[G1-Sync] Manual knowledge update

2026-05-10 22:08:15 +09:00
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+---
+id: devops-argocd-apps-deep
+title: ArgoCD Applications — App-of-Apps / ApplicationSet
+category: Coding
+status: draft
+source_trust_level: B
+verification_status: conceptual
+created_at: 2026-05-09
+updated_at: 2026-05-09
+tags: [devops, argocd, gitops, vibe-coding]
+tech_stack: { language: "YAML", applicable_to: ["DevOps"] }
+applied_in: []
+aliases: [ArgoCD, Application, ApplicationSet, App-of-Apps, GitOps, declarative deploy]
+---
+
+# ArgoCD Applications Deep
+
+> K8s GitOps 의 표준. **Application + ApplicationSet + App-of-Apps**. Declarative, drift detect, rollback.
+
+## 📖 핵심 개념
+- Git = source of truth.
+- ArgoCD 가 sync to cluster.
+- Drift detect + reconcile.
+- Multi-cluster + multi-tenant.
+
+## 💻 코드 패턴
+
+### Application
+```yaml
+apiVersion: argoproj.io/v1alpha1
+kind: Application
+metadata:
+  name: my-app
+  namespace: argocd
+spec:
+  project: default
+  source:
+    repoURL: https://github.com/me/my-app
+    targetRevision: HEAD
+    path: kubernetes
+  destination:
+    server: https://kubernetes.default.svc
+    namespace: production
+  syncPolicy:
+    automated:
+      prune: true
+      selfHeal: true
+    syncOptions:
+      - CreateNamespace=true
+```
+
+→ Git path 의 manifest 가 cluster.
+
+### Sync wave (order)
+```yaml
+metadata:
+  annotations:
+    argocd.argoproj.io/sync-wave: '-1'   # 먼저
+```
+
+→ "Database 먼저, app 다음" 식.
+
+### Hook (pre/post sync)
+```yaml
+metadata:
+  annotations:
+    argocd.argoproj.io/hook: PreSync
+    argocd.argoproj.io/hook-delete-policy: HookSucceeded
+```
+
+→ DB migration 가 sync 전.
+
+### App-of-Apps
+```yaml
+apiVersion: argoproj.io/v1alpha1
+kind: Application
+metadata:
+  name: root
+spec:
+  source:
+    path: apps/   # 매 file 가 Application
+  destination: { ... }
+```
+
+```yaml
+# apps/users.yaml
+apiVersion: argoproj.io/v1alpha1
+kind: Application
+metadata:
+  name: users-service
+spec: ...
+
+# apps/orders.yaml
+...
+```
+
+→ 1 root Application 가 모든 다른 Application 관리.
+
+### ApplicationSet (generator)
+```yaml
+apiVersion: argoproj.io/v1alpha1
+kind: ApplicationSet
+metadata:
+  name: per-cluster
+spec:
+  generators:
+    - clusters: {}     # 매 cluster 의 Application
+  template:
+    metadata:
+      name: '{{name}}-app'
+    spec:
+      source:
+        repoURL: ...
+        path: 'manifests/{{name}}'
+      destination:
+        server: '{{server}}'
+```
+
+→ 매 cluster / branch / file 가 자동 Application.
+
+### Generator types
+```
+- List: explicit list.
+- Cluster: matrix (모든 cluster).
+- Git directory: 매 dir = Application.
+- Git file: 매 file.
+- Pull request: 매 PR 가 preview env.
+- Matrix: combine.
+- Merge: combine + override.
+```
+
+### PR preview environment
+```yaml
+generators:
+  - pullRequest:
+      github:
+        owner: me
+        repo: my-app
+        labels: ['preview']
+template:
+  metadata:
+    name: 'preview-{{number}}'
+  spec:
+    source:
+      targetRevision: '{{branch}}'
+      path: kubernetes
+    destination:
+      namespace: 'preview-{{number}}'
+```
+
+→ 매 PR 의 own preview namespace.
+
+### Multi-cluster
+```yaml
+generators:
+  - clusters:
+      selector:
+        matchLabels:
+          environment: production
+template:
+  spec:
+    destination:
+      server: '{{server}}'
+```
+
+→ 매 prod cluster 에 자동 deploy.
+
+### Sync options
+```yaml
+syncPolicy:
+  automated: { prune: true, selfHeal: true }
+  syncOptions:
+    - CreateNamespace=true
+    - ServerSideApply=true
+    - ApplyOutOfSyncOnly=true
+    - RespectIgnoreDifferences=true
+```
+
+### Ignore differences
+```yaml
+ignoreDifferences:
+  - group: apps
+    kind: Deployment
+    jsonPointers:
+      - /spec/replicas    # HPA 가 manage
+```
+
+→ Replica drift 무시.
+
+### Health check
+```yaml
+# Custom resource
+metadata:
+  annotations:
+    argocd.argoproj.io/sync-options: SkipDryRunOnMissingResource=true
+```
+
+```yaml
+# Or custom health Lua script
+data:
+  resource.customizations: |
+    redis.io/Redis:
+      health.lua: |
+        if obj.status ~= nil then
+          if obj.status.phase == 'Ready' then
+            return { status = 'Healthy' }
+          end
+        end
+```
+
+### Helm + ArgoCD
+```yaml
+spec:
+  source:
+    repoURL: https://charts.example.com
+    chart: my-chart
+    targetRevision: 1.2.3
+    helm:
+      values: |
+        replicaCount: 3
+        image: ...
+```
+
+### Kustomize + ArgoCD
+```yaml
+spec:
+  source:
+    path: overlays/production
+    kustomize:
+      images:
+        - my-app:v1.2.3
+```
+
+### Notification
+```yaml
+# argocd-notifications-cm
+data:
+  service.slack: |
+    token: $slack-token
+  
+  template.app-deployed: |
+    message: 'App {{.app.metadata.name}} deployed.'
+  
+  trigger.on-deployed: |
+    when: app.status.operationState.phase == 'Succeeded'
+    send: [app-deployed]
+  
+  subscriptions: |
+    - recipients: [slack:deployments]
+      triggers: [on-deployed]
+```
+
+### RBAC
+```yaml
+data:
+  policy.csv: |
+    p, role:dev, applications, sync, default/*, allow
+    p, role:dev, applications, get, default/*, allow
+    g, dev-team, role:dev
+```
+
+→ Team 별 access.
+
+### Project (multi-tenant)
+```yaml
+apiVersion: argoproj.io/v1alpha1
+kind: AppProject
+metadata:
+  name: my-team
+spec:
+  sourceRepos:
+    - https://github.com/me/team-repo
+  destinations:
+    - namespace: 'team-*'
+      server: '*'
+  clusterResourceWhitelist:
+    - group: '*'
+      kind: '*'
+```
+
+### vs Flux
+```
+ArgoCD: UI 친화, 큰 ecosystem.
+Flux: simple, GitOps Toolkit.
+
+→ ArgoCD 가 더 popular.
+```
+
+### vs Argo Rollouts (다름)
+```
+ArgoCD: app deploy.
+Argo Rollouts: progressive delivery (canary, blue-green).
+
+→ 둘 다 같이 사용.
+```
+
+→ [[DevOps_Argo_Rollouts]].
+
+### Production patterns
+```
+1. Git monorepo 의 매 app folder.
+2. ApplicationSet 가 자동 Application 생성.
+3. PR preview 가 매 feature.
+4. Notification 가 Slack.
+5. Project 별 RBAC.
+6. Argo Rollouts 가 progressive deploy.
+```
+
+### Disaster recovery
+```
+ArgoCD 자체 가 down:
+- Cluster 의 manifest 유지 (기존 deploy).
+- 새 deploy 안 됨.
+- Restore from backup (etcd).
+
+→ ArgoCD 가 second cluster 의 backup.
+```
+
+### Cost
+```
+ArgoCD: 무료 (open source).
+Cluster compute: ArgoCD 가 작은 (작은 deployment).
+
+→ Self-host 가 cheap.
+```
+
+### Real-world
+- **Intuit** (creator).
+- **Adobe**: 큰 user.
+- **Red Hat OpenShift**: native integration.
+- **Tetrate / SAP**: 큰 deployment.
+
+### 함정
+```
+- Auto-sync 가 모든 변경 즉시: 위험.
+- No project / RBAC: 매 user 가 모든.
+- Manifest drift (manual kubectl): self-heal 가 fight.
+- Big repo (10k+ Application): 느린.
+- Helm value secret in git: 안 됨 (External Secrets).
+```
+
+### Best practice
+```
+1. App-of-Apps 또는 ApplicationSet (DRY).
+2. Self-heal + auto-sync (production).
+3. Pre-sync hook 의 migration.
+4. Notification (Slack).
+5. RBAC + Project.
+6. Argo Rollouts 가 progressive.
+7. Secret management (External Secrets / Sealed Secrets).
+```
+
+## 🤔 의사결정 기준
+| 작업 | 추천 |
+|---|---|
+| K8s GitOps | ArgoCD |
+| Simple GitOps | Flux |
+| Multi-cluster | ApplicationSet |
+| Preview env | PR generator |
+| Helm | Helm + ArgoCD |
+| Kustomize | Kustomize + ArgoCD |
+| Progressive deploy | Argo Rollouts |
+
+## ❌ 안티패턴
+- **Manual kubectl**: drift.
+- **No project**: shared cluster + 매 user.
+- **Big mono Application**: slow sync.
+- **Secret in git**: leak.
+- **No notification**: silent failure.
+- **No backup**: lost.
+
+## 🤖 LLM 활용 힌트
+- ArgoCD = K8s GitOps standard.
+- ApplicationSet 가 multi-cluster / multi-app.
+- App-of-Apps 가 hierarchy.
+- Argo Rollouts 와 함께 progressive.
+
+## 🔗 관련 문서
+- [[DevOps_ArgoCD_GitOps]]
+- [[DevOps_Argo_Rollouts]]
+- [[DevOps_K8s_Operators]]