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📄 धडा 07 — पहिले Application: प्लॅन-वहीचे एक पान

📍 तुम्ही इथे आहात: 12 पैकी धडा 07 · मागे: lesson-06-install-argocd · पुढे: lesson-08-sync-policies

📦 या ब्रँचमध्ये काय आहे

धडे 01–06, आणि त्यासोबत तो क्षण जेव्हा सगळे उमगते: एक Application resource जे ArgoCD कडून या repo चा k8s/ folder deploy करून घेते — आणि त्यानंतर तुम्ही पुन्हा कधीच app kubectl apply करत नाही. खरी file:

argocd/application.yaml — प्लॅनचे पान, जसेच्या तसे deploy करता येणारे

🧒 5 वर्षांच्या मुलाला समजावल्यासारखे

Robot कामावर ठेवलेला आहे पण रिकामा बसलेला आहे. तुम्ही त्याला प्लॅन-वहीचे एक पान 📄 देता, ज्यावर नेमक्या तीन गोष्टी आहेत:

कोणती वही 📖 — "ग्रंथालयातली learn-argocd-school नावाची वही" (git repoURL).
कोणता धडा — "k8s/ हा धडा वाचा, main आवृत्ती" (path आणि targetRevision).
कोणती खोली 🚪 — "या इमारतीतला gitops-school वर्ग त्याच्याशी जुळवा" (destination).

Robot पान वाचतो, ग्रंथालयात जातो, तो धडा घेऊन येतो, खोलीत जातो… आणि ती बांधतो: namespace, deployment, service, pods. तुम्ही काहीच वाहून नेले नाही. 🎒❌

आणि खरी जादू: हे पान कधीच कालबाह्य होत नाही. Robot दर काही मिनिटांनी, कायम, तो धडा पुन्हा वाचतो. धडा बदलणे (git commit) हाच आता ती खोली बदलण्याचा एकमेव मार्ग आहे — आणि तुम्हाला नेमके हेच हवे आहे.

🗺️ आकृती

flowchart LR
    app["📄 Application<br/>repoURL: learn-argocd-school<br/>path: k8s/ · rev: main<br/>dest: gitops-school"]
    argo["🤖 ArgoCD"]
    book["📖 git: the k8s/ chapter<br/>deployment + service + ns"]
    room["🚪 gitops-school<br/>🪑🪑 hello-school pods LIVE"]
    app -->|"1 kubectl apply -f argocd/"| argo
    argo -->|"2 clones and renders"| book
    argo -->|"3 creates everything"| room

❓ काय

Application = ArgoCD चा स्वतःचा resource type: source (repo/path/revision)
- destination (cluster/namespace) + policy (धडा 08).
ArgoCD render केलेले manifests live cluster शी तुलना करतो आणि निकाल status म्हणून दाखवतो: Synced ✅ (जुळते) किंवा OutOfSync 🟡 (वेगळे आहे) — आणि workloads कडून Healthy/Progressing/Degraded.
Public repos ना credentials लागत नाहीत — robot चे ग्रंथालय कार्ड सरळ चालते. (Private repos: एक repo secret जोडा; कल्पना अगदी तीच.)

🤔 का

ही एकच file यांची जागा घेते: तुमचा kubectl apply विधी, pipeline चा deploy job, "काय deploy झाले आहे?" ची spreadsheet, आणि पहाटे 3 वाजताची "prod git मध्ये आहे तसेच आहे का?" ही शंका. एक पान → ती खोली कायमची robot च्या ताब्यात.

✅ खड्डा 2 बुजला. Cluster च्या बाहेर आता कोणाकडेही deployment credentials नाहीत: robot आतूनच वही pull करतो, फक्त त्याचे ग्रंथालय कार्ड (git ला read access) वापरून. तुमच्या CI मधून kubeconfig पूर्णपणे काढून टाकता येते.

🔧 कसे (file, टिपांसह)

spec:
  source:
    repoURL: https://github.com/BaluRaut/learn-argocd-school.git
    targetRevision: main       # which edition to follow (branch/tag)
    path: k8s                  # which chapter (folder of manifests)
  destination:
    server: https://kubernetes.default.svc   # this very cluster
    namespace: gitops-school
  syncPolicy:
    automated: { prune: true, selfHeal: true }   # lesson 08 explains these
    syncOptions: [CreateNamespace=true]

🧪 करून पाहा

# clean slate makes the magic obvious — delete the hand-deployed app first:
kubectl delete namespace gitops-school --ignore-not-found

# hand the robot the page:
kubectl apply -f argocd/application.yaml

# watch it work (UI: the hello-school app appears and turns green), or:
kubectl -n argocd get application hello-school -w    # OutOfSync → Synced; Ctrl+C
kubectl -n gitops-school get all                     # the room, built by the robot 🎉

# proof you're out of the loop now:
kubectl -n gitops-school delete deployment hello-school
kubectl -n gitops-school get deploy -w               # ...it comes BACK. (lesson 08 tells you why)

✅ तपासा — तुम्हाला काय दिसायला हवे

kubectl -n argocd get application hello-school → एक-दोन मिनिटांत SYNC STATUS: Synced, HEALTH STATUS: Healthy. आधी ते OutOfSync / Progressing मधून जाते — ते संक्रमण हाच तर धडा आहे. kubectl -n gitops-school get all robot ने बांधलेले namespace, Deployment, Service आणि दोन pods दाखवते; UI मध्ये tile हिरवी असते, Application → Deployment → ReplicaSet → Pods या tree सह.

🧹 साफसफाई

धडे 08–12 साठी Application राहू द्या. नंतर ते काढताना: kubectl -n argocd delete application hello-school deploy झालेले resources तसेच ठेवते, जोपर्यंत Application वर resources-finalizer.argocd.argoproj.io finalizer नसतो (argocd app delete hello-school --cascade तो जोडते). मग उरलेल्या कशासाठीही kubectl delete namespace gitops-school.

⚠️ नेहमीच्या चुका

path: मध्ये typo किंवा शेवटी slash → ComparisonError — आधी tile वरचा error text तपासा
CreateNamespace=true नसणे → namespace अस्तित्वात नसल्यामुळे पहिला sync fail होतो
push नंतर लगेच sync होईल अशी अपेक्षा — default poll ~3 मिनिटांचा असतो; Refresh click करा, किंवा webhook जोडा
app चे resources हाताने बदलणे आणि आश्चर्यचकित होणे (धडा 08 selfHeal समजावतो)

🏭 प्रत्यक्ष वापरात हे का महत्त्वाचे आहे: Application फक्त YAML आहे, म्हणून तेही git मध्ये राहते (app-of-apps, धडा 11) — Applications सुद्धा कोणी हाताने kubectl apply करत नाही. Private repos ना repo credential (deploy key किंवा GitHub App) लागते, जे CI मध्ये नव्हे तर cluster मध्ये साठवले जाते.

⏭️ पुढे

automated, prune, selfHeal — robot चे घरचे नियम, आणि ते कधी सैल करायचे.

git checkout lesson-08-sync-policies

📄 Lesson 07 — The first Application: one page of the plan book

📍 You are here: Lesson 07 of 12 · Previous: lesson-06-install-argocd · Next: lesson-08-sync-policies

📦 What's in this branch

Lessons 01–06, plus the moment it all clicks: an Application resource that makes ArgoCD deploy this repo's k8s/ folder — and from then on, you never kubectl apply the app again. Real file:

argocd/application.yaml — the plan page, deployable as-is

🧒 Explain like I'm 5

The robot is hired but idle. You hand it one page of the plan book 📄 with exactly three facts on it:

Which book 📖 — "the one in the library called learn-argocd-school" (the git repoURL).
Which chapter — "read the k8s/ chapter, main edition" (the path and targetRevision).
Which room 🚪 — "make classroom gitops-school in THIS building match it" (the destination).

The robot reads the page, walks to the library, fetches the chapter, walks to the room… and builds it: namespace, deployment, service, pods. You never carried anything. 🎒❌

And the deep magic: the page never expires. The robot re-reads the chapter every few minutes, forever. Editing the chapter (a git commit) is now the ONLY way anyone changes that room — which is exactly what you want.

🗺️ Diagram

flowchart LR
    app["📄 Application<br/>repoURL: learn-argocd-school<br/>path: k8s/ · rev: main<br/>dest: gitops-school"]
    argo["🤖 ArgoCD"]
    book["📖 git: the k8s/ chapter<br/>deployment + service + ns"]
    room["🚪 gitops-school<br/>🪑🪑 hello-school pods LIVE"]
    app -->|"1 kubectl apply -f argocd/"| argo
    argo -->|"2 clones and renders"| book
    argo -->|"3 creates everything"| room

❓ What

Application = ArgoCD's own resource type: source (repo/path/revision)
- destination (cluster/namespace) + policy (lesson 08).
ArgoCD compares the rendered manifests against the live cluster and shows the result as a status: Synced ✅ (matches) or OutOfSync 🟡 (differs) — plus Healthy/Progressing/Degraded from the workloads.
Public repos need no credentials — the robot's library card just works. (Private repos: add a repo secret; the idea is identical.)

🤔 Why

This single file replaces: your kubectl apply ritual, the pipeline's deploy job, the "what's deployed?" spreadsheet, and the 3 AM "is prod what's in git?" doubt. One page → the robot owns that room forever.

✅ Gap 2 closed. Nothing outside the cluster holds deployment credentials any more: the robot pulls the book from inside, using only its library card (read access to git). Your CI can lose its kubeconfig entirely.

🔧 How (the file, annotated)

spec:
  source:
    repoURL: https://github.com/BaluRaut/learn-argocd-school.git
    targetRevision: main       # which edition to follow (branch/tag)
    path: k8s                  # which chapter (folder of manifests)
  destination:
    server: https://kubernetes.default.svc   # this very cluster
    namespace: gitops-school
  syncPolicy:
    automated: { prune: true, selfHeal: true }   # lesson 08 explains these
    syncOptions: [CreateNamespace=true]

🧪 Try it

# clean slate makes the magic obvious — delete the hand-deployed app first:
kubectl delete namespace gitops-school --ignore-not-found

# hand the robot the page:
kubectl apply -f argocd/application.yaml

# watch it work (UI: the hello-school app appears and turns green), or:
kubectl -n argocd get application hello-school -w    # OutOfSync → Synced; Ctrl+C
kubectl -n gitops-school get all                     # the room, built by the robot 🎉

# proof you're out of the loop now:
kubectl -n gitops-school delete deployment hello-school
kubectl -n gitops-school get deploy -w               # ...it comes BACK. (lesson 08 tells you why)

✅ Verify — what you should see

kubectl -n argocd get application hello-school → SYNC STATUS: Synced, HEALTH STATUS: Healthy within a minute or two. It passes through OutOfSync / Progressing first — that transition is the lesson. kubectl -n gitops-school get all shows the namespace, Deployment, Service and two pods the robot built; in the UI the tile is green with the tree Application → Deployment → ReplicaSet → Pods.

🧹 Clean up

Keep the Application for lessons 08–12. Removing it later: kubectl -n argocd delete application hello-school leaves the deployed resources in place unless the Application carries the resources-finalizer.argocd.argoproj.io finalizer (argocd app delete hello-school --cascade adds it). Then kubectl delete namespace gitops-school for anything left.

⚠️ Common mistakes

path: with a typo or a trailing slash → ComparisonError — check the tile's error text first
CreateNamespace=true missing → the first sync fails because the namespace does not exist
expecting an instant sync after a push — the default poll is ~3 minutes; click Refresh, or add a webhook
editing the app's resources by hand and being surprised (lesson 08 explains selfHeal)

🏭 Why this matters in production: the Application is just YAML, so it lives in git too (app-of-apps, lesson 11) — nobody kubectl applys Applications by hand either. Private repos need a repo credential (deploy key or GitHub App) stored in the cluster, not in CI.

⏭️ Next

automated, prune, selfHeal — the robot's house rules, and when to loosen them.

git checkout lesson-08-sync-policies

← Previousinstall argocd Next →sync policies

This page is the lesson's README from the lesson-07-first-application branch, shown here so the whole School stays on one site. Code files open on GitHub at the same branch.