Pod Deep Dive: Everything You Didn’t Know You Needed to Know

pod.yaml Sidecar Containers ● Enabled by default from v1.29 (feature gate SidecarContainers) ● “Disguised” as initContainers 🤷 ● Launched when Pod scheduled, continues running until main application containers have fully stopped, then kubelet terminates all sidecars ● Supports readinessProbe (unlike normal initContainers) and used to determine the ready state of the Pod ● apiVersion: v1 kind: Pod metadata: name: “tiny-pod” spec: initContainers: - name: logshipper image: alpine Only allowed value restartPolicy: Always On the container, not the Pod Termination handled more harshly than app containers - SIGTERM followed by SIGINT (Rest of spec omitted) before graceful exit likely 🐘 @Marcus@k8s.social | 🌐 MarcusNoble.com | 🦋 @averagemarcus.bsky.social

Pause Container ● Every Pod includes an empty pause container which bootstraps the Pod with the cgroups, reservations and namespaces before the defined containers are created ● This container can be thought of as a “parent container” for all the containers within your Pod and will remain even if workload containers crash, ensuring namespaces and networking remain available ● The pause container is always present but not visible via the Kubernetes API ● Can be seen if you query directly on the node, e.g. with containerd: ~ # ctr -n k8s.io containers list | grep pause 03bfc9fa4bd0aebb0a9f84b1aad680f4b7 gsoci.azurecr.io/giantswarm/pause:3.9 io.containerd.runc.v2 05df0356a344c23d02afbff797742c67bd gsoci.azurecr.io/giantswarm/pause:3.9 io.containerd.runc.v2 066e0c8ee2962f276c4b7bb7d505e63f5b gsoci.azurecr.io/giantswarm/pause:3.9 io.containerd.runc.v2 0a6685e4d54e94c4acc36dbbb1a2b356de gsoci.azurecr.io/giantswarm/pause:3.9 io.containerd.runc.v2 🐘 @Marcus@k8s.social | 🌐 MarcusNoble.com | 🦋 @averagemarcus.bsky.social

Image Pull Policy ● IfNotPresent - Fetches the image if not already found on the node (default if you specify a tag/sha other than “latest”) ● Always - Will always fetch the image (default if you specify the tag as “latest” or omit the tag) ○ The caching mechanism is eﬃcient enough that in this mode if first compares the image layers from the remote registry and only then downloads if not already present on node ● Never - Will not attempt to fetch the image, it must be loaded onto the node by some other means pod.yaml apiVersion: v1 kind: Pod metadata: name: “tiny-pod” spec: containers: - name: “nginx” image: “nginx:v1.2.3” imagePullPolicy: “Always” 🐘 @Marcus@k8s.social | 🌐 MarcusNoble.com | 🦋 @averagemarcus.bsky.social

RuntimeClass pod.yaml ● Allows for multiple runtimes in a single cluster ● If unset, uses default Container Runtime Interface (CRI) configured on the node ● If set, must point to a RuntimeClass resource name and have the CRI handler configured up on the node ● The scheduling property of the RuntimeClass ensures Pods are scheduled onto nodes with that runtime available (based on label selectors) apiVersion: v1 kind: Pod metadata: name: “tiny-pod” spec: runtimeClassName: “crio-runtime” containers: - name: “demo” image: “nginx:latest” —apiVersion: node.k8s.io/v1 kind: RuntimeClass metadata: name: “crio-runtime” scheduling: nodeSelector: runtime: “crio” handler: “crio” 🐘 @Marcus@k8s.social | 🌐 MarcusNoble.com | 🦋 @averagemarcus.bsky.social

RuntimeClass pod.yaml ● Allows for multiple runtimes in a single cluster ● If unset, uses default Container Runtime Interface (CRI) configured on the node ● If set, must point to a RuntimeClass resource name and have the CRI handler configured up on the node ● The scheduling property of the RuntimeClass ensures Pods are scheduled onto nodes with that runtime available (based on label selectors) ● Can be used for WASM (web assembly) runtimes, not just containers apiVersion: v1 kind: Pod metadata: name: “wasm-pod” spec: runtimeClassName: “wasmedge” containers: - name: “demo” image: “my-wasm-demo:latest” —apiVersion: node.k8s.io/v1 kind: RuntimeClass metadata: name: “wasmedge” scheduling: nodeSelector: runtime: “wasmedge” handler: “wasmedge” 🐘 @Marcus@k8s.social | 🌐 MarcusNoble.com | 🦋 @averagemarcus.bsky.social

File stored on the host node disk Static Pods /etc/kubernetes/manifests/kube-scheduler.yaml ● Managed directly by the Kubelet, not the API server ● Always bound to one Kubelet on a specific node ● Defined as static manifests either: ● ○ on disk of the node in the directory defined by —pod-manifest-path ○ or referenced from an URL using the —manifest-url flag The Kubelet automatically tries to create a “mirror Pod” on the API for each static Pod so that they are visible when querying the API server but apiVersion: v1 kind: Pod metadata: name: kube-scheduler namespace: kube-system spec: containers: - name: kube-scheduler image: kube-scheduler:v1.32.0 command: - kube-scheduler they cannot be modified via the API ● Pod names get the node name as a suﬃx (e.g. kube-scheduler-control-plane-1) ● Cannot refer to other resources (e.g. ConfigMaps) ● The Kubelet watches the static directory and reconciles when files are changed/added/removed 🐘 @Marcus@k8s.social | 🌐 MarcusNoble.com | 🦋 @averagemarcus.bsky.social

Example based on the Cilium chart provided by Bitnami Lifecycle Hooks ● cilium-agent.yaml Guaranteed to trigger at least once but may be called multiple times. ● postStart ○ Runs immediately after container is created but no guarantee that it will execute before the container’s ENTRYPOINT. ○ The container isn’t marked as “running” until this completes. ● preStop ○ ● Runs immediately before the container is terminated. Hook mechanisms available: ○ exec - perform command in container ○ httpGet - perform an HTTP GET request to the container ○ sleep - pause the container for given duration apiVersion: v1 kind: Pod metadata: name: cilium-agent spec: containers: - name: cilium-agent image: “cilium:latest” lifecycle: postStart: exec: command: - /bin/bash - -ec - | if [[ “$(iptables-save | grep -E -c ‘AWS-SNAT-CHAIN)” != “0” ]]; then iptables-save | grep -E -v ‘AWS-SNAT-CHAIN’ | iptables-restore fi preStop: exec: command: - /opt/bitnami/scripts/cilium/uninstall-cni-plugin.sh - /host (if PodLifecycleSleepAction feature gate enabled) 🐘 @Marcus@k8s.social | 🌐 MarcusNoble.com | 🦋 @averagemarcus.bsky.social

Conditions pod.yaml Kubelet manages the following Pod Conditions: ● PodScheduled - the Pod has been scheduled to a node ● PodReadyToStartContainers - (beta feature) the Pod sandbox has been created and networking configured ● ContainersReady - all containers in the Pod are ready ● Initialized - all the initContainers have completed ● Ready - all containers ready and probes successfully passing apiVersion: v1 kind: Pod metadata: name: “demo-pod” spec: … status: conditions: - type: Ready status: “False” lastProbeTime: null lastTransitionTime: 2018-01-01T00:00:00Z - type: PodScheduled status: “True” lastProbeTime: null lastTransitionTime: 2018-01-01T00:00:00Z Each status condition may also contain… ● A 🤖machine readable reason property and ● A 🧑human readable message property …that can be used for debugging. 🐘 @Marcus@k8s.social | 🌐 MarcusNoble.com | 🦋 @averagemarcus.bsky.social

Readiness Gates When container status and probes aren’t enough to determine is a Pod really is ready then there is readinessGates! These must me handled by some external application that patches the status of the Pod once the readiness gate condition is met. Example usage: AWS Load Balancer supports readiness gates to indicate a pod is registered to the ALB/NLB. pod.yaml apiVersion: v1 kind: Pod metadata: name: “aws-alb-example” spec: readinessGates: - conditionType: “target-health.elbv2.k8s.aws/k8s-readines-perf1000-7848e5026b” status: conditions: - type: “target-health.elbv2.k8s.aws/k8s-readines-perf1000-7848e5026b” status: “False” message: “Initial health checks in progress” reason: “Elb.InitialHealthChecking” lastTransitionTime: 2021-11-01T00:00:00Z 🐘 @Marcus@k8s.social | 🌐 MarcusNoble.com | 🦋 @averagemarcus.bsky.social

Environment Variables ● Hardcoded & Dynamic, leveraging other environment variables with the $(ENV) syntax ● The Downward API allows exposing properties from the Pod fields as env vars. Not all fields are valid but you can use fields from the Pod’s metadata, spec, limits and status. pod.yaml apiVersion: v1 kind: Pod metadata: name: “demo-pod” spec: containers: - name: demo image: nginx env: - name: POD_NAME valueFrom: fieldRef: fieldPath: metadata.name - name: NODE_NAME valueFrom: fieldRef: fieldPath: spec.nodeName - name: POD_IP valueFrom: fieldRef: fieldPath: status.podIP - name: CONTAINER_MEM_LIMIT valueFrom: resourceFieldRef: containerName: demo resource: limits.memory 🐘 @Marcus@k8s.social | 🌐 MarcusNoble.com | 🦋 @averagemarcus.bsky.social

Resource Requests & Limits ● Requests - How much resources are at least needed on a node to be scheduled there ● Limits - enforced amount of resources a container can use ○ CPU - enforced by CPU throttling ○ Memory - enforced by kernel out of memory (OOM) terminations ● Custom resource types can be managed by 3rd party controllers (e.g. nvidia.com/gpu) ● ○ Requests & limits must be the same ○ You cannot specify requests without limits Pod limit and requests are calculated from the sum of all the containers ○ pod.yaml apiVersion: v1 kind: Pod metadata: name: “demo-pod” spec: resources: requests: memory: “100Mi” limits: memory: “200Mi” containers: - name: demo image: nginx v1.32 introduces a new alpha (disabled by default) feature that supports pod-level resource specification 🐘 @Marcus@k8s.social | 🌐 MarcusNoble.com | 🦋 @averagemarcus.bsky.social

Node Assignment ● pod.yaml topologySpreadConstraints - more control over the spread of Pods apiVersion: v1 across a cluster when scaling replicas ○ skew = number of pods in topology - min pods in any topology ○ topologyKey - the label on nodes to use as the groupings (usually failure domains) ○ whenUnsatisfiable - Either DoNotSchedule or ScheduleAnyway ○ No guarantee the constraints remain satisfied when Pods are removed (e.g. scaling down) ○ Combined with other node assignment strategies (e.g. aﬃnity) kind: Pod metadata: name: “demo-pod” labels: app: nginx spec: topologySpreadConstraints: - maxSkew: 1 topologyKey: zone whenUnsatisfiable: DoNotSchedule labelSelector: matchLabels: app: nginx containers: - name: demo image: nginx 🐘 @Marcus@k8s.social | 🌐 MarcusNoble.com | 🦋 @averagemarcus.bsky.social

Priority & Preemption ● Pods can be given a priority to indicate their importance compared to other Pods. If a Pod is unable to be scheduled and has a higher priority than already scheduled Pods the scheduler will evict (preempt) the lower priority to make room ● PriorityClass resource is used to define possible priorities in a cluster ● PodDisruptionBudget are handled on a best-effort basis and not guaranteed to be honoured ● You can avoid preempting lower priority Pods by setting preemptionPolicy: Never on the PriorityClass ○ This effects the scheduler queue but doesn’t cause pods to be pod.yaml apiVersion: scheduling.k8s.io/v1 kind: PriorityClass metadata: name: high-priority value: 1000000 globalDefault: false description: These pods are important —apiVersion: v1 kind: Pod metadata: name: demo spec: priorityClassName: high-priority containers: - name: demo image: nginx evicted 🐘 @Marcus@k8s.social | 🌐 MarcusNoble.com | 🦋 @averagemarcus.bsky.social

DNS ● pod.yaml Usually, depending on the DNS mechanism used in the cluster (e.g. CoreDNS), each Pod also gets an A record ○ ● E.g. 172-17-0-3.default.pod.cluster.local A Pods hostname is set to its metadata.name by default but can be overridden with the spec.hostname property and an additional subdomain set with spec.subdomain ● ○ E.g. my-demo.example.default.svc.cluster.local ○ (This doesn’t mean other Pods can resolve that hostname) Add extra entries to the Pods /etc/hosts file with spec.hostAliases apiVersion: v1 kind: Pod metadata: name: demo spec: hostname: my-demo subdomain: example setHostnameAsFQDN: true hostAliases: - ip: “127.0.0.1” hostnames: - “demo.local” containers: - name: demo image: nginx 🐘 @Marcus@k8s.social | 🌐 MarcusNoble.com | 🦋 @averagemarcus.bsky.social

DNS Config pod.yaml ● More control over the DNS settings used within a Pod ● nameservers - a list of IPs to use as the DNS servers (max 3) ● searches - list of DNS search domains to use for hostname lookup, merged into the base search domains generated (max 32) ● options - a list of name/value pairs to define extra DNS configuration options /etc/resolv.conf nameserver 192.0.2.1 search ns1.svc.cluster-domain.example my.dns.search.suffix options ndots:2 edns0 apiVersion: v1 kind: Pod metadata: name: demo spec: containers: - name: busybox Only use the below config image: nginx dnsPolicy: “None” dnsConfig: nameservers: - 192.0.2.1 searches: - ns1.svc.cluster-domain.example - my.dns.search.suffix options: - name: ndots value: “2” - name: edns0 🐘 @Marcus@k8s.social | 🌐 MarcusNoble.com | 🦋 @averagemarcus.bsky.social