Note_Tech

All technological notes.

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Kubernetes - Services

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• Kubernetes - Services
  • Pod Communication
  • Service
    • Types of Services
    • Imperative Commands
    • Declarative Manifest
  • ClusterIP
  • Load Balancer
  • Common Commands
  • Service Networking
  • Lab: Get network info

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Pod Communication

• Pods are linked via a flat network that requires no NAT

  • packets exchange between Pods only needs the source IP and port, and the destination IP and port,

• constraint

  • Pods are ephemeral.
  • A pod gets its IP address only when it’s assigned to a node.
  • A simgle IP / DNS to is required; but Each of replicas Pod has its own IP address.

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Service

• Service

  • an object to provide a single, stable access point to a set of pods that provide the same service.
    • Each service has a stable IP address that doesn’t change for as long as the service exists.
    • A service acts as a load balancer in front of those pods.
  • Services use labels and selectors to find the right Pods.

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Types of Services

• ClusterIP
  • default
  • Exposes Pods on a virtual internal IP.
  • Accessible only within the cluster.
    • e.g. backend services for microservices.
• NodePort
  • Exposes the Service on a static port (30000–32767) on each Node’s IP.
  • Accessible from outside the cluster using NodeIP:NodePort.
  • Good for testing, not ideal for production.
• LoadBalancer
  • Integrates with cloud provider load balancers (AWS ELB, GCP LB, Azure LB).
  • Exposes the Service to the internet.
  • Cloud provider assigns a public IP.

• ExternalName

  • Maps the Service to an external DNS name (like api.example.com).
  • No selector, just returns a CNAME.

• Service Discovery

  • Inside the cluster, Services are automatically registered with DNS.
  • e.g., nginx-service.default.svc.cluster.local
  • Other Pods can talk to it using this name instead of IP.
• Cannot ping a serivce:
  • because svc is virtual.

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Imperative Commands

Command	Description
kubectl get svc	List all Services in the current namespace.
kubectl describe svc service_name	Show detailed information about a specific Service.
kubectl create svc clusterip svc_name --tcp=80	Create a ClusterIP service
kubectl create svc nodeport svc_name --tcp=80	Create a NodePort service
kubectl create svc loadbalancer svc_name --tcp=80	Create a LoadBalancer service
kubectl delete svc svc_name	Delete a Service by name.

Declarative Manifest

• basic fields

Field	Description
clusterIP	The internal IP address within the cluster. blank:k8s assign; None:headless service.
type	the type of Service object: ClusterIP(default), NodePort, LoadBalancer, and ExternalName.
selector	the label keys and values
ports	List of ports exposed by this service. can specify the name, protocol, appProtocol, port, nodePort, and targetPort.

• spec.sessionAffinity field
  • whether it should forward all connections from the same client to the same pod.
  • None:
    • default
    • no guarantee to which pod each connection will be forwarded
  • ClientIP:
    • all connections originating from the same IP will be forwarded to the same pod.

• spec.sessionAffinityConfig.clientIP.timeoutSeconds field:

  • specify how long the session will persist.
  • default: 3 hours
• service doesn’t provide cookie-based session affinity

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ClusterIP

• ClusterIP

  • default type of service
  • exposes the service within the defined Kubernetes cluster.
  • enable communication between services
  • a type of Service that provides an internal, cluster-wide IP address to enable communication between different components (typically Pods) within the same Kubernetes cluster.
  • forward the requests to one of the pods under the service randomly

• The service canbe aacessed by other pods using the cluster IP/service name

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• Definition

apiVersion: v1
kind: Service
metadata:
  name: back-end
spec:
  type: ClusterIP
  ports:
    - targetPort: 80 # the port exposed on backend
      port: 80 # the port exposed on service
  selector: # link the service to the pods
    app: myapp
    type: back-end

• Create

kubectl create -f service-cip-def.yaml

kubectl get svc

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Load Balancer

• LoadBalancer
  • a type of Service that provides external access to applications running in a Kubernetes cluster.
  • Only works with supported cloud platforms.

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• Definition

apiVersion: v1
kind: Service
metadata:
  name: myservice
spec:
  type: LoadBalancer
  ports:
    - targetPort: 80
      port: 80
      nodePort: 30008

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Common Commands

• how many services exist on system
  • kubectl get svc
• the type of default kubernetes service
  • ClusterIP
• what targetPort the default kubernetes is configured
  • kubectl describe svc kubernetes
  • 6443/TCP
• how many labels are configured on the default kubernetes service
  • kubectl describe svc kubernetes, labels
• how many endpoint are attached on the default kubernetes service
  • kubectl describe svc kubernetes, Endpoints

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Service Networking

• kubelet, the agent on each node, monitor the change on API Server for pod
  • create pod
  • invoke CNI plugin to configure networking for the pod
• kube proxy, the agent on each node, monitor the change on API server for services.
  • get the predeined IP address for updated service object
    • kubectl-api-server --service-cluster-ip-range ipNet
  • manage forwarding rules to the predefined IP address on each node
    • once the rule created, the request for the service get forwarded to the predefined IP address and port.
    • 3 method to manage the routing rules:
      • userspace
      • iptables
      • ipvs
      • can modify by kube-proxy --proxy-mode userspace|iptables|ipvs

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Lab: Get network info

• ge the primary network range

# get master node ip
kubectl get node -o wide
# NAME           STATUS   ROLES           AGE   VERSION   INTERNAL-IP      EXTERNAL-IP   OS-IMAGE             KERNEL-VERSION    CONTAINER-RUNTIME
# controlplane   Ready    control-plane   39m   v1.34.0   192.168.81.31    <none>        Ubuntu 22.04.5 LTS   5.15.0-1083-gcp   containerd://1.6.26

# get master node ip range
ip a
# 4: eth0@if22929: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1410 qdisc noqueue state UP group default
#     link/ether 22:8d:d9:de:30:0b brd ff:ff:ff:ff:ff:ff link-netnsid 0
#     inet 192.168.81.31/32 scope global eth0
#        valid_lft forever preferred_lft forever
#     inet6 fe80::208d:d9ff:fede:300b/64 scope link
#        valid_lft forever preferred_lft forever

• get ip range for pods

# get the cidr from controller manager conf
cat /etc/kubernetes/manifests/kube-controller-manager.yaml | grep cluster-cidr
# - --cluster-cidr=172.17.0.0/16

• Get the ip range for services

# get the service config from the aip server
cat /etc/kubernetes/manifests/kube-apiserver.yaml | grep service-cluster-ip-range
# - --service-cluster-ip-range=172.20.0.0/16

• Get the proxy on the node

kubectl get pod -A | grep proxy
# kube-system   kube-proxy-6c4gt                           1/1     Running   0          59m
# kube-system   kube-proxy-sjt5p                           1/1     Running   0          59m

# get the type of proxy
k logs kube-proxy-sjt5p    -n kube-system
# I1203 18:43:42.896019       1 server_linux.go:132] "Using iptables Proxier"

# get how the proxy is deployed
kubectl get all -A | grep kube-proxy  # use daemonset
# kube-system   pod/kube-proxy-6c4gt                           1/1     Running   0          66m
# kube-system   pod/kube-proxy-sjt5p                           1/1     Running   0          65m
# kube-system   daemonset.apps/kube-proxy   2         2         2       2            2           kubernetes.io/os=linux   66m

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