# k8s Bullet Notes for Beginners ## k8s Architecture ### Control Plane #### What does the control plane do? * Makes decisions across the entire cluster * Contains the core components of k8s #### Where does the control plane exist? * The control plane can run on any machine in the cluster #### Control Plane Components * api-server * etcd * scheduler * controller manager ### Node Components * kube-proxy * kubelet ## k8s Resources ### Pod #### Why did k8s define Pods? * To provide a unified interface for accessing containers #### What is a Pod? * A Pod is an abstraction of a node, allowing us to not be restricted by the number of physical instances * The smallest unit of object in k8s * Each Pod can contain multiple containers * The same application should not exist in the same Pod * Containers in the same Pod can communicate through their own network * Containers in the same Pod can share the same volume #### How do we prepare the environment before starting our app? * Define an init container in the Pod and execute the prerequisite processes inside it ### Deployment #### Why did k8s design the Deployment object? * To provide a declarative way to deploy applications #### Serving applications in real-world scenarios * Services should run 24/7 * Single points of failure should be avoided * The number of Pods should remain constant #### What elements make up a Deployment? * Deployment strategy * ReplicaSet #### Deployment strategy ##### Recreate * First terminate all Pods, then create new ones ##### Rolling Update * Ensure there is always at least one existing Pod * This is the default deployment strategy ### ReplicaSet * Load balances the Pods * Ensures the correct number of Pods * Manages Pod-level operations ### DaemonSet #### What does a DaemonSet do? * Ensures that each node runs the required Pods #### Examples * Network plugins, like Calico * Pod communication proxy, like kube-proxy * Monitoring exporters * Logging collectors ## Network Prerequisite Knowledge ### Link ```bash ip link ``` * Inspect network interfaces at the data link layer ### Switch ```bash ip addr ``` * Inspect network interfaces at the network layer ### Route * Subnetwork resolution * 192.168.1.10/24 -> 255.255.255.0 -> from 192.168.1.10 to 192.168.1.255 ### Hostname and Domain Name * A host can request the IP through its name * If the name is not found in the hostname configuration, it will be searched through the domain name ### NAT * Translates internal IPs to external IPs * IP addresses from external sources don't have router settings, so NAT is needed ### ARP * Translates IP addresses to MAC addresses ### Network Namespace * Isolates IP definitions from the physical node * Processes running under the namespace can communicate through network interfaces * Network interfaces set up by different namespaces can link to each other ### How do IPs of each virtual network communicate with each other? * By setting up a virtual switch (Linux bridge) ### How does an IP in a virtual network connect to the internet through the physical host? * By adding routing through a gateway ## k8s Network ### Types of IP * Cluster IP * External IP ### How does k8s discover dynamically adjusting Pods? * By allocating CoreDNS ### What is the responsibility of kube-proxy? * Bypass request restrictions from firewalls and whitelists * Load balance incoming requests at the Pod level * Maintain IP tables on each node