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package mapreduce
// Please do not modify this file.
import (
// Master holds all the state that the master needs to keep track of.
type Master struct {
sync.Mutex //匿名参数,表示这个struct 具有sync.Mutex的接口
//因此Master 也能调用sync.Mutex的函数. 所以当调用master.Lock()的时候也不足为奇
address string
doneChannel chan bool
// protected by the mutex
newCond *sync.Cond // signals when Register() adds to workers[]
workers []string // each worker's UNIX-domain socket name -- its RPC address
// Per-task information
jobName string // Name of currently executing job
files []string // Input files
nReduce int // Number of reduce partitions
shutdown chan struct{}
l net.Listener
stats []int
// Register is an RPC method that is called by workers after they have started
// up to report that they are ready to receive tasks.
func (mr *Master) Register(args *RegisterArgs, _ *struct{}) error {
defer mr.Unlock()
debug("Register: worker %s\n", args.Worker)
mr.workers = append(mr.workers, args.Worker)
// tell forwardRegistrations() that there's a new workers[] entry.
return nil
// newMaster initializes a new Map/Reduce Master
func newMaster(master string) (mr *Master) {
mr = new(Master)
mr.address = master
mr.shutdown = make(chan struct{})
mr.newCond = sync.NewCond(mr)
mr.doneChannel = make(chan bool)
// Sequential runs map and reduce tasks sequentially, waiting for each task to
// complete before running the next.
func Sequential(jobName string, files []string, nreduce int,
mapF func(string, string) []KeyValue,
reduceF func(string, []string) string,
) (mr *Master) {
mr = newMaster("master")
go, files, nreduce, func(phase jobPhase) {
switch phase {
case mapPhase:
for i, f := range mr.files {
doMap(mr.jobName, i, f, mr.nReduce, mapF)
case reducePhase:
for i := 0; i < mr.nReduce; i++ {
doReduce(mr.jobName, i, mergeName(mr.jobName, i), len(mr.files), reduceF)
}, func() {
mr.stats = []int{len(files) + nreduce}
// helper function that sends information about all existing
// and newly registered workers to channel ch. schedule()
// reads ch to learn about workers.
func (mr *Master) forwardRegistrations(ch chan string) {
i := 0
for {
if len(mr.workers) > i {
// there's a worker that we haven't told schedule() about.
w := mr.workers[i]
go func() { ch <- w }() // send without holding the lock.
i = i + 1
} else {
// wait for Register() to add an entry to workers[]
// in response to an RPC from a new worker.
// 当mr.newCond.Broadcast()被调用,此处就被唤醒,否则一直阻塞
// Distributed schedules map and reduce tasks on workers that register with the
// master over RPC.
func Distributed(jobName string, files []string, nreduce int, master string) (mr *Master) {
mr = newMaster(master)
go, files, nreduce,
func(phase jobPhase) {
ch := make(chan string)
go mr.forwardRegistrations(ch)
//任务分配到 已经连接的 worker,
schedule(mr.jobName, mr.files, mr.nReduce, phase, ch)
func() {
mr.stats = mr.killWorkers()
// run executes a mapreduce job on the given number of mappers and reducers.
// First, it divides up the input file among the given number of mappers, and
// schedules each task on workers as they become available. Each map task bins
// its output in a number of bins equal to the given number of reduce tasks.
// Once all the mappers have finished, workers are assigned reduce tasks.
// When all tasks have been completed, the reducer outputs are merged,
// statistics are collected, and the master is shut down.
// Note that this implementation assumes a shared file system.
func (mr *Master) run(jobName string, files []string, nreduce int,
schedule func(phase jobPhase),
finish func(),
) {
mr.jobName = jobName
mr.files = files
mr.nReduce = nreduce
fmt.Printf("%s: Starting Map/Reduce task %s\n", mr.address, mr.jobName)
fmt.Printf("%s: Map/Reduce task completed\n", mr.address)
mr.doneChannel <- true
// Wait blocks until the currently scheduled work has completed.
// This happens when all tasks have scheduled and completed, the final output
// have been computed, and all workers have been shut down.
func (mr *Master) Wait() {
// killWorkers cleans up all workers by sending each one a Shutdown RPC.
// It also collects and returns the number of tasks each worker has performed.
func (mr *Master) killWorkers() []int {
defer mr.Unlock()
ntasks := make([]int, 0, len(mr.workers))
for _, w := range mr.workers {
debug("Master: shutdown worker %s\n", w)
var reply ShutdownReply
ok := call(w, "Worker.Shutdown", new(struct{}), &reply)
if ok == false {
fmt.Printf("Master: RPC %s shutdown error\n", w)
} else {
ntasks = append(ntasks, reply.Ntasks)
return ntasks