Kubernetes কেন — control plane, node আর reconciliation loop

$ kubectl get pods -n kube-system --no-headers
etcd-docker-desktop                      1/1   Running   0     4m52s
kube-apiserver-docker-desktop            1/1   Running   0     4m52s
kube-controller-manager-docker-desktop   1/1   Running   0     4m16s
kube-proxy-j6w94                         1/1   Running   0     4m16s
kube-scheduler-docker-desktop            1/1   Running   0     4m52s
…

apiVersion: apps/v1
kind: Deployment
metadata:
  name: web
  namespace: abcl-ka
  labels:
    app: web
spec:
  replicas: 3
  selector:
    matchLabels:
      app: web
  template:
    metadata:
      labels:
        app: web
    spec:
      containers:
        - name: nginx
          image: nginx:1.27-alpine
          imagePullPolicy: IfNotPresent
          resources:
            requests:
              cpu: 10m
              memory: 16Mi
            limits:
              cpu: 100m
              memory: 32Mi

$ kubectl get deploy web -n abcl-ka -o yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  …
  generation: 3
  name: web
  namespace: abcl-ka
  resourceVersion: "1303"
  uid: 152895a9-4ff0-43cd-88c8-9e8e2228b19c
spec:
  progressDeadlineSeconds: 600
  replicas: 3
  …
status:
  availableReplicas: 3
  …
  observedGeneration: 3
  readyReplicas: 3
  replicas: 3
  updatedReplicas: 3

$ kubectl create -f web.yaml
Error from server (AlreadyExists): error when creating "web.yaml":
deployments.apps "web" already exists
$ kubectl apply -f web.yaml
deployment.apps/web unchanged

$ kubectl scale deploy/web --replicas=5 -n abcl-ka
deployment.apps/web scaled
$ kubectl get deploy web -n abcl-ka
NAME   READY   UP-TO-DATE   AVAILABLE   AGE
web    5/5     5            5           109s
$ kubectl apply -f web.yaml
deployment.apps/web configured
$ kubectl get deploy web -n abcl-ka
NAME   READY   UP-TO-DATE   AVAILABLE   AGE
web    3/3     3            3           111s

const START_TICKS = 2;        // a new Pod needs 2 ticks to become Running
const EVENTS = {
  4:  { kill: 1, note: "1 Pod crashed" },
  7:  { desired: 5, note: "user: replicas=5" },
  11: { kill: 2, note: "2 Pods lost with a node" },
  15: { desired: 2, note: "user: replicas=2" },
};
let desired = 3, pods = [], nextId = 1;

console.log("tick desired running starting  action");
for (let t = 0; t <= 17; t++) {
  const ev = EVENTS[t];
  if (ev && ev.desired) desired = ev.desired;     // spec changed
  if (ev && ev.kill) pods.splice(0, ev.kill);     // world changed

  // OBSERVE: count what really exists now
  const running = pods.filter(p => p.readyAt <= t).length;
  const starting = pods.length - running;

  // DIFF and ACT: only the difference, then stop
  const diff = desired - pods.length;
  let action = "nothing to do";
  if (diff > 0) {
    for (let i = 0; i < diff; i++)
      pods.push({ name: "pod-" + nextId++, readyAt: t + START_TICKS });
    action = "create " + diff;
  } else if (diff < 0) {
    pods.splice(0, -diff);
    action = "delete " + (-diff);
  }
  const line = String(t).padStart(4) + String(desired).padStart(8) +
    String(running).padStart(8) + String(starting).padStart(9) + "  " + action;
  console.log(line + (ev ? "   <- " + ev.note : ""));
}
console.log("final pods:", pods.map(p => p.name).join(" "));

$ kubectl get deploy,rs,pods -n abcl-ka
NAME                  READY   UP-TO-DATE   AVAILABLE   AGE
deployment.apps/web   3/3     3            3           2s

NAME                             DESIRED   CURRENT   READY   AGE
replicaset.apps/web-69f4d599d6   3         3         3       2s

NAME                       READY   STATUS    RESTARTS   AGE
pod/web-69f4d599d6-92562   1/1     Running   0          2s
pod/web-69f4d599d6-jszbc   1/1     Running   0          2s
pod/web-69f4d599d6-wbths   1/1     Running   0          2s

const POD = { cpu: 500, mem: 512, nodeSelector: { disk: "ssd" }, image: "shop:1.4" };

const NODES = [ // cpu in millicores, mem in MiB, free = capacity - requested
  { name: "node-a", ready: true,  cap: [2000, 4096], used: [1800, 1024], disk: "ssd", images: [] },
  { name: "node-b", ready: true,  cap: [2000, 4096], used: [ 500, 1024], disk: "ssd", images: [] },
  { name: "node-c", ready: true,  cap: [4000, 8192], used: [2000, 6000], disk: "ssd", images: ["shop:1.4"] },
  { name: "node-d", ready: true,  cap: [4000, 8192], used: [ 200,  512], disk: "hdd", images: [] },
  { name: "node-e", ready: false, cap: [4000, 8192], used: [   0,    0], disk: "ssd", images: [] },
];

function why(n) {               // FILTER: any "no" removes the node
  if (!n.ready) return "node not Ready";
  if (POD.nodeSelector.disk !== n.disk) return "selector disk=ssd";
  if (n.cap[0] - n.used[0] < POD.cpu) return "not enough cpu";
  if (n.cap[1] - n.used[1] < POD.mem) return "not enough memory";
  return "";
}
function score(n) {             // SCORE: most free share after placing
  const cpu = (n.cap[0] - n.used[0] - POD.cpu) / n.cap[0];
  const mem = (n.cap[1] - n.used[1] - POD.mem) / n.cap[1];
  const free = Math.round(((cpu + mem) / 2) * 100);
  const bonus = n.images.includes(POD.image) ? 10 : 0;   // image is cached
  return { free, bonus, total: free + bonus };
}

const left = NODES.filter(n => {
  console.log("filter", n.name, why(n) ? "OUT: " + why(n) : "ok");
  return !why(n);
});
const ranked = left.map(n => ({ n: n.name, ...score(n) }))
  .sort((a, b) => b.total - a.total || a.n.localeCompare(b.n));
for (const r of ranked)
  console.log("score ", r.n, "free", r.free, "+ image", r.bonus, "=", r.total);
console.log(ranked.length ? "result: bind Pod to " + ranked[0].n
                          : "result: Pending (no node fits)");

$ kubectl get pod needs-ssd -n abcl-ka
NAME        READY   STATUS    RESTARTS   AGE
needs-ssd   0/1     Pending   0          4s

$ kubectl get events -n abcl-ka \
    --field-selector involvedObject.name=needs-ssd
LAST SEEN   TYPE      REASON             OBJECT          MESSAGE
4s          Warning   FailedScheduling   pod/needs-ssd   0/1 nodes are available:
1 node(s) didn't match Pod's node affinity/selector. no new claims to deallocate,
preemption: 0/1 nodes are available: 1 Preemption is not helpful for scheduling.

$ kubectl get pods -l app=web -n abcl-ka --watch-only --output-watch-events &
$ kubectl delete pod web-69f4d599d6-92562 -n abcl-ka

 সময়    EVENT      NAME                   READY   STATUS
+0.07s  MODIFIED   web-69f4d599d6-92562   1/1     Terminating
+0.08s  ADDED      web-69f4d599d6-ns5z4   0/1     Pending
+0.09s  MODIFIED   web-69f4d599d6-ns5z4   0/1     Pending
+0.09s  MODIFIED   web-69f4d599d6-92562   1/1     Terminating
+0.11s  MODIFIED   web-69f4d599d6-ns5z4   0/1     ContainerCreating
+0.92s  MODIFIED   web-69f4d599d6-92562   0/1     Completed
+1.94s  MODIFIED   web-69f4d599d6-92562   0/1     Completed
+1.95s  MODIFIED   web-69f4d599d6-92562   0/1     Completed
+1.95s  DELETED    web-69f4d599d6-92562   0/1     Completed
+1.99s  MODIFIED   web-69f4d599d6-ns5z4   1/1     Running

$ kubectl get pods -n abcl-ka
NAME                   READY   STATUS    RESTARTS   AGE
web-69f4d599d6-jszbc   1/1     Running   0          36s
web-69f4d599d6-ns5z4   1/1     Running   0          10s
web-69f4d599d6-wbths   1/1     Running   0          36s

$ kubectl get --raw '/apis/apps/v1/namespaces/abcl-ka/deployments?watch=true&timeoutSeconds=12&fieldSelector=metadata.name%3Dweb'
{"type":"ADDED","object":{"kind":"Deployment","apiVersion":"apps/v1","metadata":{"name":"web","namespace":"abcl-ka",…,"resourceVersion":"1062","generation":1,…
…

# script-এ প্রতিটি লাইনের কয়েকটি ক্ষেত্র ছেঁকে নিলে (scale কমান্ড থেকে সময়):
  সময়   type      rv    gen spec  replicas ready observedGen
-1.44s  ADDED     1062  1   3     3        3     1
+0.08s  MODIFIED  1190  2   4     3        3     1
+0.09s  MODIFIED  1193  2   4     3        3     2
+0.10s  MODIFIED  1195  2   4     3        3     2
+0.12s  MODIFIED  1201  2   4     4        3     2
+2.38s  MODIFIED  1238  2   4     4        4     2

$ kubectl explain deployment.spec.replicas
GROUP:      apps
KIND:       Deployment
VERSION:    v1

FIELD: replicas <integer>

DESCRIPTION:
    Number of desired pods. This is a pointer to distinguish between explicit
    zero and not specified. Defaults to 1.

$ kubectl describe pod web-69f4d599d6-jszbc -n abcl-ka
Name:             web-69f4d599d6-jszbc
Namespace:        abcl-ka
…
Labels:           app=web
                  pod-template-hash=69f4d599d6
Status:           Running
IP:               10.1.0.9
Controlled By:  ReplicaSet/web-69f4d599d6
Containers:
  nginx:
    Image:          nginx:1.27-alpine
    …
    Ready:          True
    Restart Count:  0
    Limits:
      cpu:     100m
      memory:  32Mi
    Requests:
      cpu:        10m
      memory:     16Mi
…
Tolerations:                 node.kubernetes.io/not-ready:NoExecute op=Exists for 300s
                             node.kubernetes.io/unreachable:NoExecute op=Exists for 300s
Events:
  Type    Reason     Age   From               Message
  ----    ------     ----  ----               -------
  Normal  Scheduled  24s   default-scheduler  Successfully assigned abcl-ka/web-69f4d599d6-jszbc to docker-desktop
  Normal  Pulled     23s   kubelet            Container image "nginx:1.27-alpine" already present on machine
  Normal  Created    23s   kubelet            Created container: nginx
  Normal  Started    23s   kubelet            Started container nginx

$ kubectl get pods -n default
No resources found in default namespace.

$ kubectl get pods -n abcl-ka -L app
NAME                   READY   STATUS    RESTARTS   AGE    APP
web-69f4d599d6-jszbc   1/1     Running   0          107s   web
web-69f4d599d6-ns5z4   1/1     Running   0          81s    web
web-69f4d599d6-wbths   1/1     Running   0          107s   web

$ kubectl label pod web-69f4d599d6-wbths app=debug --overwrite -n abcl-ka
pod/web-69f4d599d6-wbths labeled

$ kubectl get pods -n abcl-ka -L app        # 4 সেকেন্ড পরে
NAME                   READY   STATUS    RESTARTS   AGE    APP
web-69f4d599d6-gk8vl   1/1     Running   0          4s     web
web-69f4d599d6-jszbc   1/1     Running   0          111s   web
web-69f4d599d6-ns5z4   1/1     Running   0          85s    web
web-69f4d599d6-wbths   1/1     Running   0          111s   debug

$ kubectl get pods -n abcl-ka -l app=web
NAME                   READY   STATUS    RESTARTS   AGE
web-69f4d599d6-gk8vl   1/1     Running   0          4s
web-69f4d599d6-jszbc   1/1     Running   0          111s
web-69f4d599d6-ns5z4   1/1     Running   0          85s
$ kubectl get pods -n abcl-ka -l 'app in (web,debug)' --no-headers | wc -l
4

// etcd quorum math for 1 to 7 members. Change DOWN and P_DOWN.
const DOWN = 2;        // members that are down right now
const P_DOWN = 0.01;   // assumed: each member down 1% of the time, independently

function choose(n, k) {
  let c = 1;
  for (let i = 1; i <= k; i++) c = c * (n - i + 1) / i;
  return c;
}
function pNoQuorum(n, q) {     // lost if more than n - q members are down
  let sum = 0;
  for (let k = n - q + 1; k <= n; k++)
    sum += choose(n, k) * P_DOWN ** k * (1 - P_DOWN) ** (n - k);
  return sum;
}

console.log("members quorum tolerate  down=" + DOWN + "      P(no quorum)");
for (let n = 1; n <= 7; n++) {
  const q = Math.floor(n / 2) + 1;
  const alive = Math.max(n - DOWN, 0);
  const state = alive >= q ? "writes ok" : "BLOCKED";
  console.log(String(n).padStart(7) + String(q).padStart(7) +
    String(n - q).padStart(9) + "  " + state.padEnd(10) +
    pNoQuorum(n, q).toExponential(2).padStart(13));
}