# Shadowing, Blue-Green and Progressive Delivery — API Gateway and Service Mesh

Source: https://www.geekswithgeeks.com/en/api-gateway-service-mesh/p-shadow-bluegreen

> Test new versions with mirrored traffic and switch over safely.

## Prove it before users depend on it

**Traffic shadowing (mirroring)** copies live requests to a new version and discards its responses, so you can compare behaviour and load with zero user impact (be careful with requests that cause side effects). **Blue-green** runs two full environments: switch the gateway from blue to green in one step and switch back instantly if something is wrong; it needs double capacity and careful database compatibility. **Canary** shifts traffic gradually with metric checks. **Progressive delivery** tools (such as Argo Rollouts or Flagger) automate these steps: shift 5%, check error rate and latency, continue or roll back. Always keep the database migration **backward compatible** during rollouts, so both versions can run at once.

## Mirroring in nginx (illustrative)

The `mirror` directive sends a copy of each request to the shadow upstream; its response is ignored. Not run here.

```nginx
location /orders/ {
  mirror /shadow;
  mirror_request_body on;
  proxy_pass http://orders_stable/;
}
location = /shadow { internal; proxy_pass http://orders_canary$request_uri; }
```

## An automated rollout plan

Write the promotion rules down so the tool, not a person at 2 a.m., decides.

```text
step 1   mirror 100% (no user impact)   compare errors + latency for 15 min
step 2   canary 5%                       continue if 5xx < 0.5% and p99 < 1.2x baseline
step 3   canary 25% -> 50% -> 100%       hold 10 min at each step
abort    any check fails                 shift back to 0%, page the owner
after    keep old version warm 24 h      then remove; schema changes stay backward compatible
```

**Quiz:** What is traffic mirroring used for?

- [ ] Encrypting traffic
- [ ] Doubling user-visible responses
- [x] Testing a new version with real traffic copies without affecting users
- [ ] Caching static files

*Answer:* Testing a new version with real traffic copies without affecting users. Mirrored responses are discarded, so only the test target is exercised.