Pod

• Pods are the smallest deployable units of computing that you can create and manage in Kubernetes.

• Abstraction over a container

• Usually single Pod can runs 1 Application pod, it can also run multiple applications (not recommended),
• Pod is running environment or a layer on top of the container, (Kubernetes replace the container runtime or container technologies, so that there is no need to directly work with docker)

https://kubernetes.io/docs/concepts/workloads/pods/ (opens in a new tab)

Communication between Pods

• Each pod is assigned with an IP address, so that pods can communicate with each other using the internal IP address.

What happens when a pod dies ?

• Pods die very easily, a new IP address in assigned to the pod.

• A pod dies, when the an application has crashed *(maybe the application node ran out of resources)

• A new pod will be created at its place and when that happens the new pod is assigned with a new IP Address.

  https://kubernetes.io/docs/concepts/workloads/pods/pod-lifecycle/#pod-fault-recovery (opens in a new tab)

Service

Problem:

• When communication only happens based on IP Address, it need to be adjusted every time a pod dies, and a new IP Address is created.
• To avoid this problem, Service is used

Solution:

• Service is a static IP address, that is attached to each pod.
• Lifecycle of service and pod are not connected, i.e. even if the pod dies the IP address will not change.

Types of Service:

External

• An external service is used to make the application accessible to a browser.

• But database service, should not be open for public request.

Internal

• So, internal service is used, only for internal communication

Ingress

• before ingress: http://<ip_address>:port
• after ingress: http://application.com

Config Map

Problem:

• Normally environment variables are usually in the built application
• i.e. To change any environment variable
  1. Re-built Container
  2. Push it to Repository
  3. Pull it to Pod

Solution:

• Config Map is an external configuration of your application, it contains configuration data like environment variables, services.

Secret

Problem:

• Storing username and password in config map is in-secure.

Solution:

• Secret is like a config map, but it is used to user credentials.
• it is base64 encoded

Volumes

Problem:

• If the database container dies, then it losses all its data.
• !! Kubernetes doesn't manage data persistence !!

Solution

• Volumes attaches a physical storage(local / cloud) to pod.

Deployments

Problem:

• If the application node dies, then users will face downtime (site cannot be reached).

Solution:

• ReplicaSet is used to maintain a stable set of replica Pods running at any given time.
• To create ReplicaSets, blueprint: Deployment is used
• Blueprint is an abstraction of pods.

https://kubernetes.io/docs/concepts/workloads/controllers/replicaset/ (opens in a new tab)

• Creating a replica or clone of node would prevent this issue.
• All node should be connected to the service.
• Service assigns a permanent IP address, and also works as a load balancer (it catches the request and forwards it to the least busy node).

Stateful Set

Problem:

• If the database node dies, application will face downtime
• DEPLOYMENT cannot create replicas of database node.
• To create replicas of database node, all the nodes has to be synced and has to access the same shared data storage. To avoid inconsistency

Solution:

• StatefulSet is the workload API object used to manage stateful applications.

• Deploying StatefulSet is k8s cluster is not easy

Recommended: Keep the database outside of the k8s cluster