programming kubernetes chapter 3 - client-go

Tags: hausenblas - programming kubernetes

Overview: controllers can be constructed by calling the client-go library, which provides a set of sensible defaults such as work queues and informers
https://github.com/kubernetes/client-go
- most api types can be found under https://github.com/kubernetes/client-go/tree/master/listers
api definitions: https://github.com/kubernetes/api
https://github.com/kubernetes/apimachinery - generic building blocks to kube-like API, not restricted to container management
- most of the generic API types (ObjectMeta, TypeMeta, GetOptions, ListOptions) are in https://github.com/kubernetes/apimachinery/tree/master/pkg/apis/meta/v1

// basic client

import (
    metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
    "k8s.io/client-go/tools/clientcmd"
    "k8s.io/client-go/kubernetes"
)

kubeconfig = flag.String("kubeconfig, "~/.kube/config", "kubeconfig file")
flag.Parse()
config, err := clientcmd.BuildConfigFromFlags("", *kubeconfig)
clientset, err := kubernetes.NewForConfig(config)

pod, err := clientset.CoreV1().Pods("book").Get("example", metav1.GetOptions{})

This parses the appropriate kubeconfig and then gets the pods using the GetOptions

If you run a binary inside of a pod inside a cluster, kubelet automatically mounts a sservice account into /var/run/secrets/kubernetes.io/serviceaccount

CRUD operations are run through the XOptions, such as CreateOptions or DeleteOptions. ALl of htem are part of ObjectMeta

alpha/beta/vN verisons are what you would expect (alpha changes, beta is unlikely to break, vN is stable)
each object stored in etcd is stored as a specific version known as the storage version of that resource
anything served as a kind is a struct
- all objects can be retrived and deepcopied
- each object stores its groupversionkind

type meta

https://github.com/kubernetes/apimachinery/blob/10b38829b6211c0d672e14a126479221312d3368/pkg/apis/meta/v1/types.go#L41

example of a pod declaration

type Pod struct {
    metav1.TypeMeta `json:",inline"`

    metav1.ObjectMeta `json:"metadata,omitempty"`

    Spec PodSpec `json:"spec,omitempty"`

    Status PodStatus `json:"status,omitempty"`
}

Note that the type and version are not set by the user, they are set when being marshaled in the versionserializer

`ObjectMeta`

corresponds to the metadata section in yaml
rarely read or written from in client-go code, but very key to the system

Client Sets

gives access to multiple API groups and resources
also gives access to the discovery client

Status subresources

each deployment has a status subresource, updatestatus uses an additional http endpoint with /status
used with /apis/apps/v1beta1/namespaces/ns/deployments/name/status can change the status of an object

Listing and Deletions

ALlows to delete multiple objects of a namespace at once

Watches

Gives an event interface for all changes, returns a channel
- actually discouraged, should try to use informers (k8s) instead

Client Options

can set:
- the version
- wire format
- user agent (allows for debugging and better log readability)

informers (k8s) and caching

work queue

example: https://github.com/kubernetes/client-go/blob/master/examples/workqueue/main.go
premade implementation of queue for building controllers

api machinery (k8s)

vendoring

typical golang vendoring, or modules