Ensure Continuous Delivery On Kubernetes With GitOps’ Argo CD
GitOps is a Continuous Deployment model for cloud-native applications. In GitOps, the Git repositories which contain the declarative descriptions of the infrastructure are considered as the single source of truth for the desired state of the system and we need to have an automated way to ensure that the deployed state of the system always matches the state defined in the Git repository. All the changes (deployment/upgrade/rollback) on the environment are triggered by changes (commits) made on the Git repository
“The artifacts that we run on any environment always have a corresponding code for them on some Git repositories. Can we say the same thing for our infrastructure code?”
Infrastructure as code tools, completely declarative orchestration tools like Kubernetes allow us to represent the entire state of our system in a declarative way. GitOps intends to make use of this ability and make infrastructure-related operations more developer-centric.
Role of Infrastructure as Code (IaC) in GitOps
The ability to represent the infrastructure as code is at the core of GitOps. But just having versioned controlled infrastructure as code doesn’t mean GitOps, we also need to have a mechanism in place to keep (try to keep) our deployed state in sync with the state we define in the Git repository.
“Infrastructure as Code is necessary but not sufficient to achieve GitOps”
GitOps does pull-based deployments
Most of the deployment pipelines we see currently, push the changes in the deployed environment. For example, consider that we need to upgrade our application to a newer version then we will update its docker image tag in some repository which will trigger a deployment pipeline and update the deployed application. Here the changes were pushed on the environment.
In GitOps, we just need to update the image tag on the Git repository for that environment and the changes will be pulled to the environment to match the updated state in the Git repository. The magic of keeping the deployed state in sync with state-defined on Git is achieved with the help of operators/agents. The operator is like a control loop which can identify differences between the deployed state and the desired state and make sure they are the same.
Key benefits of GitOps:
- All the changes are verifiable and auditable as they make their way into the system through Git repositories.
- Easy and consistent replication of the environment as Git repository is the single source of truth. This makes disaster recovery much quicker and simpler.
- More developer-centric experience for operating infrastructure. Also a smaller learning curve for deploying dev environments.
- Consistent rollback of application as well as infrastructure state.
Introduction to Argo CD
Argo CD is a continuous delivery tool that works on the principles of GitOps and is built specifically for Kubernetes. The product was developed and open-sourced by Intuit and is currently a part of CNCF.
Key components of Argo CD:
- API Server: Just like K8s, Argo CD also has an API server that exposes APIs that other systems can interact with. The API server is responsible for managing the application, repository and cluster credentials, enforcing authentication and authorization, etc.
- Repository server: The repository server keeps a local cache of the Git repository, which holds the K8s manifest files for the application. This service is called by other services to get the K8s manifests.
- Application controller: The application controller continuously watches the deployed state of the application and compares it with the desired state of the application, reports the API server whenever they are not in sync with each other and seldom takes corrective actions as well. It is also responsible for executing user-defined hooks for various lifecycle events of the application.
Key objects/resources in Argo CD:
- Application: Argo CD allows us to represent the instance of the application which we want to deploy in an environment by creating Kubernetes objects of a custom resource definition(CRD) named Application. In the specification of Application type objects, we specify the source (repository) of our application’s K8s manifest files, the K8s server where we want to deploy those manifests, namespace, and other information.
- AppProject: Just like Application, Argo CD provides another CRD named AppProject. AppProjects are used to logically group related-applications.
- Repo Credentials: In the case of private repositories, we need to provide access credentials. For credentials, Argo CD uses the K8s secrets and config map. First, we create objects of secret types and then we update a special-purpose configuration map named argocd-cm with the repository URL and the secret which contains the credentials.
- Cluster Credentials: Along with Git repository credentials, we also need to provide the K8s cluster credentials. These credentials are also managed using K8s secret, we are required to add the label argocd.argoproj.io/secret-type: cluster to these secrets.
Enough of theory, let’s try out the things we discussed above. For the demo, I have created a simple app named message-app. This app reads a message set in the environment variable named MESSAGE. We will populate the values of this environment variable using a K8s config map. I have kept the K8s manifest files for the app in a separate repository. We have the application and the K8s manifest files ready. Now we are all set to install Argo CD and deploy our application.
Installing Argo CD:
For installing Argo CD, we first need to create a namespace named argocd.
Applying the files from the argo-cd repo directly is fine for demo purposes, but in actual environments, you must copy the file in your repository before applying them.
We can see that this command has created the core components and CRDs we discussed earlier in the blog. There are some additional resources as well but we can ignore them for the time being.
Accessing the Argo CD GUI
We have the Argo CD running in our cluster, Argo CD also provides a GUI which gives us a graphical representation of our k8s objects. It allows us to view events, pod logs, and other configurations.
By default, the GUI service is not exposed outside the cluster. Let us update its service type to LoadBalancer so that we can access it from outside.
After this, we can use the external IP of the argocd-server service and access the GUI.
The initial username is admin and the password is the name of the api-server pod. The password can be obtained by listing the pods in the argocd namespace or directly by this command.
Deploy the app:
Now let’s go ahead and create our application for the staging environment for our message app.
In the application spec, we have specified the repository, where our manifest files are stored and also the path of the files in the repository.
We want to deploy our app in the same k8s cluster where ArgoCD is running so we have specified the local k8s service URL in the destination. We want the resources to be deployed in the staging namespace, so we have set it accordingly.
In the sync policy, we have enabled automated sync. We have kept the project as default.
Adding the resources-finalizer.argocd.argoproj.io ensures that all the resources created for the application are deleted when the Application is deleted. This is fine for our demo setup but might not always be desirable in real-life scenarios.
Our git repos are public so we don’t need to create secrets for git repo credentials.
We are deploying in the same cluster where Argo CD itself is running. As this is a demo setup, we can use the admin user created by Argo CD, so we don’t need to create secrets for cluster credentials either.
Now let’s go ahead and create the application and see the magic happen.
As soon as the application is created, we can see it on the GUI.
By clicking on the application, we can see all the Kubernetes objects created for it.
It also shows the objects which are indirectly created by the objects we create. In the above image, we can see the replica set and endpoint object which are created as a result of creating the deployment and service respectively.
We can also click on the individual objects and see their configuration. For pods, we can see events and logs as well.
As our app is deployed now, we can grab public IP of message-app service and access it on the browser.
We can see that our app is deployed and accessible.
Updating the app
For updating our application, all we need to do is commit our changes to the GitHub repository. We know the message-app just displays the message we pass to it via. Config map, so let’s update the message and push it to the repository.
Once the commit is done, Argo CD will start to sync again.
Once the sync is done, we will restart our message app pod, so that it picks up the latest values in the config map. Then we need to refresh the browser to see updated values.
As we discussed earlier, for making any changes to the environment, we just need to update the repo which is being used as the source for the environment and then the changes will get pulled in the environment.
We can follow an exact similar approach and deploy the application to the production environment as well. We just need to create a new application object and set the manifest path and deployment namespace accordingly.
It’s still early days for GitOps, but it has already been successfully implemented at scale by many organizations. As the GitOps tools mature along with the ever-growing adoption of Kubernetes, I think many organizations will consider adopting GitOps soon. GitOps is not limited only to Kubernetes, but the completely declarative nature of Kubernetes makes it simpler to achieve GitOps. Argo CD is a deployment tool that’s tailored for Kubernetes and allows us to do deployments in a Kubernetes native way while following the principles of GitOps.I hope this blog helped you in understanding how what and why of GitOps and gave some insights to Argo CD.
Originally published at https://www.velotio.com.