Argo CD - Architectural Overview

Components

API Server

The API server is a gRPC/REST server which exposes the API consumed by the Web UI, CLI, and CI/CD systems. It has the following responsibilities:

application management and status reporting
invoking of application operations (e.g. sync, rollback, user-defined actions)
repository and cluster credential management (stored as K8s secrets)
authentication and auth delegation to external identity providers
RBAC enforcement
listener/forwarder for git webhook events

Repository Server

The repository server is an internal service which maintains a local cache of the git repository holding the application manifests. It is responsible for generating and returning the Kubernetes manifests when provided the following inputs:

repository URL
git revision (commit, tag, branch)
application path
template specific settings: parameters, ksonnet environments, helm values.yaml

Application Controller

The application controller is a Kubernetes controller which continuously monitors running applications and compares the current, live state against the desired target state (as specified in the git repo). It detects OutOfSync application state and optionally takes corrective action. It is responsible for invoking any user-defined hooks for lifcecycle events (PreSync, Sync, PostSync)

Application CRD (Custom Resource Definition)

The Application CRD is the Kubernetes resource object representing a deployed application instance in an environment. It is defined by two key pieces of information:

source reference to the desired state in git (repository, revision, path, environment)
destination reference to the target cluster and namespace.

An example spec is as follows:

spec:
  project: default
  source:
    repoURL: https://github.com/argoproj/argocd-example-apps.git
    targetRevision: HEAD
    path: guestbook
    environment: default
  destination:
    server: https://kubernetes.default.svc
    namespace: default

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