Kubernetes Overview for MariaDB Users
Kubernetes, or K8s, is software to orchestrate containers. It is released under the terms of an open source license, Apache License 2.0.
Kubernetes was originally developed by Google. Currently it is maintained by the Cloud Native Computing Foundation (CNCF), with the status of Graduated Project.
For information about how to setup a learning environment or a production environment, see Getting started in Kubernetes documentation.
Architecture
Kubernetes runs in a cluster. A cluster runs a workload: a set of servers that are meant to work together (web servers, database servers, etc).
A Kubernetes cluster consists of the following components:
Nodes run containers with the servers needed by our applications.
Controllersconstantly check the cluster nodes current state, and compare it with the desired state.
A Control Plane is a set of different components that store the cluster desired state and take decisions about the nodes. The Control Plane provides an API that is used by the controllers.
For more information on Kubernetes architecture, see Concepts and Kubernetes Components in Kubernetes documentation.
Nodes
A node is a system that is responsible to run one or more pods. A pod is a set of containers that run a Kubernetes workload or part of it. All containers that run in the same pod are also located on the same node. Usually identical pods run on different nodes for fault tolerance.
For more details, see Nodes in the Kubernetes documentation.
Every node must necessarily have the following components:
kubelet
kubelet has a set of PodSpecs which describe the desired state of pods. It checks that the current state of the pods matches the desired state. It especially takes care that containers don't crash.
kube-proxy
In a typical Kubernetes cluster, several containers located in different pods need to communicate with other containers within the same pods for performance and fault tolerance reasons. When developing and deploying applications, the IP addresses of these containers are unknown in advance. For instance, an application server may need to connect to a MariaDB instance, but the MariaDB's IP will vary for each pod.
The primary role of kube-proxy is to manage Kubernetes services. When an application needs to connect to MariaDB, it connects to the MariaDB service instead.
kube-proxy then receives the request and redirects it to a running MariaDB container within the same pod.
Container Runtime
Kubernetes orchestrates containers within a pod using a container runtime that complies with the Kubernetes Container Runtime Interface (CRI). Approved container runtimes are detailed on the Container Runtimes page of the Kubernetes documentation. Additional details about the CRI can be found on GitHub.
Initially, Kubernetes defaulted to using Docker as its container runtime. Though now deprecated, Docker images remain viable with any compatible container runtime.
Controllers
Controllers constantly check if there are differences between the pod's current state and their desired state. When differences are found, controllers try to fix them. Each node type controls one or more resource types. Several types of controllers are needed to run a cluster.
Most of the actions taken by the controllers user the API server in the Control Plane. However, this is not necessarily true for custom controllers. Also, some actions cannot be performed via the Control Plane. For example, if some nodes crashed, adding new nodes involves taking actions outside of the Kubernetes cluster, and controllers will have to do this themselves.
It is possible to write custom controllers to perform checks that require knowledge about a specific technology. For example, a MariaDB custom controller may want to check if replication is working by issuing SHOW REPLICA STATUS commands. This logic is specific to the way MariaDB works, and can only be implemented in a customer controller. Custom controllers are usually part of operators.
For more information, see Controllers in the Kubernetes documentation.
Control Plane
The control plane consists of the following components.
For more information about the control plane, see Control Plane Components in Kubernetes documentation.
API Server
An API Server exposes API functions both internally and externally. It is essential to coordinate Kubernetes components so that they react to node's change of state, and it allows the user to send commands.
The default implementation of the API Server is kube-apiserver. It is able to scale horizontally and to balance the load between its instances.
kube-controller-manager
Most controllers run in this component.
etcd
Contains all data used by a Kubernetes cluster. Taking regular backups of etcd data is advisable.
kube-scheduler
Decides which node should host a new pod based on criteria like resource requirements.
cloud-controller-manager
Implements logic and API of a cloud provider. Handles requests from the API Server and performs actions specific to a cloud vendor, like creating instances.
Clients and Tools
Kubernetes comes with a set of tools that allow us to communicate with the API server and test a cluster.
kubectl
Enables communication with the API server and allows running commands on a Kubernetes cluster.
kubeadm
Facilitates the creation of a Kubernetes cluster ready to receive commands from kubectl.
kind
Used to create and manage test clusters on a personal machine. It creates a cluster consisting of Docker containers, requiring Docker to be installed. Available on Linux, MacOS, and Windows.
minikube
Runs a single-node cluster on the local machine. Available on Linux, MacOS, and Windows.
Kubernetes Resources and References
Kubernetes on Wikipedia.
Kubernetes organization on GitHub.
(video) MariaDB database clusters on Kubernetes, by Pengfei Ma, at MariaDB Server Fest 2020.
Series of posts by Anel Husakovic on the MariaDB Foundation blog:
Content initially contributed by Vettabase Ltd.
This page is licensed: CC BY-SA / Gnu FDL
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