分类：Kubernetes笔记

Node（也称之为minion）运行docker container，而master则负责调度管理这些container。

Master运行下列服务：

1. API Server—nearly all the components on the master and nodes accomplish their respective tasks by making API calls. These are handled by the API Server running on the master.
2. Etcd—Etcd is a service whose job is to keep and replicate the current configuration and run state of the cluster. It is implemented as a lightweight distributed key-value store and was developed inside the CoreOS project.
3. Scheduler and Controller Manager—These processes schedule containers (actually, pods—but more on them later) onto target nodes. They also make sure that the correct numbers of these things are running at all times.

Node运行下列进程：

1. Kubelet—A special background process (daemon that runs on each node whose job is to respond to commands from the master to create, destroy, and monitor the containers on that host.
2. Proxy—This is a simple network proxy that’s used to separate the IP address of a target container from the name of the service it provides.
3. cAdvisor (optional)—http://bit.ly/1izYGLi[Container Advisor (cAdvisor)] is a special daemon that collects, aggregates, processes, and exports information about running containers. This information includes information about resource isolation, historical usage, and key network statistics.

A service is a process that:
1. is designed to do a small number of things (often just one).
2. has no user interface and is invoked solely via some kind of API.
An application, on the other hand, is pretty much the opposite of that. It has a user interface (even if it’s just a command line) and often performs lots of different tasks. It can also expose an API, but that’s just bonus points.

个人理解，service一般专注做一件事，没有用户界面，并且通过API与application交互。而application会有用户界面，并且通常可以运行很多任务。举个例子，web browser就是application，而web server即为service。

A Kubernetes cluster does not manage a fleet of applications. It manages a cluster of services.A service running in a container managed by Kubernetes is designed to do a very small number of discrete things.

If your services are small and of limited purpose, then they can more easily be scheduled and re-arranged as your load demands. Otherwise, the dependencies become too much to manage and either your scale or your stability suffers.

K8s即是用来管理service的。

k8s中用到一个hyperkube模块，其功能如下：

// Package hyperkube is a framework for kubernetes server components. It
// allows us to combine all of the kubernetes server components into a single
// binary where the user selects which components to run in any individual
// process.
//
// Currently, only one server component can be run at once. As such there is
// no need to harmonize flags or identify logs across the various servers. In
// the future we will support launching and running many servers — either by
// managing processes or running in-proc.
//
// This package is inspired by https://github.com/spf13/cobra. However, as
// the eventual goal is to run multiple servers from one call, a new package
// was needed.

通俗地讲，hyperkube模块就是把各种功能集成到一个可执行文件，然后在运行时指定模块。比如km程序就用到了hyperkube：

$ km --help
This is an all-in-one binary that can run any of the various Kubernetes-Mesos
servers.

Usage

  km <server> [flags]

Servers

  apiserver
    The main API entrypoint and interface to the storage system. The API server
    is also the focal point for all authorization decisions.
......

hyperkube结构体定义：

type HyperKube struct {
    Name string // The executable name, used for help and soft-link invocation
    Long string // A long description of the binary.  It will be world wrapped before output.

    servers     []Server
    baseFlags   *pflag.FlagSet
    out         io.Writer
    helpFlagVal bool
}

其中用到的Server结构体的定义：

// Server describes a server that this binary can morph into.
type Server struct {
    SimpleUsage string        // One line description of the server.
    Long        string        // Longer free form description of the server
    Run         serverRunFunc // Run the server.  This is not expected to return.

    flags *pflag.FlagSet // Flags for the command (and all dependents)
    name  string
    hk    *HyperKube
}

参看km程序的main函数：

func main() {
    hk := HyperKube{
        Name: "km",
        Long: "This is an all-in-one binary that can run any of the various Kubernetes-Mesos servers.",
    }

    hk.AddServer(NewKubeAPIServer())
    hk.AddServer(NewControllerManager())
    hk.AddServer(NewScheduler())
    hk.AddServer(NewKubeletExecutor())
    hk.AddServer(NewKubeProxy())
    hk.AddServer(NewMinion())

    hk.RunToExit(os.Args)
}

main函数用到了hyperkube的一个重要方法AddServer：

// AddServer adds a server to the HyperKube object.
func (hk *HyperKube) AddServer(s *Server) {
    hk.servers = append(hk.servers, *s)
    hk.servers[len(hk.servers)-1].hk = hk
}

可以看到，在这个方法中，hk.servers[len(hk.servers)-1].hk = hk可以让Server结构体的hk字段指向同一个binary中的hyperkube，这样就把这些功能集成到一起。 接下来调用hyperkube的RunToExit运行相应的功能。