Cloud Custodian Tutorial: Features, Use Cases, How It Works

TL;DR, What is Cloud Custodian?

Cloud Custodian is an open-source tool for cloud governance automation. For DevSecOps and cloud engineering teams, Cloud Custodian solves the challenge of managing separate scripts and manual processes across multi-cloud environments. The tool replaces these fragmented approaches with a consistent policy-as-code framework. Using a simple YAML DSL, you can define and automatically enforce rules for security, multi-cloud compliance, and cloud cost optimization. The framework enables real-time policy enforcement, helping you maintain a consistent security posture and manage resource lifecycles without complex, custom tooling. Originally developed by Capital One, Cloud Custodian is now a Cloud Native Computing Foundation (CNCF) incubating project, trusted by organizations for governance at scale.

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At-A-Glance

• GitHub: https://github.com/cloud-custodian/cloud-custodian

• License: Apache-2.0

• Primary Language: Python

• Stars: 5.8k ⭐

• Last Release: September 2025

• Topics/Tags: cloud-security, policy-as-code, cost-optimization, governance, aws, azure, gcp, serverless

Common use cases

1. Real-time Security Remediation: Automatically enforce security best practices by deploying event-driven policies that can remove public access from storage buckets, enforce encryption on new databases, or ensure security groups do not have overly permissive ingress rules, closing security gaps quickly.

2. Proactive Cloud Cost Optimization: Implement policies that identify and manage cloud spend. Proactive policies can terminate idle resources, schedule non-production instances to shut down after business hours, and rightsize over-provisioned compute resources based on utilization metrics to reduce waste.

3. Automated Governance and Tagging: Enforce consistent organizational tagging standards across all cloud resources. Policies can identify non-compliant resources and automatically apply correct tags, ensuring accurate cost allocation, resource ownership, and efficient asset management in large-scale environments.

4. Shift-Left Governance in CI/CD: Integrate policy checks into CI/CD pipelines to validate infrastructure-as-code templates before deployment. Integrating policy checks prevents non-compliant infrastructure from being provisioned, embedding governance directly into the DevOps workflow and promoting policy-as-code practices.

5. Automated Operational Hygiene: Maintain a clean cloud environment with policies that automatically clean up orphaned resources. Policies can delete unattached storage volumes, remove old snapshots, and de-register unused machine images, which reduces both operational clutter and unnecessary storage costs.

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How does Cloud Custodian work?

Cloud Custodian operates as a stateless rules engine to automate cloud governance. The process starts when its policy engine parses a YAML policy and uses resource managers to query cloud provider APIs for relevant assets. The filter framework then applies the policy's conditional logic to this data, identifying any non-compliant resources. Finally, the action framework executes prescribed remediation—such as tagging, stopping, or deleting the resource—while output handlers generate detailed logs, metrics, and reports for auditing and monitoring purposes.

• Policy Engine: Parses human-readable YAML policies to orchestrate the discovery, filtering, and remediation workflow.

• Resource Discovery & Filtering: Interfaces with cloud APIs to enumerate resources, then applies nested boolean logic to identify assets matching policy criteria.

• Action Framework: Executes a wide range of actions on non-compliant resources, from sending notifications and applying tags to stopping or deleting them.

• Outputs: Generates structured logs for audit trails, metrics for cloud monitoring systems, and reports stored in cloud object storage.

Core Capabilities:

1. Policy-as-Code Engine: Cloud Custodian uses a simple, declarative YAML DSL, making governance accessible to both development and operations teams. This policy-as-code approach allows rules to be version-controlled, peer-reviewed, and tested like application code. The language supports complex logic and reusable components, enabling detailed cloud governance automation while ensuring policies remain readable and maintainable.

2. Real-Time, Event-Driven Enforcement: By integrating natively with cloud provider event streams like AWS EventBridge (formerly CloudWatch Events), the tool provides immediate, automated enforcement. Policies are triggered in real time as resources are created or modified, allowing remediation to occur within seconds of a violation. A serverless model ensures low operational overhead while effectively preventing compliance drift and security gaps.

3. Unified Multi-Cloud Management: Cloud Custodian supports AWS, Azure, GCP, and Kubernetes through one interface. The tool abstracts away provider-specific complexities such as authentication, API pagination, and rate limiting. This abstraction simplifies creating consistent multi-cloud compliance policies, helping teams manage diverse cloud estates without deep expertise in each provider's native APIs.

4. Advanced Resource Filtering: A filtering system enables targeting of resources using nested boolean logic. Policies can identify resources based on a wide array of criteria including tags, configuration attributes, security states, cost metrics, and operational data. The system allows for the creation of highly specific and context-aware c7n policies that align with complex business and regulatory requirements.

5. Comprehensive Reporting and Observability: Cloud Custodian generates structured JSON reports, integrates with cloud-native metrics, and supports centralized logging. The generated output creates a clear audit trail for compliance, enables detailed cost analysis, and feeds into monitoring platforms for alerting and trend analysis. The observability features support both day-to-day operational management and formal regulatory reporting.

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Limitations

1. Complexity at Scale: While the YAML DSL is simple for individual policies, managing hundreds of interdependent policies across a large enterprise can become challenging. Ensuring consistency and avoiding conflicts at scale requires disciplined organization and may rely on external tooling.

2. Risk of Automated Misconfiguration: Real-time remediation is effective but risky. A misconfigured policy can cause significant, unintended changes, requiring rigorous testing and dry-run procedures to prevent accidental modification or termination of critical production resources.

3. Requires High-Privilege IAM Roles: The tool needs high-privilege permissions to perform remediation actions. The high-privilege identity becomes a critical security asset that must be diligently secured and monitored to prevent potential misuse or compromise by malicious actors.

4. Lacks a Centralized Management UI: Cloud Custodian operates as a command-line tool without a native GUI for policy management or aggregated reporting. A consolidated compliance dashboard requires integrating its structured output with external visualization and monitoring platforms.

5. Stateless Execution Model: The tool's stateless execution model re-evaluates resources on each run. A stateless approach can be inefficient for tracking resource changes over time and may require external data sources or more complex policy logic to manage state-dependent workflows.

Getting Started:

To get started with Cloud Custodian, ensure you have Python 3.10.2+ and pip installed.

1. First, install Cloud Custodian, then add optional cloud provider packages as needed:

python3 -m venv custodian

source custodian/bin/activate

pip install c7n # AWS support is included in core

2.  Create a policy file named custodian.yml with a simple policy to identify running EC2 instances with a specific tag:

policies:

name: my-first-policy

resource: aws.ec2

filters:

"tag:Environment": "Dev"

State.Name: running

3. Run Cloud Custodian from your terminal, providing your AWS credentials and specifying the output directory:

export AWS_ACCESS_KEY_ID="YOUR_KEY"

export AWS_SECRET_ACCESS_KEY="YOUR_SECRET"

custodian run --output-dir=. custodian.yml

FAQ:

Verified Cloud Custodian User Reviews

"Custodian is working really great for us. We deploy it (c7n-org container image) to a kubernetes cron job, with the account config and policy yaml files managed as configMaps. Via a service account / IAM role, it scans all accounts in our AWS Org, for any resources in the specified regions. Policy matches are written to an SQS queue, and then we run the c7n-mailer image as another cron, to read the messages from SQS and send them to specified slack channels." [bcdady] - [r/aws]

"Custodian also has c7n-left which operates on iac (terraform) that can do single policy for multi resource type tag governance pre deployment in ci pipelines." [kapilt] - [r/cloudcustodian]

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