golang实现mapreduce单进程版本

元旦放假的第一天,在家没事干,用golang实现了一下mapreduce的单进程版本,github地址。处理对大文件统计最高频的10个单词,因为功能比较简单,所以设计没有解耦合。
本文先对mapreduce大体概念进行介绍,然后结合代码介绍一下,如果接下来几天有空,我会实现一下分布式高可用的mapreduce版本。

1. Mapreduce大体架构

pic1 上图是论文中mapreduce的大体架构。总的来说Mapreduce的思想就是分治思想:对数据进行分片,然后用mapper进行处理,以key-value形式输出中间文件;然后用reducer进行对mapper输出的中间文件进行合并:将key一致的合到一块,并输出结果文件;如果有需要,采用Combiner进行最后的合并。
归纳来说主要分为5部分:用户程序、Master、Mapper、Reducer、Combiner(上图未给出)。

1. 用户程序。用户程序主要对输入数据进行分割,制定Mapper、Reducer、Combiner的代码。
2. Master:中控系统。控制分发Mapper、Reduer的个数,比如生成m个进程处理Mapper,n个进程处理Reducer。其实对Master来说,Mapper和Reduer都属于worker,只不过跑的程序不一样,Mapper跑用户输入的map代码,Reduer跑用户输入的reduce代码。Master还作为管道负责中间路径传递,比如将Mapper生成的中间文件传递给Reduer,将Reduer生成的结果文件返回,或者传递给Combiner(如果有需要的话)。由于Master是单点,性能瓶颈,所以可以做集群:主备模式或者分布式模式。可以用zookeeper进行选主,用一些消息中间件进行数据同步。Master还可以进行一些策略处理:比如某个Worker执行时间特别长,很有可能卡住了,对分配给该Worker的数据重新分配给别的Worker执行,当然需要对多份数据返回去重处理。
3. Mapper:负责将输入数据切成key-value格式。Mapper处理完后,将中间文件的路径告知Master,Master获悉后传递给Reduer进行后续处理。如果Mapper未处理完,或者已经处理完但是Reduer未读完其中间输出文件,分配给该Mapper的输入将重新被别的Mapper执行。
4. Reducer: 接受Master发送的Mapper输出文件的消息,RPC读取文件并处理,并输出结果文件。n个Reduer将产生n个输出文件。
5. Combiner: 做最后的归并处理,通常不需要。

总的来说,架构不复杂。组件间通信用啥都可以,比如RPC、HTTP或者私有协议等。

2. 实现代码介绍

该版本代码实现了单机单进程版本,Mapper、Reducer和Combiner的实现用协程goroutine实现,通信采用channel。代码写的比较随意,没有解耦合。

• 功能:统计给定文件中出现的最高频的10个单词
• 输入:大文件
• 输出:最高频的10个单词
• 实现:5个Mapper协程、2个Reducer、1个Combiner。

为了方便起见,Combiner对最高频的10个单词进行堆排序处理,按规范来说应该放在用户程序处理。
文件目录如下,其中bin文件夹下的big_input_file.txt为输入文件,可以调用generate下的main文件生成,caller文件为入口的用户程序,master目录下分别存放master、mapper、reducer、combiner代码:

.
├── README.md
├── bin
│  └── file-store
│  └── big_input_file.txt
└── src
 ├── caller
 │  └── main.go
 ├── generate
 │  └── main.go
 └── master
 ├── combiner.go
 ├── mapper.go
 ├── master.go
 └── reducer.go
6 directories, 8 files 

2.1 caller

用户程序,读入文件并按固定行数进行划分;然后调用master.Handle进行处理。

package main
import ( 
 "os"
 "path"
 "path/filepath"
 "bufio"
 "strconv"
 "master"
 "github.com/vinllen/go-logger/logger"
)
const ( 
 LIMIT int = 10000 // the limit line of every file
)
func main() { 
 curDir, err := filepath.Abs(filepath.Dir(os.Args[0]))
 if err != nil {
 logger.Error("Read path error: ", err.Error())
 return
 }
 fileDir := path.Join(curDir, "file-store")
 _ = os.Mkdir(fileDir, os.ModePerm)
 // 1. read file
 filename := "big_input_file.txt"
 inputFile, err := os.Open(path.Join(fileDir, filename))
 if err != nil {
 logger.Error("Read inputFile error: ", err.Error())
 return
 }
 defer inputFile.Close()
 // 2. split inputFile into several pieces that every piece hold 100,000 lines
 filePieceArr := []string{}
 scanner := bufio.NewScanner(inputFile)
 piece := 1
Outter: 
 for {
 outputFilename := "input_piece_" + strconv.Itoa(piece)
 outputFilePos := path.Join(fileDir, outputFilename)
 filePieceArr = append(filePieceArr, outputFilePos)
 outputFile, err := os.Create(outputFilePos)
 if err != nil {
 logger.Error("Split inputFile error: ", err.Error())
 continue
 }
 defer outputFile.Close()
 for cnt := 0; cnt < LIMIT; cnt++ {
 if !scanner.Scan() {
 break Outter
 }
 _, err := outputFile.WriteString(scanner.Text() + "\n")
 if err != nil {
 logger.Error("Split inputFile writting error: ", err.Error())
 return
 }
 }
 piece++
 }
 // 3. pass to master
 res := master.Handle(filePieceArr, fileDir)
 logger.Warn(res)
}

2.2 master

Master程序,依次生成Combiner、Reducer、Mapper,处理消息中转,输出最后结果。

package master
import (
 "github.com/vinllen/go-logger/logger"
)
var ( 
 MapChanIn chan MapInput // channel produced by master while consumed by mapper
 MapChanOut chan string // channel produced by mapper while consumed by master
 ReduceChanIn chan string // channel produced by master while consumed by reducer
 ReduceChanOut chan string // channel produced by reducer while consumed by master
 CombineChanIn chan string // channel produced by master while consumed by combiner
 CombineChanOut chan []Item // channel produced by combiner while consumed by master
)
func Handle(inputArr []string, fileDir string) []Item { 
 logger.Info("handle called")
 const(
 mapperNumber int = 5
 reducerNumber int = 2
 )
 MapChanIn = make(chan MapInput)
 MapChanOut = make(chan string)
 ReduceChanIn = make(chan string)
 ReduceChanOut = make(chan string)
 CombineChanIn = make(chan string)
 CombineChanOut = make(chan []Item)
 reduceJobNum := len(inputArr)
 combineJobNum := reducerNumber
 // start combiner
 go combiner()
 // start reducer
 for i := 1; i <= reducerNumber; i++ {
 go reducer(i, fileDir)
 }
 // start mapper
 for i := 1; i <= mapperNumber; i++ {
 go mapper(i, fileDir)
 }
 go func() {
 for i, v := range(inputArr) {
 MapChanIn <- MapInput{
 Filename: v,
 Nr: i + 1,
 } // pass job to mapper
 }
 close(MapChanIn) // close map input channel when no more job
 }()
 var res []Item
outter: 
 for {
 select {
 case v := <- MapChanOut:
 go func() {
 ReduceChanIn <- v
 reduceJobNum--
 if reduceJobNum <= 0 {
 close(ReduceChanIn)
 }
 }()
 case v := <- ReduceChanOut:
 go func() {
 CombineChanIn <- v
 combineJobNum--
 if combineJobNum <= 0 {
 close(CombineChanIn)
 }
 }()
 case v := <- CombineChanOut:
 res = v
 break outter
 }
 }
 close(MapChanOut)
 close(ReduceChanOut)
 close(CombineChanOut)
 return res
}

2.3 mapper

Mapper程序,读入并按key-value格式生成中间文件,告知Master。

package master
import ( 
 "fmt"
 "path"
 "os"
 "bufio"
 "strconv"
 "github.com/vinllen/go-logger/logger"
)
type MapInput struct { 
 Filename string
 Nr int
}
func mapper(nr int, fileDir string) { 
 for {
 val, ok := <- MapChanIn // val: filename
 if !ok { // channel close
 break
 }
 inputFilename := val.Filename
 nr := val.Nr
 file, err := os.Open(inputFilename)
 if err != nil {
 errMsg := fmt.Sprintf("Read file(%s) error in mapper(%d)", inputFilename, nr)
 logger.Error(errMsg)
 MapChanOut <- ""
 continue
 }
 mp := make(map[string]int)
 scanner := bufio.NewScanner(file)
 scanner.Split(bufio.ScanWords)
 for scanner.Scan() {
 str := scanner.Text()
 //logger.Info(str)
 mp[str]++
 }
 outputFilename := path.Join(fileDir, "mapper-output-" + strconv.Itoa(nr))
 outputFileHandler, err := os.Create(outputFilename)
 if err != nil {
 errMsg := fmt.Sprintf("Write file(%s) error in mapper(%d)", outputFilename, nr)
 logger.Error(errMsg)
 } else {
 for k, v := range mp {
 str := fmt.Sprintf("%s %d\n", k, v)
 outputFileHandler.WriteString(str)
 }
 outputFileHandler.Close()
 }
 MapChanOut <- outputFilename
 }
}

2.4 reducer

Reducer程序,读入Master传递过来的中间文件并归并。

package master
import ( 
 "fmt"
 "bufio"
 "os"
 "strconv"
 "path"
 "strings"
 "github.com/vinllen/go-logger/logger"
)
func reducer(nr int, fileDir string) { 
 mp := make(map[string]int) // store the frequence of words
 // read file and do reduce
 for {
 val, ok := <- ReduceChanIn
 if !ok {
 break
 }
 logger.Debug("reducer called: ", nr)
 file, err := os.Open(val)
 if err != nil {
 errMsg := fmt.Sprintf("Read file(%s) error in reducer", val)
 logger.Error(errMsg)
 continue
 }
 scanner := bufio.NewScanner(file)
 for scanner.Scan() {
 str := scanner.Text()
 arr := strings.Split(str, " ")
 if len(arr) != 2 {
 errMsg := fmt.Sprintf("Read file(%s) error that len of line(%s) != 2(%d) in reducer", val, str, len(arr))
 logger.Warn(errMsg)
 continue
 }
 v, err := strconv.Atoi(arr[1])
 if err != nil {
 errMsg := fmt.Sprintf("Read file(%s) error that line(%s) parse error in reduer", val, str)
 logger.Warn(errMsg)
 continue
 }
 mp[arr[0]] += v
 }
 if err := scanner.Err(); err != nil {
 logger.Error("reducer: reading standard input:", err)
 }
 file.Close()
 }
 outputFilename := path.Join(fileDir, "reduce-output-" + strconv.Itoa(nr))
 outputFileHandler, err := os.Create(outputFilename)
 if err != nil {
 errMsg := fmt.Sprintf("Write file(%s) error in reducer(%d)", outputFilename, nr)
 logger.Error(errMsg)
 } else {
 for k, v := range mp {
 str := fmt.Sprintf("%s %d\n", k, v)
 outputFileHandler.WriteString(str)
 }
 outputFileHandler.Close()
 }
 ReduceChanOut <- outputFilename
}

2.5 combiner

Combiner程序,读入Master传递过来的Reducer结果文件并归并成一个,然后堆排序输出最高频的10个词语。

package master
import ( 
 "fmt"
 "strings"
 "bufio"
 "os"
 "container/heap"
 "strconv"
 "github.com/vinllen/go-logger/logger"
)
type Item struct { 
 key string
 val int
}
type PriorityQueue []*Item
func (pq PriorityQueue) Len() int { 
 return len(pq)
}
func (pq PriorityQueue) Less(i, j int) bool { 
 return pq[i].val > pq[j].val
}
func (pq PriorityQueue) Swap(i, j int) { 
 pq[i], pq[j] = pq[j], pq[i]
}
func (pq *PriorityQueue) Push(x interface{}) { 
 item := x.(*Item)
 *pq = append(*pq, item)
}
func (pq *PriorityQueue) Pop() interface{} { 
 old := *pq
 n := len(old)
 item := old[n - 1]
 *pq = old[0 : n - 1]
 return item
}
func combiner() { 
 mp := make(map[string]int) // store the frequence of words
 // read file and do combine
 for {
 val, ok := <- CombineChanIn
 if !ok {
 break
 }
 logger.Debug("combiner called")
 file, err := os.Open(val)
 if err != nil {
 errMsg := fmt.Sprintf("Read file(%s) error in combiner", val)
 logger.Error(errMsg)
 continue
 }
 scanner := bufio.NewScanner(file)
 for scanner.Scan() {
 str := scanner.Text()
 arr := strings.Split(str, " ")
 if len(arr) != 2 {
 errMsg := fmt.Sprintf("Read file(%s) error that len of line != 2(%s) in combiner", val, str)
 logger.Warn(errMsg)
 continue
 }
 v, err := strconv.Atoi(arr[1])
 if err != nil {
 errMsg := fmt.Sprintf("Read file(%s) error that line(%s) parse error in combiner", val, str)
 logger.Warn(errMsg)
 continue
 }
 mp[arr[0]] += v
 }
 file.Close()
 }
 // heap sort
 // pq := make(PriorityQueue, len(mp))
 pq := make(PriorityQueue, 0)
 heap.Init(&pq)
 for k, v := range mp {
 node := &Item {
 key: k,
 val: v,
 }
 // logger.Debug(k, v)
 heap.Push(&pq, node)
 }
 res := []Item{}
 for i := 0; i < 10 && pq.Len() > 0; i++ {
 node := heap.Pop(&pq).(*Item)
 res = append(res, *node)
 }
 CombineChanOut <- res
}

3. 总结

不足以及未实现之处:

1. 各模块间耦合性高
2. master单点故障未扩展
3. 未采用多进程实现,进程间采用RPC通信
4. 未实现单个Workder时间过长,另起Worker执行任务的代码。

接下来要是有空,我会实现分布式高可用的代码,模块间采用RPC通讯。

说明

转载请注明出处:http://vinllen.com/golangshi-xian-mapreducedan-jin-cheng-ban-ben/

参考

https://research.google.com/archive/mapreduce.html

本文来自:vinllen.com

感谢作者:vinllen.com

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