Argo CD is a declarative continuous delivery (CD) tool for Kubernetes. Argo CD pulls Kubernetes configurations (such as manifests, Helm charts, and Kustomize overlays) from a Git repository and applies them to a Kubernetes cluster.

With Argo CD, developers can automatically deploy changes to their Kubernetes environments by updating their Git repository. Argo CD continuously monitors Kubernetes deployments and ensures their state matches the configuration declared in Git.

By using Git as the single source of truth, Argo CD enables teams to version control and manage Kubernetes applications efficiently. This approach eliminates the need for DevOps teams to manually apply Kubernetes configurations, ensuring a more reliable and automated deployment process.

How does Argo CD work?

Argo CD is a Kubernetes resource controller that modifies Kubernetes configurations to keep them synchronized with the state declared in a Git repository.

Let’s look at a typical Argo CD workflow:

1. Kubernetes configuration settings are defined in a YAML manifest and pushed to a Git repository.
2. Argo CD monitors the Git repository looking for any changes.
3. When a change is made, Argo CD detects it and pulls the new configuration from the repository. Argo CD automatically applies the new configuration to the Kubernetes cluster.

In addition to monitoring the Git repository, Argo CD also monitors the Kubernetes cluster for configuration drift. If changes are made to the cluster outside of Git (such as through the Kubernetes CLI), Argo CD detects this drift and can automatically apply settings from the repository.

Equipped with a rollback feature, Argo CD can revert the Kubernetes cluster to its last working state. Should a deployment fail, this feature provides a quick remediation strategy.

Argo CD provides a CLI and a web interface. The web interface features a visualization of the Kubernetes configuration on an interactive dashboard.

GitOps and Argo CD

GitOps is an approach to CD and infrastructure automation that uses Git as the single source of truth for declarative infrastructure and application configurations. It enables teams to manage and deploy applications and infrastructure in a version-controlled, automated, and auditable manner.

Argo CD extends the principles of GitOps to Kubernetes clusters. Treating Git as the single source of truth for Kubernetes configurations, Argo CD ensures the declarative state defined in the Git repository is reflected in the actual cluster.

Continuous delivery in Kubernetes

Continuous integration and continuous delivery (CI/CD) automates merging code, running tests, and building container images for applications. At the end of many teams’ CI/CD pipeline is a container image pushed to a registry and available for deployment on Kubernetes.

When making changes to the application, it may also be necessary to update the Kubernetes cluster to reflect changes in the code. This is where Argo CD comes into play. Developers can update the manifests stored in the Git repository which will be automatically reflected in the Kubernetes cluster when Argo CD synchronizes the two.

This eliminates the need to manually apply changes to the Kubernetes cluster by running a command like kubectl apply or creating custom scripts.

Argo CD is a continuous delivery tool, ensuring that application configurations are always in a deployable state. However, Argo CD can also enable a continuous deployment strategy, where new changes are automatically applied to Kubernetes after being merged into the repository, without manual intervention.

While the differences between continuous delivery and continuous deployment can be subtle, Argo CD can play a role in both approaches.

Using Kubernetes configuration management tools with Argo CD

Argo CD natively supports Helm and Kustomize, two popular Kubernetes configuration management tools.

When using Helm with Argo CD, Helm Charts replace YAML/JSON manifests in your Git repository. Helm Charts package multiple Kubernetes resources (Deployments, Services, ConfigMaps) into a reusable format, reducing complexity.

Argo CD can directly pull, template, and deploy Helm charts from Git repositories or Helm chart repositories, ensuring GitOps workflows remain declarative.

Kustomize allows you to create overlays for different environments (e.g., dev, staging, production) without duplicating base Kubernetes manifests. Unlike Helm, Kustomize uses pure YAML and makes adjustments through overlays, making it more straightforward to understand and maintain without needing templating syntax.

Kustomize fits well with GitOps workflows, as Argo CD can track and apply the YAML files directly, ensuring the cluster state matches what’s stored in Git.

Combining Helm and Kustomize in Argo CD allows teams to use Helm for packaging and templating complex applications while leveraging Kustomize for environment-specific customizations without modifying the base charts. This approach enables reusable, versioned application configurations with flexible, maintainable customizations across multiple environments.

Argo CD best practices

Here are some general guidelines and best practices for any Argo CD environment:

• Always test new manifests: Validate manifests as a step in your CI/CD pipeline before pushing them to Git. You can validate manifests using the kubectl dry-run flag or with a tool like kubeval.
• Separate code and configuration repositories: A general GitOps best practice is to utilize a multi-repo strategy that segments application code, infrastructure as code, and Kubernetes manifests into separate repositories. If everything is in a shared repository, changing your Kubernetes manifests requires changing the application code’s repository and potentially triggering the CI/CD pipeline. With a multi-repo approach, developers can modify individual repositories without running the entire CI/CD pipeline.
• Set your synchronization policies: Automatic synchronization may not be right for every environment. You may want to configure Argo CD to alert you of any changes or configuration drift before synchronizing. This is particularly useful in production environments where you want manual control over any synchronization.

Adding Argo CD to a CI/CD pipeline

Argo CD’s automation capabilities make it an ideal complement for a CI/CD pipeline. Any team utilizing a continuous integration (CI) tool to build images for deployment to Kubernetes can easily add Argo CD to the end of their pipeline.

Deploying to Kubernetes with Argo CD and CircleCI is a straightforward process. Adding Argo CD to a CircleCI pipeline optimizes the CI/CD workflow by providing a declarative, automated, and consistent deployment process for Kubernetes applications. It improves deployment speed, reduces errors, and enhances collaboration while ensuring environment consistency and automated rollback options.

Conclusion

Argo CD is a useful means of applying GitOps principles to Kubernetes environments. It utilizes Git repositories as a single source of truth for Kubernetes configurations and automatically applies configuration settings to Kubernetes clusters to synchronize with the state declared in the repository.

Argo CD is a natural partner for CI/CD tools and principles. It turns Kubernetes configuration into a declarative process and automates key steps, ensuring consistency and reducing toil for DevOps teams. Argo CD enjoys a comfortable position near the end of a CI/CD pipeline, ensuring container images have a properly configured Kubernetes cluster for deployment.

Ready to integrate Argo CD into your pipeline? Start by signing up for a free CircleCI account.