Strangler fig: splitting a monolith

Key points

• Never rewrite in one go. Put a facade in front of the monolith first, usually your gateway, so routing becomes something you control.
• Choose the first slice by pain and independence: read-heavy, low-risk, and owning as little shared data as possible.
• Run both sides at once. Send a small share of traffic to the new service, compare the answers, then raise the share.
• The data is the hard half. Copy the tables, keep them in sync for a while, then flip the writes and let the monolith read the service.
• Delete the old path once the route is at 100%. The migration is not done while the dead code still compiles.

Example

# The facade owns the migration. Each route moves on its own schedule.
spring:
  cloud:
    gateway:
      routes:
        - id: catalogue-migrated       # step 3: fully moved, old code deleted
          uri: http://catalogue:8080
          predicates:
            - Path=/catalogue/**

        - id: orders-canary            # step 2: 5% to the new service
          uri: http://orders:8080
          predicates:
            - Path=/orders/**
            - Weight=orders, 5

        - id: orders-monolith          # the other 95%, and the way back
          uri: http://monolith:8080
          predicates:
            - Path=/orders/**
            - Weight=orders, 95

        - id: everything-else          # step 1: nothing moved yet
          uri: http://monolith:8080
          predicates:
            - Path=/**

Strangle the monolith route by route, and keep a way back until the old code is gone.