How would you replace a legacy service without a big-bang migration?

package com.bank.migration;

public class BigBangRisk {
    public static void main(String[] args) {
        System.out.println("Big-bang rewrite:");
        System.out.println("  Build new system for 6 months");
        System.out.println("  Switch everything on one Saturday night");
        System.out.println("  If something is wrong → no safe way back");
        System.out.println("  All risk in one moment, no gradual learning");
        System.out.println();
        System.out.println("Strangler-fig:");
        System.out.println("  Move one slice at a time");
        System.out.println("  Validate each slice in parallel");
        System.out.println("  Instant rollback for each increment");
        System.out.println("  Risk spread across many small, reversible steps");
    }
}

package com.bank.migration;

public class StranglerFacade {

    private final boolean balanceEnabled = true;  // feature flag
    private final boolean transferEnabled = false; // not migrated yet

    String route(String capability, String request) {
        if (capability.equals("balance") && balanceEnabled) {
            return "NEW service: " + request;
        }
        return "LEGACY service: " + request;
    }

    public static void main(String[] args) {
        StranglerFacade facade = new StranglerFacade();
        System.out.println(facade.route("balance", "SAV2001"));
        System.out.println(facade.route("transfer", "SAV2001 → SAV3002"));
    }
}

package com.bank.migration;

public class ShadowTraffic {

    static String legacyBalance(String accountId) {
        return "75000.00";
    }

    static String newBalance(String accountId) {
        return "75000.00"; // should match; log if different
    }

    public static void main(String[] args) {
        String accountId = "SAV2001";
        String legacyResp = legacyBalance(accountId);
        String newResp = newBalance(accountId);

        if (!legacyResp.equals(newResp)) {
            System.out.println("DIVERGENCE on " + accountId
                    + ": legacy=" + legacyResp + ", new=" + newResp);
        } else {
            System.out.println("Match on " + accountId + ": " + legacyResp);
        }
        System.out.println("Caller receives LEGACY response (safe). New response discarded.");
    }
}

Match on SAV2001: 75000.00
Caller receives LEGACY response (safe). New response discarded.

package com.bank.migration;

import java.util.concurrent.ThreadLocalRandom;

public class PercentageRamp {

    static String route(String request, int newServicePercent) {
        int roll = ThreadLocalRandom.current().nextInt(100);
        if (roll < newServicePercent) {
            return "NEW service: " + request;
        }
        return "LEGACY service: " + request;
    }

    public static void main(String[] args) {
        int percent = 10; // 10% to new service
        for (int i = 0; i < 5; i++) {
            System.out.println(route("balance SAV2001", percent));
        }
        System.out.println("At " + percent + "% ramp, watch error rate and latency");
    }
}

LEGACY service: balance SAV2001
LEGACY service: balance SAV2001
NEW service: balance SAV2001
LEGACY service: balance SAV2001
LEGACY service: balance SAV2001
At 10% ramp, watch error rate and latency

package com.bank.migration;

public class ReturnPath {
    public static void main(String[] args) {
        System.out.println("Before cutover: test the rollback");
        System.out.println("  1. Route balance to NEW (feature flag ON)");
        System.out.println("  2. Observe for 1 hour");
        System.out.println("  3. Bug found! Flip feature flag OFF");
        System.out.println("  4. Balance routes back to LEGACY instantly");
        System.out.println("  5. No data lost because both read from shared store");
        System.out.println();
        System.out.println("If there is no designed return path, it is a big-bang in disguise.");
    }
}

Before cutover: test the rollback
  1. Route balance to NEW (feature flag ON)
  2. Observe for 1 hour
  3. Bug found! Flip feature flag OFF
  4. Balance routes back to LEGACY instantly
  5. No data lost because both read from shared store

If there is no designed return path, it is a big-bang in disguise.

package com.bank.migration;

public class SharedSourceOfTruth {
    public static void main(String[] args) {
        System.out.println("Option A: Shared database (both old and new read/write same DB)");
        System.out.println("  Rollback: flip route, data is always current");
        System.out.println();
        System.out.println("Option B: Dual-write (write to both old and new stores)");
        System.out.println("  Rollback: stop writing to new, old store is current");
        System.out.println();
        System.out.println("The guarantee: flipping back NEVER loses data");
    }
}

Option A: Shared database (both old and new read/write same DB)
  Rollback: flip route, data is always current

Option B: Dual-write (write to both old and new stores)
  Rollback: stop writing to new, old store is current

The guarantee: flipping back NEVER loses data

package com.bank.migration;

// WRONG: migrate to new service, decommission legacy database
// Bug found → no way back, data only in new store

// CORRECT: keep legacy fully operational until migration is COMPLETE
// Test rollback BEFORE cutover, not during an incident