# Canary Deployment

Canary deployment is a technique to reduce the risk of introducing a new software version in production by slowly rolling out the change to a small subset of users before rolling it out to the entire infrastructure.

## 1. The Concept

The term comes from the "canary in a coal mine" practice. Miners would carry a canary into mine tunnels. If dangerous gases collected, the canary would die first, giving the miners a warning to escape.

In software, the "canary" is the small percentage of users who receive the new update. If the update is buggy, only a few users are affected, and you can rollback quickly.

## 2. How it Works

1.  **Stage 1:** Deploy the new version (v2) to a small percentage of your infrastructure (e.g., 5% of servers) or route 5% of traffic to it.
2.  **Stage 2:** Monitor key metrics (error rates, latency, CPU usage) specifically for the v2 instances.
3.  **Stage 3:** If metrics are healthy, gradually increase the traffic (10%, 25%, 50%).
4.  **Stage 4:** Once 100% of traffic is on v2, decommission the old version (v1).

## 3. Canary vs. Blue-Green Deployment

*   **Blue-Green:** You have two identical environments (Blue and Green). You switch 100% of traffic from Blue to Green instantly.
    *   *Pros:* Fast rollback, no version mixing.
    *   *Cons:* Requires double the infrastructure (costly).
*   **Canary:** You gradually replace old instances with new ones.
    *   *Pros:* Cheaper (no need to double resources), limits blast radius of bugs.
    *   *Cons:* Slow rollout, you have two versions running simultaneously (version compatibility issues).

## 4. Implementation Strategies

*   **Load Balancer:** Configure the load balancer (e.g., NGINX, HAProxy, AWS ALB) to split traffic based on weight.
*   **Feature Flags:** Use feature flags to enable the new functionality for specific users (e.g., internal employees or beta testers) regardless of the server they hit.
*   **Kubernetes:** Tools like Istio or Flagger can automate canary rollouts based on metrics.

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[[programming/ci-cd-pipelines]]
[[programming/software-testing-basics]]