Netflix's Simian Army: Chaos Engineering Explained

Netflix has developed a suite of automated tools known as the Simian Army to test the resilience of its cloud infrastructure. The primary tool, Chaos Monkey, randomly terminates virtual machine instances and services within the production environment to ensure that the system can withstand unexpected failures without impacting users.

This approach forces engineers to design fault-tolerant systems from the ground up. The Simian Army includes other tools like Janitor Monkey, which cleans up unused resources, and Chaos Gorilla, which simulates availability zone outages. By embracing failure as a constant, Netflix aims to build a more robust and reliable streaming platform that can survive the inevitable faults that occur in complex cloud environments.

The move to Amazon Web Services (AWS) presented Netflix with both opportunities and challenges. While the cloud offered unprecedented scalability, it also introduced a new class of failures that traditional data centers did not face. Hardware failures, network partitions, and availability zone outages became part of daily operations.

To address this, Netflix engineers realized they needed to proactively test their systems against these failures. Instead of waiting for things to break, they decided to break them on purpose. This philosophy led to the creation of the Simian Army, a collection of tools designed to simulate various failure scenarios.

The goal was not to create chaos for its own sake, but to build confidence in the system's ability to survive real-world disruptions. By constantly testing in production, Netflix could identify weaknesses before they caused customer-facing outages.

Chaos Monkey is the most well-known member of the Simian Army. Its job is simple yet terrifying: it randomly selects a virtual machine or service in the production environment and terminates it. This happens during normal business hours when engineers are available to respond.

The presence of Chaos Monkey forces every service to be resilient. If a service cannot handle the sudden loss of one of its instances, it is considered broken and must be fixed immediately. This ensures that the loss of any single component does not cascade into a larger outage.

Key principles behind Chaos Monkey include:

• Randomness: The timing and target of failures are unpredictable
• Automation: The tool runs continuously without manual intervention
• Production Environment: Testing happens in the real environment where it matters
• Non-disruptive: Failures should be handled gracefully without customer impact

The Simian Army has expanded to include specialized tools for different types of failure scenarios. Chaos Gorilla extends the concept from individual instances to entire availability zones, simulating what happens when a whole data center goes offline.

Janitor Monkey takes a different approach by focusing on resource management. It identifies and cleans up unused resources, helping to prevent the accumulation of technical debt and reducing costs. This ensures the infrastructure remains lean and efficient.

Other tools in the army address specific concerns:

• Conformity Monkey: Checks for compliance with best practices
• Doctor Monkey: Monitors health checks and symptoms
• Lawyer Monkey: Ensures legal and regulatory requirements are met

Each tool serves a specific purpose in maintaining the overall health and resilience of the Netflix ecosystem.

The Simian Army represents more than just tools; it embodies a cultural shift at Netflix toward embracing failure. The company operates under the assumption that failures are inevitable and must be designed for, not avoided.

This chaos engineering mindset requires teams to build systems that can self-heal. Services must be able to detect failures, route around them, and recover automatically. Monitoring and alerting become critical components of this architecture.

The approach has proven successful. Netflix has survived numerous real-world AWS outages with minimal customer impact. The constant testing ensures that when real failures occur, the system has already been hardened against them.

By making failure a daily practice, Netflix has created one of the most resilient streaming platforms in the world, capable of serving millions of users simultaneously even when parts of its infrastructure are under stress.