Kubernetes is a portable, extensible, open source platform for managing containerized workloads and services, that facilitates both declarative configuration and automation. It has a large, rapidly growing ecosystem. Kubernetes services, support, and tools are widely available.
Traditional deployment era
- run applications on physical servers.
Virtualized deployment era
- run multiple VM on a single physical server's CPU
- applications are isolated ==> more security
- better utilisation of resources ==> better scalability ==> reduce hardware costs
- each VM is a full machine running all the components: OS on top of hardware
Container deployment era
- Share the OS among the applications
- Lightweight
- own filesystem, share of CPU, memore, process space and more
- decoupled from the underlying infrastructure, portable accross clouds and OS distrib
Advantages of containers
- easier to create and deploy application
- continuous dev, integration and deploy (CI/CD), (efficient rollback)
- decouple app and infra
- observability: application health and other signals
- cloud and OS portability
- focus on applicative management: raise level of abstraction
- loosely coupled: micro-services applications, not a monolithic stack
- resource isolation, predicablt application perf
- resource utilisation, high efficiency
It takes care of scaling and failover, provides deployment patterns. e.g.: Manage canary deployment
Service discovery and load balancing
- expose a container using the DNS name or using their own IP address
- traffic is high, able to load balance and distribute the network traffic so that the deployment is stable
Storage orchestration
- automatically mount a storage system of your choice: e.g.: such as local storages, public cloud providers, ...
Automated rollouts and rollbacks
- describe the desired state for your deployed containers ==> change the actual state to the desired one
- automate to create new containers for your deployment, remove existing containers and adopt all their resources to the new container
Automatic bin packing
- a cluster of nodes that it can use to run containerized tasks
- describe how much CPU and memory (RAM) each container need ==> Kubernetes can fit containers onto your nodes to make the best use of your resources
Self-healing
- restarts containers that fail
- replaces containers
- kills containers that don't respond to your user-defined health check
Secret and configuration management
- store and manage sensitive information, such as passwords, OAuth tokens, and SSH keys. Y
- deploy and update secrets and application configuration without rebuilding your container images, and without exposing secrets in your stack configuration
Additionally, Kubernetes is not a mere orchestration system. In fact, it eliminates the need for orchestration. Kubernetes comprises a set of independent, composable control processes that continuously drive the current state towards the provided desired state. It shouldn't matter how you get from A to C. Centralized control is also not required. This results in a system that is easier to use and more powerful, robust, resilient, and extensible.
https://kubernetes.io/docs/concepts/architecture/nodes/
- A node may be a virtual or physical machine, depending on the cluster.
- Each node is managed by the control plane and contains services necessary to run Pods.
- Several nodes in a cluster
A node is composed of the following components:
- kubelet
- container runtime
- kube-proxy
Two kind of nodes:
- Master (Control plane): often abstracted
- Workers
Add Nodes to API Server:
- The kubelet on a node self-registers to the control plane
- You (or another human user) manually add a Node object
A Node is healthy:
- Kubernetes checks kubelet has registered to the API server with label
metadata.nameof the Node - ✅ All necessary services are running ==> run a Pod
- ❌ Node is ignored for any cluster
- Kubernetes keeps the invalid node and checks if it becomes ✅
- Must explicitly delete the Node to stop health checking 🔫
Node name uniqueness
- The name identifies a Node.
- Two nodes cannot have the same name ❌ 👯
- Remove the previous object to create a new one
Self-registration of Nodes When the kubelet flag --register-node is true (the default), the kubelet will attempt to register itself with the API server. This is the preferred pattern, used by most distros.
- Use kubectl
- set the kubelet flag --register-node=false.
- Addresses
- Conditions
- Capacity and Allocatable
- Info
Addresses
- Hostname
- ExternalIP: IP address available outside the cluster
- InternalIP: IP address routable only within the cluster
Conditions
conditions: the status of all Running nodes- Examples:
Ready,DiskPressure,MemoryPressure,PIDPressure,NetworkUnavailable
Capacity and Allocatable
- Describes the resources available on the node:
- CPU
- Memory
- Max number
Info
- general information about the node
- kernel version
- Kubernetes version (kubelet and kube-proxy version)
- container runtime details
- operating system the node uses
The kubelet gathers this information from the node and publishes it into the Kubernetes API.
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