in two places
WHAT TO MIGRATE FIRST
Something self-contained and low risk.
WHY
It proves the arrangement before anything critical depends on it.
WHAT TO MIGRATE LAST
Anything handling money.
WHAT TO ESTABLISH
What is genuinely still used.
Frequently substantial parts of a legacy system are not.
WHAT NOT TO MIGRATE
Features nobody uses.
WHAT TO PLAN
The point at which the old system is retired.
