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A deeper understanding of how Argo CD works

Popularity：811 ℃/2024-09-09 08:13:31

1. Architecture of ArgoCD

ArgoCD is a Kubernetes-native continuous delivery tool that automates the deployment of applications to Kubernetes clusters by monitoring application definitions in Git repositories. Its core architecture consists of the following key components:

  • API ServerAPI Server is the API entry point for ArgoCD, providing an external interface for users or external tools to interact with ArgoCD.

  • Repository ServerRepository Server is responsible for interacting with Git repositories. It pulls application definitions from the repository and transforms them into Kubernetes manifest files.The Repository Server caches the files fetched from the Git repository to speed up subsequent operations.

  • Controller: A core controller that continuously monitors the difference between the current state of the Kubernetes cluster and the desired state (defined in the Git repository).The controller is responsible for aligning the state of the cluster with the desired state in Git.

  • Application Controller: Responsible for processing user-defined ArgoCD Application resources. It checks the definitions in the Git repository and ensures that they are synchronized with the application state in the Kubernetes cluster.

  • Redis Server: Used to cache data and improve system performance, especially important when dealing with large numbers of applications and frequent synchronization operations.

  • Web UI: Provides a user-friendly graphical interface that allows users to view application status, synchronization status, and perform manual operations through the Web UI.

2. How ArgoCD works

The core concept of ArgoCD is GitOps, which is to use a Git repository as a single source of truth, and synchronize the application configurations in the repository to the Kubernetes cluster through automation.

  1. Define the application: Users define the application's Kubernetes resource manifests in a Git repository and submit these manifest files to the Git repository.

  2. Creating an ArgoCD Application: Create aApplication resource, which describes where the application is located in the Git repository and where it is deployed in the Kubernetes cluster.

  3. Synchronized status monitoringThe ArgoCD Controller continuously monitors the configuration in the Git repository and compares it to the current cluster state. Every time it detects a change in the configuration of an application in the Git repository, the Controller will automatically update the resources in the cluster to maintain consistency with the Git repository.

  4. Automatic vs. manual synchronization: ArgoCD supports both automatic and manual synchronization. In automatic synchronization mode, ArgoCD will automatically update the resources in the Kubernetes cluster once it detects a change in the Git repository. In manual synchronization mode, you need to trigger the synchronization operation manually.

  5. Rollback function: If an application update causes problems, ArgoCD provides a rollback feature that allows users to easily revert to a previous state.

3. Mechanism for automatic pulling of updates by ArgoCD

ArgoCD automates the pulling and updating mechanism by continuously monitoring changes in the Git repository. Here's how it works:

  1. timed polling: The ArgoCD Controller polls a specified Git repository periodically to check for new commits. By default, this polling cycle is every 3 minutes.

  2. Webhook Trigger: In order to respond faster to updates, ArgoCD supports triggering synchronization via a Git repository's Webhook. When a commit or merge request occurs in a repository, GitLab, GitHub, etc. can notify ArgoCD via a webhook to synchronize immediately.

  3. status comparison: Each time the latest Git repository status is pulled, ArgoCD compares it with the current state of the applications in the cluster. If any discrepancies are found, ArgoCD will automatically perform a synchronization operation to ensure that the cluster and Git repositories are configured the same way.

4. How to integrate with GitLab

ArgoCD seamlessly integrates with GitLab to automate deployments through Webhook and CI/CD pipelines. Here are the integration steps:

1. Configure the GitLab project

Create or select a project in GitLab, make sure the project contains Kubernetes resource manifest files, and store these files in a directory in the repository, for examplemanifests/。

2. Configuring the ArgoCD Application

In ArgoCD, create aApplication resource, point to the GitLab repository, and set up the associated path and target cluster. Example:

apiVersion: /v1alpha1
kind: Application
metadata:
  name: my-app
  namespace: argocd
spec:
  project: default
  source:
    repoURL: '/your-username/'
    path: 'manifests'
    targetRevision: HEAD
  destination:
    server: ''
    namespace: my-namespace
  syncPolicy:
    automated:
      prune: true
      selfHeal: true

 

3. Configuring the GitLab Webhook

Go to the GitLab project'sSettings -> WebhooksIf you want to add a new Webhook, fill in the URL with the address of the ArgoCD's Webhook, which is usually/api/webhookSelectionPush events maybeMerge request events Trigger.

4. Validating integration

Whenever there is a new commit or merge request in a GitLab repository, ArgoCD receives a webhook notification and triggers a synchronization operation immediately. You can view the progress and results of the synchronization through ArgoCD's Web UI.

5. Example code

Below is a complete example of an ArgoCD Application configuration, integrated with GitLab and automatically synchronized:

apiVersion: /v1alpha1
kind: Application
metadata:
  name: demo-app
  namespace: argocd
spec:
  project: default
  source:
    repoURL: '/your-username/'
    path: 'k8s-manifests'
    targetRevision: HEAD
  destination:
    server: ''
    namespace: demo
  syncPolicy:
    automated:
      prune: true
      selfHeal: true
  webhook:
    gitlab:
      - url: https:///api/webhook
        secret: your-webhook-secret

 

In the above configuration, thesyncPolicy hit the nail on the headautomated parameter enables auto-sync and auto-repair, which means that ArgoCD automatically pulls the latest configuration from GitLab and applies it to the Kubernetes cluster.

With the above content, you should have understood the architecture of ArgoCD, how it works, the mechanism of pulling updates automatically, and how to integrate it with GitLab.ArgoCD's powerful automation and GitOps capabilities make your application deployment more efficient and reliable.

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