6.824 MapReduce lab1 2020(一）

MIT 6.824 分布式系统 lab1：MapReduce

文档 https://pdos.csail.mit.edu/6.824/labs/lab-mr.html
MapReduce论文 https://pdos.csail.mit.edu/6.824/schedule.html
知乎笔记 zhuanlan.zhihu.com/p/54243727
博客 https://www.cnblogs.com/haoweizh/p/10395016.html

Notes

单词计数模块：We also provide you with a couple of MapReduce applications: word-count in mrapps/wc.go. a simple sequential mapreduce implementation

[kou@python mrapps]$ pwd
/home/kou/MIT/6.824-golabs-2020/src/mrapps
[kou@python mrapps]$ ls
wc.go

wordcount’s MapReduce Model look like

[kou@python mrapps]$ cat wc.go 
package main

// a word-count application "plugin" for MapReduce.
//
// go build -buildmode=plugin wc.go

import "../mr"
import "unicode"
import "strings"
import "strconv"

// The map function is called once for each file of input. The first
// argument is the name of the input file, and the second is the
// file's complete contents. You should ignore the input file name,
// and look only at the contents argument. The return value is a slice
// of key/value pairs.
//
func Map(filename string, contents string) []mr.KeyValue {
	// function to detect word separators.
	ff := func(r rune) bool { return !unicode.IsLetter(r) }

	// split contents into an array of words.
	words := strings.FieldsFunc(contents, ff)

	kva := []mr.KeyValue{}
	for _, w := range words {
		kv := mr.KeyValue{w, "1"}
		kva = append(kva, kv)
	}
	return kva
}

// The reduce function is called once for each key generated by the
// map tasks, with a list of all the values created for that key by
// any map task.

func Reduce(key string, values []string) string {
	// return the number of occurrences of this word.
	return strconv.Itoa(len(values))
}

a simple sequential mapreduce implementation(mrsequential.go)

mrsequential.go is a imple sequential MapReduce and leaves its output in the file mr-out-0. The input is from the text files named pg-xxx.txt.

[kou@python main]$ cat mrsequential.go 
package main

// simple sequential MapReduce.

// go run mrsequential.go wc.so pg*.txt

import "fmt"
import "../mr"
import "plugin"
import "os"
import "log"
import "io/ioutil"
import "sort"

// for sorting by key.
type ByKey []mr.KeyValue

// for sorting by key.
func (a ByKey) Len() int           { return len(a) }
func (a ByKey) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
func (a ByKey) Less(i, j int) bool { return a[i].Key < a[j].Key }

func main() {
	if len(os.Args) < 3 {
		fmt.Fprintf(os.Stderr, "Usage: mrsequential xxx.so inputfiles...\n")
		os.Exit(1)
	}

	mapf, reducef := loadPlugin(os.Args[1])// os.Args[1] wc.so

	//
	// read each input file,
	// pass it to Map,
	// accumulate the intermediate Map output.
	//
	intermediate := []mr.KeyValue{}
    for _, filename := range os.Args[2:] {//os.Args[2:] pg*.txt
		file, err := os.Open(filename)
		if err != nil {
			log.Fatalf("cannot open %v", filename)
		}
		content, err := ioutil.ReadAll(file)
		if err != nil {
			log.Fatalf("cannot read %v", filename)
		}
		file.Close()
		kva := mapf(filename, string(content))
		intermediate = append(intermediate, kva...)
	}

	//
	// a big difference from real MapReduce is that all the
	// intermediate data is in one place, intermediate[],
	// rather than being partitioned into NxM buckets.
	//

	sort.Sort(ByKey(intermediate))

	oname := "mr-out-0"
	ofile, _ := os.Create(oname)

	//
	// call Reduce on each distinct key in intermediate[],
	// and print the result to mr-out-0.
	//
	i := 0
	for i < len(intermediate) {
		j := i + 1
		for j < len(intermediate) && intermediate[j].Key == intermediate[i].Key {
			j++
		}
		values := []string{}
		for k := i; k < j; k++ {
			values = append(values, intermediate[k].Value)
		}
		output := reducef(intermediate[i].Key, values)

		// this is the correct format for each line of Reduce output.
		fmt.Fprintf(ofile, "%v %v\n", intermediate[i].Key, output)

		i = j
	}

	ofile.Close()
}

//
// load the application Map and Reduce functions
// from a plugin file, e.g. ../mrapps/wc.so
//

func loadPlugin(filename string) (func(string, string) []mr.KeyValue, func(string, []string) string) {
	p, err := plugin.Open(filename)
	if err != nil {
		log.Fatalf("cannot load plugin %v", filename)
	}
	xmapf, err := p.Lookup("Map")
	if err != nil {
		log.Fatalf("cannot find Map in %v", filename)
	}
	mapf := xmapf.(func(string, string) []mr.KeyValue)
	xreducef, err := p.Lookup("Reduce")
	if err != nil {
		log.Fatalf("cannot find Reduce in %v", filename)
	}
	reducef := xreducef.(func(string, []string) string)

	return mapf, reducef
}