Skip to main content

Synchronization strategies

Table of Contents​

  1. Manual vs automatic sync
  2. Auto-sync
  3. Self-heal
  4. Prune
  5. Sync Options
  6. Hands-on exercises

1 - Manual vs automatic sync​

Manual sync​

spec:
  syncPolicy: {}  # Pas de politique = manuel
# Synchroniser manuellement
argocd app sync my-app

Automatic sync​

spec:
  syncPolicy:
    automated: {}

Comparison​

AspectManualAutomatic
ControlTotalDelegated to Argo CD
SpeedDepends on the humanInstant
RiskReview possibleImmediate deployment
Use caseCritical productionDev, staging

🔝 Back to table of contents​

2 - Auto-sync​

Enabling​

spec:
  syncPolicy:
    automated:
      prune: false      # Ne pas supprimer les ressources orphelines
      selfHeal: false   # Ne pas corriger le drift

Behavior​

  1. Argo CD detects a change in Git (polling every 3 min)
  2. Compares with the cluster state
  3. If OutOfSync → synchronizes automatically

Polling configuration​

# argocd-cm ConfigMap
apiVersion: v1
kind: ConfigMap
metadata:
  name: argocd-cm
  namespace: argocd
data:
  timeout.reconciliation: 180s  # 3 minutes par défaut

Webhook for instant sync​

# Configurer un webhook GitHub/GitLab
# POST https://argocd.example.com/api/webhook

# Argo CD synchronise immédiatement au lieu d'attendre le polling

🔝 Back to table of contents​

3 - Self-heal​

Definition​

Self-heal automatically corrects drift (manual modification of the cluster).

spec:
  syncPolicy:
    automated:
      selfHeal: true

Drift example​

# Git dit : replicas=3
# Quelqu'un fait :
kubectl scale deployment my-app --replicas=5

# Avec selfHeal=true :
# Argo CD détecte le drift et remet à 3 replicas

# Avec selfHeal=false :
# L'application reste OutOfSync jusqu'au prochain sync manuel

Workflow​

When to use​

ScenarioselfHeal
Stable productiontrue
Debugging in progressfalse
Manual testsfalse
Strict compliancetrue

🔝 Back to table of contents​

4 - Prune​

Definition​

Prune deletes cluster resources that are no longer in Git.

spec:
  syncPolicy:
    automated:
      prune: true

Example​

# Git contient :
# - deployment.yaml
# - service.yaml

# Vous supprimez service.yaml de Git

# Avec prune=true :
# Le Service est supprimé du cluster

# Avec prune=false :
# Le Service reste dans le cluster (orphelin)

Protection against deletion​

metadata:
  annotations:
    # Cette ressource ne sera jamais prunée
    argocd.argoproj.io/sync-options: Prune=false

PruneLast​

spec:
  syncPolicy:
    syncOptions:
      - PruneLast=true  # Prune après le sync des autres ressources

🔝 Back to table of contents​

5 - Sync Options​

Available options​

spec:
  syncPolicy:
    syncOptions:
      - CreateNamespace=true      # Créer le namespace si absent
      - PruneLast=true            # Prune à la fin
      - ApplyOutOfSyncOnly=true   # Appliquer seulement les ressources OutOfSync
      - PrunePropagationPolicy=foreground  # Politique de suppression
      - Replace=true              # Utiliser replace au lieu d'apply
      - ServerSideApply=true      # Server-side apply (K8s 1.22+)

CreateNamespace​

spec:
  destination:
    namespace: new-namespace  # Sera créé automatiquement
  syncPolicy:
    syncOptions:
      - CreateNamespace=true

Retry​

spec:
  syncPolicy:
    automated:
      prune: true
      selfHeal: true
    retry:
      limit: 5
      backoff:
        duration: 5s
        factor: 2
        maxDuration: 3m

Sync Waves​

# Déployer dans un ordre spécifique
metadata:
  annotations:
    argocd.argoproj.io/sync-wave: "1"  # Déployé en premier

# Ordre : -1 → 0 → 1 → 2 → ...

🔝 Back to table of contents​

6 - Hands-on exercises​

Exercise 1: Test Self-heal​

# 1. Créer une application avec self-heal
argocd app create test-heal \
  --repo https://github.com/argoproj/argocd-example-apps.git \
  --path guestbook \
  --dest-server https://kubernetes.default.svc \
  --dest-namespace default \
  --sync-policy automated \
  --self-heal

# 2. Synchroniser
argocd app sync test-heal

# 3. Modifier manuellement
kubectl scale deployment guestbook-ui --replicas=5

# 4. Observer
argocd app get test-heal
# Après quelques secondes, replicas revient à 1

Exercise 2: Complete configuration​

apiVersion: argoproj.io/v1alpha1
kind: Application
metadata:
  name: production-app
  namespace: argocd
spec:
  project: default
  source:
    repoURL: https://github.com/org/gitops.git
    path: apps/my-app
    targetRevision: main
  destination:
    server: https://kubernetes.default.svc
    namespace: production
  syncPolicy:
    automated:
      prune: true
      selfHeal: true
    syncOptions:
      - CreateNamespace=true
      - PruneLast=true
    retry:
      limit: 3
      backoff:
        duration: 5s
        factor: 2
        maxDuration: 1m

Quiz​

Q1. What does prune: true do?

Answer

Prune deletes from the cluster resources that are no longer present in Git. If you delete a YAML file from the repo, the corresponding resource will be deleted from the cluster.

Q2. What is the difference between selfHeal and auto-sync?

Answer
  • auto-sync: Synchronizes when Git changes
  • selfHeal: Synchronizes when the cluster changes (drift)

Both are complementary: auto-sync reacts to Git changes, selfHeal corrects manual cluster modifications.

🔝 Back to table of contents​

Key takeaways​

  • Auto-sync: automatic deployment on Git change
  • Self-heal: automatic drift correction
  • Prune: deletion of orphaned resources
  • Use Sync Waves for deployment order
  • CreateNamespace to auto-create namespaces

← Previous chapter | Next chapter: UI and CLI →