分类： 技术

Go语言import语句的位置

以下摘自The Go Programming Language：

The import declarations must follow the package declaration.After that, a program consists of the declarations of functions, variables, constants, and types (introduced by the key words func, var , const , and type ); for the most part, the order of declarations does not matter.

import语句必须跟在package定义后面。请看下例：

package main

var str string = "Hello world!\n"

import "fmt"

func main(){
    // your code goes 
    fmt.Println(str)
}

编译如下：

./prog.go:5: syntax error: unexpected import

调整一下语句位置：

package main

import "fmt"

var str string = "Hello world!\n"

func main(){
    // your code goes 
    fmt.Println(str)
}

编译执行成功：

Hello world!

 

Go语言的“main package”

以下摘自The Go Programming Language：

Package main is special. It defines a standalone executable program, not a library. Within package main the function main is also special—it’s where execution of the program begins. Whatever main does is what the program does.

main package不同于其它library package，它定义了一个可执行程序。其中的main函数即是可执行文件的入口函数。

 

Kubernetes笔记（2）—— 编译时的workspace

编译k8s代码时，会在k8s根目录下生成一个_output文件夹，同时这个文件夹下还包含local文件夹：

~/kubernetes/_output/local$ ls
bin  go

go文件夹下就是一个标准的Go语言workspace：

:~/kubernetes/_output/local/go$ ls -alt
total 20
drwxrwxr-x 4 nan nan 4096 Dec  9 22:09 ..
drwxrwxr-x 2 nan nan 4096 Dec  9 22:09 bin
drwxrwxr-x 4 nan nan 4096 Dec  9 22:08 pkg
drwxrwxr-x 5 nan nan 4096 Dec  9 22:07 .
drwxrwxr-x 3 nan nan 4096 Dec  9 22:04 src

进入src文件夹：

~/kubernetes/_output/local/go/src$ ls -alt
total 12
drwxrwxr-x 5 nan nan 4096 Dec  9 22:07 ..
drwxrwxr-x 2 nan nan 4096 Dec  9 22:06 k8s.io
drwxrwxr-x 3 nan nan 4096 Dec  9 22:04 .
nan@ubuntu:~/kubernetes/_output/local/go/src$ cd k8s.io/
nan@ubuntu:~/kubernetes/_output/local/go/src/k8s.io$ ls -alt
total 8
drwxrwxr-x 2 nan nan 4096 Dec  9 22:06 .
lrwxrwxrwx 1 nan nan   20 Dec  9 22:06 kubernetes -> /home/nan/kubernetes
drwxrwxr-x 3 nan nan 4096 Dec  9 22:04 ..

可以看到，src/k8s.io/kubernetes就是一个指向外层工作目录的软链接。

至此，可以理解代码里下面import语句为什么能工作了：

import (
    "k8s.io/kubernetes/contrib/mesos/pkg/controllermanager"
    "k8s.io/kubernetes/contrib/mesos/pkg/hyperkube"
)

 

Go语言的workspace

一个典型的Golang workspace包含3个文件夹：

src contains Go source files organized into packages (one package per directory),  
pkg contains package objects, and  
bin contains executable commands.

src包含这个工程的所有源码文件（包含依赖的第三方package的源码），pkg包含编译生成的package目标文件，而bin包含最后生成的可执行文件。下面是一个例子：

bin/
    hello                          # command executable
    outyet                         # command executable
pkg/
    linux_amd64/
        github.com/golang/example/
            stringutil.a           # package object
src/
    github.com/golang/example/
        .git/                      # Git repository metadata
    hello/
        hello.go               # command source
    outyet/
        main.go                # command source
        main_test.go           # test source
    stringutil/
        reverse.go             # package source
        reverse_test.go        # test source

参考资料：
Workspaces。

 

Kubernetes笔记（1）—— hyperkube

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。

 

etcd简介

最近在搭建kubernetes on Mesos，用到了etcd。etcd是一个分布式的，可靠的，key-value数据库。其项目主页在这里。

etcd的搭建很简单，有现成的binary可以使用。不过让我感到与其它项目不同的是，它会为某项服务同时启动两个端口。比如：

-listen-peer-urls

List of URLs to listen on for peer traffic. This flag tells the etcd to accept incoming requests from its peers on the specified scheme://IP:port combinations. Scheme can be either http or https.If 0.0.0.0 is specified as the IP, etcd listens to the given port on all interfaces. If an IP address is given as well as a port, etcd will listen on the given port and interface. Multiple URLs may be used to specify a number of addresses and ports to listen on. The etcd will respond to requests from any of the listed addresses and ports.

default: “http://localhost:2380,http://localhost:7001”
env variable: ETCDLISTENPEER_URLS
example: “http://10.0.0.1:2380”
invalid example: “http://example.com:2380” (domain name is invalid for binding)

-listen-client-urls

List of URLs to listen on for client traffic. This flag tells the etcd to accept incoming requests from the clients on the specified scheme://IP:port combinations. Scheme can be either http or https. If 0.0.0.0 is specified as the IP, etcd listens to the given port on all interfaces. If an IP address is given as well as a port, etcd will listen on the given port and interface. Multiple URLs may be used to specify a number of addresses and ports to listen on. The etcd will respond to requests from any of the listed addresses and ports.
default: “http://localhost:2379,http://localhost:4001”
env variable: ETCDLISTENCLIENT_URLS
example: “http://10.0.0.1:2379”
invalid example: “http://example.com:2379” (domain name is invalid for binding)

-listen-peer-urls监听peer的请求，默认端口为2380和7001，而-listen-client-urls监听client的请求，默认端口为2379和4001。

另外一个有趣的配置项是-advertise-client-urls，它是把当前运行的etcd的client URL地址通知给cluster中的其它成员。

-advertise-client-urls

List of this member’s client URLs to advertise to the rest of the cluster. These URLs can contain domain names. default: “http://localhost:2379,http://localhost:4001”
env variable: ETCDADVERTISECLIENT_URLS
example: “http://example.com:2379, http://10.0.0.1:2379”
Be careful if you are advertising URLs such as http://localhost:2379 from a cluster member and are using the proxy feature of etcd. This will cause loops, because the proxy will be forwarding requests to itself until its resources (memory, file descriptors) are eventually depleted.

参考资料：
Configuration Flags。

 

Haskell笔记 （7）—— 类型

（1）

Haskell requires type names to start with an uppercase letter, and variable names must start with a lowercase letter.

举例如下：

> let C = "foo"

<interactive>:20:5: Not in scope: data constructor ‘C’
> let c = "foo"
> c
"foo"
> :type c
c :: [Char]

（2）

The combination of :: and the type after it is called a type signature.

举例如下：

> :type 'a'
'a' :: Char

:: Char就被称之为type signature。

 

Haskell笔记 （5）—— ghci

（1）

ghci stores the result of the last expression into a variable whose name is “it”. (This isn’t a Haskell language feature; it’s specific to ghci alone.)

举例如下：

> "foo"
"foo"
> it ++ "bar"
"foobar"
> it
"foobar"

（2）可以使用:set <option>和:unset <option>来设置和取消一些选项。以打印计算的结果类型为例：

> :set +t
> fst ('a', 'b')
'a'
it :: Char
> :unset +t
> fst ('a', 'b')
'a'

（3）在ghci中定义变量与源文件中定义变量不一样，需要在变量前加上let：

ghci> let a = 10
ghci> b = 100

<interactive>:9:3: parse error on input ‘=’

 

Mesos和Kubernetes的区别

stackocerflow上的这篇帖子描述了kubernetes和mesos的区别：

Kubernetes is an open source project that brings ‘Google style’ cluster management capabilities to the world of virtual machines, or ‘on the metal’ scenarios. It works very well with modern operating system environments (like CoreOS or Red Hat Atomic) that offer up lightweight computing ‘nodes’ that are managed for you. It is written in Golang and is lightweight, modular, portable and extensible. We (the Kubernetes team) are working with a number of different technology companies (including Mesosphere who curate the Mesos open source project) to establish Kubernetes as the standard way to interact with computing clusters. The idea is to reproduce the patterns that we see people needing to build cluster applications based on our experience at Google. Some of these concepts include:

pods — a way to group containers together
replication controllers — a way to handle the lifecycle of containers
labels — a way to find and query containers, and
services — a set of containers performing a common function.
So with Kubernetes alone you will have something that is simple, easy to get up-and-running, portable and extensible that adds ‘cluster’ as a noun to the things that you manage in the lightest weight manner possible. Run an application on a cluster, and stop worrying about an individual machine. In this case, cluster is a flexible resource just like a VM. It is a logical computing unit. Turn it up, use it, resize it, turn it down quickly and easily.

With Mesos, there is a fair amount of overlap in terms of the basic vision, but the products are at quite different points in their lifecycle and have different sweet spots. Mesos is a distributed systems kernel that stitches together a lot of different machines into a logical computer. It was born for a world where you own a lot of physical resources to create a big static computing cluster. The great thing about it is that lots of modern scalable data processing application run well on Mesos (Hadoop, Kafka, Spark) and it is nice because you can run them all on the same basic resource pool, along with your new age container packaged apps. It is somewhat more heavy weight than the Kubernetes project, but is getting easier and easier to manage thanks to the work of folks like Mesosphere.

Now what gets really interesting is that Mesos is currently being adapted to add a lot of the Kubernetes concepts and to support the Kubernetes API. So it will be a gateway to getting more capabilities for your Kubernetes app (high availability master, more advanced scheduling semantics, ability to scale to a very large number of nodes) if you need them, and is well suited to run production workloads (Kubernetes is still in an alpha state).

When asked, I tend to say:

Kubernetes is a great place to start if you are new to the clustering world; it is the quickest, easiest and lightest way to kick the tires and start experimenting with cluster oriented development. It offers a very high level of portability since it is being supported by a lot of different providers (Microsoft, IBM, Red Hat, CoreOs, MesoSphere, VMWare, etc).

If you have existing workloads (Hadoop, Spark, Kafka, etc), Mesos gives you a framework that let’s you interleave those workloads with each other, and mix in a some of the new stuff including Kubernetes apps.

Mesos gives you an escape valve if you need capabilities that are not yet implemented by the community in the Kubernetes framework.

简而言之，Mesos是一个distributed systems kernel，它的作用可以理解成把一组physical机器抽象成一台logic机器供程序使用。Kubernetes是一个运行在Mesos上的framework，它是一个管理container的cluster manager。