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Kubernetes

Components

Node

  • A physical or virtual machine running pods
  • Worker servers in K8 cluster

Pod

  • Smallest unit of Kubernetes
  • Abstraction over a container
  • Creates a running environment over the container so that we only interact with the Kubernetes layer. This allows us to replace the container technology like Docker.
  • Usually, only 1 application is run on a pod
  • Each pod gets a unique internal IP address for communicating with each other (virtual network created by K8)
  • If a pod is restarted (maybe after the application running on it crashed), its IP address changes
  • Image
    • attachments/Pasted image 20220828102630.jpg

Service

  • Static IP address that can be attached to pods
  • Pods communicate with each other using services
  • Lifecycle of pod and service are not connected. So even if a pod dies, we can restart it and attach the original service to have the same IP.
  • Service could be:
    • ClusterIP
      • Enables access to the service from within the K8s cluster (internal)
      • Every service created in Kubernetes has a cluster IP
    • NodePort
      • Every node in the cluster has a public IP
      • Assigns a port to the service (external requests going to any node at that port will be routed to the service)
      • Limitations
        • If a node goes down it's public IP might change when restarted
        • One port per service
      • Image
        • attachments/Pasted image 20220912100744.jpg
    • LoadBalancer
      • Depends on how the cloud provider is providing their Kubernetes service
      • Spins up a Network Load Balancer for each service. Requests going to the NLB's IP will be routed to the service.
      • Limitations:
        • One NLB IP per service
        • If the number of services increase, need to add a lot of NLBs (expensive)
  • Multiple pods could be connected to a service. In this case, the service acts as a load balancer.

Ingress

  • DNS for K8
  • Used to route traffic into the K8 cluster
  • Allows us to use domain names instead of IPs
  • The request first goes to ingress and is then forwarded to service
  • Image
    • attachments/Pasted image 20220904222518.jpg

ConfigMap

  • External configuration to the application
  • Used to store parameters like DB_URL
  • Not for storing secrets (DB_USER, DB_PASS)
  • Can be connected to the pod to get parameters (configs)

Secret

  • Used to store secrets (DB_USER, DB_PASS)
  • Base 64 encoded
  • Built-in security mechanism is not enabled by default
  • Can be connected to the pod to get secrets
  • Image
    • attachments/Pasted image 20220828110943.jpg

Volume

  • If the database container or pod gets restarted, the stored data would be lost
  • A volume (persistent storage) could be attached to the database pod
  • Volume could be:
    • local (on the same node as the DB pod)
    • remote (outside the K8 cluster, could be a cloud storage)
  • Image
    • attachments/Pasted image 20220828111746.jpg

Deployment

  • Blueprint for stateless pods (application layer)
  • Deployment specifies how many replicas of a pod will be running
  • Databases can't be replicated using deployments (because they are stateful)

StatefulSet

  • Blueprint for stateful pods (database layer)
  • Database pods should be created using StatefulSets (not Deployments)
  • Would take care of replicating the DB pods or scaling them while ensuring database consistency
  • Working with StatefulSets is tedious. So, it is a common practice to host databases outside the K8 cluster and use K8 for hosting stateless applications.

Last updated: 2022-09-12