Strangler Fig

The Big-Bang Trap

 rewrite-the-monolith projects: famously lethal.

 18-month rewrites meet reality:
 - business keeps changing DURING the rewrite
 - feature freeze kills the business's patience
 - the old system keeps evolving → moving target
 - big-bang cutover = one enormous untestable bet

 THE STRANGLER FIG (Fowler, from the vine that grows around
 a tree until the tree is gone):
 route traffic through a facade; migrate ENDPOINT BY ENDPOINT;
 old system shrinks incrementally until it's removable.

The Mechanics

 STEP 1: put a FACADE in front of everything:

 [clients]──►[facade/router]──►[legacy monolith]  (100%)

 STEP 2: pick ONE capability; build it as a new service;
         route its paths to it:

 [facade]──/orders/*──────►[orders-svc: NEW]
          ──everything else─►[legacy]

 STEP 3: repeat per capability. legacy share shrinks:

 [facade]──/orders,/inventory,/users──►[new services]
          ──/reports──►[legacy: shrinking...]

 STEP 4: last path migrates; delete the trunk.

What Makes It Work

 □ ROUTING IS REVERSIBLE: new service misbehaves? flip back
   instantly. risk per step = tiny and bounded ✓
 □ DATA COEXISTENCE plan per migration:
     - shared-read phase: new service reads legacy DB
       (temporary, read-only — the sanctioned exception!)
     - then data moves; legacy reads via events/API
     - write-ownership flips LAST, behind the router
 □ INCREMENTAL VALUE: every migrated endpoint ships real
   improvements NOW; no 18-month faith-based program
 □ PARITY TESTING: shadow-traffic comparisons before each
   flip (same request → same response?) catches drift

 anti-goals to name out loud:
 ✗ "rewrite while also building the roadmap" without
   capacity math — strangler still needs TEAM BUDGET
 ✗ migrating the hardest domain first for glory —
   start with edges, learn the machinery.

The Hard Part: Data

Phase	Orders data lives	Writes	Reads
Pre-migration	legacy DB	legacy	legacy
Dual-read pilot	both	legacy	compare both
Ownership flip	NEW db ← migrated	new svc only	new (+events)
Legacy retirement	gone from legacy	n/a	n/a

 the ownership-flip moment is the risky beat:
 freeze window or CDC-synced dual-write with verification,
 THEN cut. reversibility shrinks here — rehearse it.

Interview Framing

“We must modernize our 12-year-old monolith but can’t stop shipping” scored shape: reject big-bang explicitly with reasons, strangler mechanics drawn (facade → incremental routing), data-coexistence phases tabled, reversibility emphasized as THE property, sequencing advice (edges first), parity/shadow testing named. Migration questions grade RISK ARCHITECTURE — answers that eliminate the possibility of total failure outrank answers that promise speed.