Kubernetes Architecture, POD, ReplicaSet & Replica Controller
1. Introduction
What is Node ?
- Node is a machine - Physical or Virtual on which Kubernetes is installed,
- Node is where containers will be launched by Kubernetes.
- Application goes down -- Means Kubernetes goes down.
Why do we need to have more than one node OR Why do we need CLUSTER?
- Because of one node on which Kuberneres is installed goes down then application will also go down. So for redundancy, we need to have more than one node.
- So we have more than one node grouped together.
- So even if one node goes down, application is still accessible from other nodes.
Now we have CLUSTER ?
- So who is managing cluster ?
- Where is the information about members of clusters stored ?
- How are nodes monitored ?
- When a node failed, how do u move workload of one node to other worked node ?
- This is the work of Master node. ( M, S, M, F )
- Manage, Store, Monitor, Failure Handling
- Api server
- Acts as Front end for kubernetes
- User, Management Devices, Command Line interfaces
- talk to API server to interact with Kubernetes cluster.
- etcd
- distributed, reliable key-value storage to store
- all data used to manage cluster.
- Also implements locks in the cluster, so that there is no conflict
- kubelet
- agent that runs on each node of the cluster.
- makes sure that conatiners are running on nodes as expected.
- container runtime
- Underlying software that run containers.
- For example - docker.
- controller
- Brain behind orchestration.
- Responds when nodes, containers or end points goes down.
- Take decision to bringup new containers in case of failure.
- schedular
- For distributing work across containers on various nodes.
Master & Worker Nodes
How some node became master and some node became worker nodes ?
- Master node that has
- Kube-API server, becomes master node.
- All information is stored in KEY-VALUE ( etcd ) store in master.
- Master also has
- controller and
- schedular
- Worked node has
- kubelet agent, so it becomes worker node.
- Through kubelet-agent worker node can interact with MASTER.
- TO provide health information of worker node.
- And CARRY out instruction from MASTER.
What is kubectl
- This is KUBE command line control
- kubectl run
- to install cluster
- kubectl cluster-info
- View information about cluster
- kubectl get nodes
- To list all nodes of the cluster
Docker Vs Container
Docker
- container solution.
Kubernetes
- kubernetes came to orchestrate docker. and they were tightly coupled.
- But later Kubernetes provided a layer Container Runtime Interface ( CRI ) that can take into any container solution ex - docker, rkt, as long as they adhere to OCI standard (Open Container standard)
- CRTICTL
- This is command line interface (CLI command) for CRI
- This will work for any container intarface.
POD
Steps -
- Application is built
- Has been put in docker image
- And Kubernetes can pull the image.
Aim
- Deploy APplication
- In the form of containers
- On a set of machines
- That are configured as worker nodes.
- In a cluster
But
- Kubernetes does not deploy container directly, it has to be put in POD.
Scale up/down
- We can't add new container inside POD
- We need to create new POD in same node.
- We can create new POD and new Node.
Concept of multi-container pod
- POD can have multiple container.
- But generally they dont have multiple container of same kind
- We can have hlper container
- Ex - Processing file,
YAML with Kubernetes
Creating POD using YAML based configuration file
Command to create a POD
kubectl create -f pod.yml
Mandatory Values in K8 yaml file
- apiVersion
- Not a KEY/VALUE, we just pass a string
- API version that we are using to create Kubernetes
- Example value - v1 , app/v1
- kind
- Not a KEY/VALUE, we just pass a string
- type of objext we are trying to create
- Example - POD, Service, ReplicaSet, Deployment
- Metadata
- This is dictionary.
- This is data of the object.
-- Example - Name, Label etc
-- Here 'name' and labels are dictionary. (have KEY/Value)
- NAME and IMAGE are sibling
- Under "MetaData" we can have only "name" and "label"
- But under "label" we can define any "key". "value" pair.
- Spec
- 'Spec' is a dictionary, So Add property under it.
- Below 'spec', there is 'container'. 'container' is also a list/array
- because there can be multiple containers in a pod.
- So first element in a list indicate that it is first item in the list ( - name )
- Here name & spec is element inside 'container'
Replica Sets / Replica Controllers.
Replica Controllers
- Are the brain behind Kubernetes
- Have processes that
- Monitor Kubernetes objects
- And manage accordingly.
Question - What is a replica and why do we need replication-controller ?
- Suppose we have a node + POD + Container running our application
- What will happen if our application crashes ?
- So users will no longer to access our application.
Solution
- If we don't want any disruption, we will need to have multiple POD so that there will not disruption. Even if one fails, other POD will still be accessible.
- helps running multiple instance of same POD in a Kubernetes cluster.
Two reasons for Replication Controller
- High Availability
- And so it provides high availability.
- So if one pod goes down, replication controller quickly instantiates another pod.
- Load Balancing
- If demand increases, replication controller creates Pods
- On same Node
- Or No multiple Nodes.
Two terms - Replication Controller vs Replica Set
There is minor difference between both but more or less they are same.
- Replication Controller - older way
- Replica Set - newer way.
How to create Replication Controller - using Yaml file.
apiVersion: v1
kind: Replication Controller
metadata:
name: myapp-rc
labels:
app: myapp
type: fromt-end
spec: spec defines what in inside the KIND. As POD is inside Replication Controller. So provide template of the Pod.
template: <<< Here need to provide template for POD. We just to insert metadata + spec section of POD.
metadata:
name: myapp-pod
labels:
app: myapp
type: front-end
spec:
containers:
name: nginx-container
image: nginx
But we have something still missing.
- We have not specified how many PODs to be created?
- Or how many replicas of POD to be available all the time?
- Solution : Add 'replicas: 3
apiVersion: v1
kind: Replication Controller
metadata:
name: myapp-rc
labels:
app: myapp
type: fromt-end
spec: spec defines what in inside the KIND. As POD is inside Replication Controller. So provide template of the Pod.
template: <<< Here need to provide template for POD. We just to insert metadata + spec section of POD.
metadata:
name: myapp-pod
labels:
app: myapp
type: front-end
spec:
containers:
name: nginx-container
image: nginx
replicas: 3 <<< This is sibling of template
How to create ReplicaSet - using Yaml file.
apiVersion: apps/v1 << Note for ReplicationController this is - v1
kind: ReplicaSet
metadata:
name: myapp-replicaset
labels:
app: myapp
type: front-end
spec:
template: <<< Here need to provide template for POD. We just to insert metadata + spec section of POD.
metadata:
name: myapp-pod
labels:
app: myapp
type: front-end
spec:
containers:
name: nginx-container
image: nginx
replicas: 3 <<< This is sibling of template
selector: << Only got replicaSet
matchLabels:
type: front-end
selector
Defines which POD fall in it.We can define multiple labels.
- Can have labels of PODS define in the same definition
- Can have labels defined somewhere else.
Labels & Selectors
- Monitor POD of specified LABEL specified as part of matchLabels
- Always keep the 3 replicas of the POD instance.
- ReplicaSet is 'process' that monitors the PODS and if any POD fails, it creates new POD.
How ReplicaSet knows which POD to monitor ?
- This is done by labels provided as part of matchLabels.
How to change replicas ?
Two commands
- replace
- scale
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