2020 6.624 的 MapReduce

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前言

做2020的6.824，刚刚开始了第一个实验，是MapReduce，这个实验难度（过测试）还行，前提是需要把要求理清楚。过程也是参考了网上好几个博客的，下面是我的solution

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一、MapReduce

主要就是实现paper中提到的MapReduce
paper 地址

流程大概是这样的

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二、Master

1. 数据结构

Master只有有两个个数据结构， Master， Task
Task包含了Map的task跟Reduce的task，对于Map task，会有FileIndex，对于Reduce task，会有ReduceIndex

type Task struct {
	StartTime   time.Time
	TaskType    TaskType
	FileName    string
	FileIndex 	int
	ReduceIndex int
	State       TaskState
	ReduceNum   int
}

type FileMap map[string]int

type Master struct {
	// Your definitions here.
	files              FileMap
	UnstartedMapTask   ConcurrentList
	MapRunningTasks    ConcurrentList
	ReduceTasks        ConcurrentList
	ReduceRunningTasks ConcurrentList
	nReduce            int
	state              MasterState
}

const (
	MapTask    MasterState = 0
	ReduceTask MasterState = 1
	Finish     MasterState = 2
)

const (
	Assigned     TaskState = 1
	WorkerFinish TaskState = 2
)

const (
	Map    TaskType = 1
	Reduce TaskType = 2
	Close  TaskType = 3
)

我用的是List结构保存需要分配map task以及reduce task, List 可以选择泛型 interface{}，但是考虑到只给这个实验使用，就限定了类型MapReduceItem，里面组合了name, Task, Master

type MapReduceItem struct {
	name   string
	task   *Task
	master *Master
}

type ConcurrentList struct {
	mu    sync.Mutex
	items []MapReduceItem
}

func (s *ConcurrentList) Add(item MapReduceItem) {
	s.mu.Lock()
	defer s.mu.Unlock()
	s.items = append(s.items, item)
}

func (s *ConcurrentList) GetLast() MapReduceItem {
	s.mu.Lock()
	defer s.mu.Unlock()
	item := s.items[len(s.items)-1]
	s.items = s.items[:len(s.items)-1]
	return item
}

func (s *ConcurrentList) Remove(name string) {
	s.mu.Lock()
	defer s.mu.Unlock()
	for i := 0; i < len(s.items); i++ {
		if s.items[i].name == name {
			s.items = append(s.items[:i], s.items[i+1:]...)
		}
	}
}

2.部分核心函数

Master 的调度函数，就是先分配MapTask，然后分配ReduceTask

//Return a task for the worker
func (m *Master) GetTask(args *NilArgs, reply *Task) error {

	//if master have map task to assign, return map task
	//if master have all map task assigned but NOT ALL map tasks are done, TODO
	//if all map tasks are done, assign reduce task

	if m.state == MapTask {
		reply = m.handleMapTask(reply)
	}

	if m.state == ReduceTask {
		m.handleReduceTask(reply)
	}

	//fmt.Println("GetTask return nil")
	return nil
}

handleMapTask

func (m *Master) handleMapTask(reply *Task) *Task {
	if m.UnstartedMapTask.Size() > 0 {
		popMapItem := m.UnstartedMapTask.Pop()
		*reply = Task{StartTime: time.Now(), TaskType: Map, FileName: popMapItem.name, State: Assigned, ReduceNum: m.nReduce, FileIndex: m.GetFileIndex(popMapItem.name)}
		m.MapRunningTasks.Add(MapReduceItem{name: reply.FileName, task: reply})
	} else if m.isMapTasksAssignedNotDone() == true {
		m.checkDeadTask(Map)
	} else if m.isMapTasksDone() == true {
		m.state = ReduceTask
	}
	return reply
}

handleReduceTask 类似

func (m *Master) handleReduceTask(reply *Task) {
	if m.ReduceTasks.Size() > 0 {
		popReduceItem := m.ReduceTasks.Pop()
		*reply = Task{StartTime: time.Now(), TaskType: Reduce, State: Assigned, ReduceNum: m.nReduce, FileIndex: m.GetReduceIndex(popReduceItem.name)}
		m.ReduceRunningTasks.Add(MapReduceItem{name: strconv.Itoa(reply.FileIndex), task: reply})
	} else if m.isReduceTasksAssignedNotDone() == true {
		m.checkDeadTask(Reduce)
	} else if m.isReduceTasksDone() == true {
		*reply = Task{TaskType: Close}
		m.state = Finish
	}
}

处理deadTask

func (m *Master) checkDeadTask(taskType TaskType) {
	switch taskType {
	case Map:
		m.checkMapTask()
		break
	case Reduce:
		m.checkReduceTask()
		break
	default:
		break
	}
}

处理Worker发过来的UpdateTask

func (m *Master) UpdateTask(task *Task, reply *NilReply) error {
	// update task: map
	// update task: reduce
	//fmt.Println("update task is called for task in master ", task.FileName)
	if task.TaskType == Map {
		if task.State == WorkerFinish {
			//fmt.Println("remove file ", task.FileName)
			m.MapRunningTasks.Remove(task.FileName)
		}
	}
	if task.TaskType == Reduce {
		if task.State == WorkerFinish {
			//fmt.Println("it is a reduce task for file name ", task.FileIndex)
			m.ReduceRunningTasks.Remove(strconv.Itoa(task.FileIndex))
		}
	}
	return nil
}

三、Worker

部分核心函数

向Mater发送task请求

func Worker(mapf func(string, string) []KeyValue, reducef func(string, []string) string) {
	for {
		task := CallGetTask()
		switch task.TaskType {
		case Map:
			doMap(mapf, task)
			break
		case Reduce:
			doReduce(reducef, task)
			break
		case Close:
			return
		}
	}
}

doMap, 其中文件读写比较复杂一些，我也是参考（复制粘贴）了网上的做法，当然Lab的文件也有相关提示，好好阅读Lab的要求以及提示

/*
* read input file from task.
* FileName pass it to Map accumulate the intermediate Map output.
 */
func doMap(mapf func(string, string) []KeyValue, task *Task) {
	constent := fileHandler(task.FileName)
	kva := mapf(task.FileName, constent)

	//prepare file for reduce task
	mapGroupName := prefix + strconv.Itoa(task.FileIndex)
	outFiles := make([]*os.File, task.ReduceNum)
	EncodeFile := make([]*json.Encoder, task.ReduceNum)

	for i := 0; i < task.ReduceNum; i++ {
		f, err := ioutil.TempFile("mr-tmp", "tmp")
		if err != nil {
			panic("fail to create temp file")
		}
		outFiles[i] = f
		EncodeFile[i] = json.NewEncoder(f)
	}

	//kva: all kv by map function
	for _, kv := range kva {
		index := ihash(kv.Key) % task.ReduceNum
		enc := EncodeFile[index]
		err := enc.Encode(&kv)
		if err != nil {
			fmt.Println("err is ", err)
			fmt.Println("File encode fail", kv)
			panic("fail encode")
		}
	}

	for i, file := range outFiles {
		outName := mapGroupName + hyphen + strconv.Itoa(i)
		oldpath := filepath.Join(file.Name())
		os.Rename(oldpath, outName)
		file.Close()
	}

	task.State = WorkerFinish
	CallUpdateTask(task)

doReduce，这个跟mrsequential 的做法就像了，直接把核心代码搬过来就行，不一样的就是系统的读写，需要把intermediate files读出来然后根据最后的数字决定生成文件的数字编号，比如mr-X-Y 最后生成就是mr-out-Y

func doReduce(reducef func(string, []string) string, task *Task) {
	// get relate intermediate
	// do reduce

	patternPrefix := "mr-tmp/mr-*-"
	pattern := patternPrefix + strconv.Itoa(task.FileIndex)
	fileList, err := filepath.Glob(pattern)
	if err != nil {
		fmt.Println("cannot read the dir")
	}

	intermediate := []KeyValue{}

	for _, fname := range fileList {
		//fmt.Printf("fname is %v\n", fname)
		file, err := os.Open(fname)
		if err != nil {
			fmt.Println("cannot read the dir")
		}
		dec := json.NewDecoder(file)
		for {
			var kv KeyValue
			if err = dec.Decode(&kv); err != nil {
				break
			}
			intermediate = append(intermediate, kv)
		}
	}

	sort.Sort(ByKey(intermediate))
	oname := "mr-out-" + strconv.Itoa(task.FileIndex)
	ofile, _ := os.Create(oname)
	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)
		fmt.Fprintf(ofile, "%v %v\n", intermediate[i].Key, output)
		i = j
	}

	task.State = WorkerFinish
	CallUpdateTask(task)
}

总结

自己的solution还是有很多可以改善的地方的，代码有点冗余，其中一个像改进的地方就是想通过channel通信换掉自己实现的concurrentList，毕竟channel是Go的特点之一。总的来说MapReduce流程清楚了，代码部分不是太难写，Go还是很灵活的语言，但是没有了传统面向对象的特点，有时候方法实现起来还是有点不适应