Kubernetes Job Controller源码分析

Author: [email protected], WaltonWang@csdn

摘要：对于一般用户，学习和使用Job，官方文档就足够了，但如果你是个变态，那你总会去想，Job Controller和Deployment Controller在管理Pod上，除了RestartPolicy等不同外，还有哪些不同呢？其实是因为最近在搞TensorFlow on Kubernetes项目，想通过Job映射分布式TensorFlow中的worker task，达到训练完数据，自动回收资源资源等目的。本博文通过Job Controller代码分析其内部主要流程。

实现流程图

New JobController

type JobController struct {
    kubeClient clientset.Interface
    podControl controller.PodControlInterface

    // To allow injection of updateJobStatus for testing.
    updateHandler func(job *batch.Job) error
    syncHandler   func(jobKey string) (bool, error)
    // podStoreSynced returns true if the pod store has been synced at least once.
    // Added as a member to the struct to allow injection for testing.
    podStoreSynced cache.InformerSynced
    // jobStoreSynced returns true if the job store has been synced at least once.
    // Added as a member to the struct to allow injection for testing.
    jobStoreSynced cache.InformerSynced

    // A TTLCache of pod creates/deletes each rc expects to see
    expectations controller.ControllerExpectationsInterface

    // A store of jobs
    jobLister batchv1listers.JobLister

    // A store of pods, populated by the podController
    podStore corelisters.PodLister

    // Jobs that need to be updated
    queue workqueue.RateLimitingInterface

    recorder record.EventRecorder
}


func NewJobController(podInformer coreinformers.PodInformer, jobInformer batchinformers.JobInformer, kubeClient clientset.Interface) *JobController {
    eventBroadcaster := record.NewBroadcaster()
    eventBroadcaster.StartLogging(glog.Infof)
    // TODO: remove the wrapper when every clients have moved to use the clientset.
    eventBroadcaster.StartRecordingToSink(&v1core.EventSinkImpl{Interface: v1core.New(kubeClient.CoreV1().RESTClient()).Events("")})

    if kubeClient != nil && kubeClient.CoreV1().RESTClient().GetRateLimiter() != nil {
        metrics.RegisterMetricAndTrackRateLimiterUsage("job_controller", kubeClient.CoreV1().RESTClient().GetRateLimiter())
    }

    jm := &JobController{
        kubeClient: kubeClient,
        podControl: controller.RealPodControl{
            KubeClient: kubeClient,
            Recorder:   eventBroadcaster.NewRecorder(scheme.Scheme, v1.EventSource{Component: "job-controller"}),
        },
        expectations: controller.NewControllerExpectations(),
        queue:        workqueue.NewNamedRateLimitingQueue(workqueue.NewItemExponentialFailureRateLimiter(DefaultJobBackOff, MaxJobBackOff), "job"),
        recorder:     eventBroadcaster.NewRecorder(scheme.Scheme, v1.EventSource{Component: "job-controller"}),
    }

    jobInformer.Informer().AddEventHandler(cache.ResourceEventHandlerFuncs{
        AddFunc:    jm.enqueueController,
        UpdateFunc: jm.updateJob,
        DeleteFunc: jm.enqueueController,
    })
    jm.jobLister = jobInformer.Lister()
    jm.jobStoreSynced = jobInformer.Informer().HasSynced

    podInformer.Informer().AddEventHandler(cache.ResourceEventHandlerFuncs{
        AddFunc:    jm.addPod,
        UpdateFunc: jm.updatePod,
        DeleteFunc: jm.deletePod,
    })
    jm.podStore = podInformer.Lister()
    jm.podStoreSynced = podInformer.Informer().HasSynced

    jm.updateHandler = jm.updateJobStatus
    jm.syncHandler = jm.syncJob

    return jm
}

• 构造JobController，并初始化相关数据，比如rate limiter queue;
• watch pod and job object;
• 注册podInformer的add/del/update EventHandler；
• 注册jobInformer的add/del/update EventHandler;
• 注册updataHandler为updateJobStatus，用来更新Job状态；
• 注册syncHandler为syncJob，用来进行处理queue中的Job；

JobController Run

// Run the main goroutine responsible for watching and syncing jobs.
func (jm *JobController) Run(workers int, stopCh <-chan struct{}) {
    defer utilruntime.HandleCrash()
    defer jm.queue.ShutDown()

    glog.Infof("Starting job controller")
    defer glog.Infof("Shutting down job controller")

    if !controller.WaitForCacheSync("job", stopCh, jm.podStoreSynced, jm.jobStoreSynced) {
        return
    }

    for i := 0; i < workers; i++ {
        go wait.Until(jm.worker, time.Second, stopCh)
    }

    <-stopCh
}

// worker runs a worker thread that just dequeues items, processes them, and marks them done.
// It enforces that the syncHandler is never invoked concurrently with the same key.
func (jm *JobController) worker() {
    for jm.processNextWorkItem() {
    }
}

func (jm *JobController) processNextWorkItem() bool {
    key, quit := jm.queue.Get()
    if quit {
        return false
    }
    defer jm.queue.Done(key)

    forget, err := jm.syncHandler(key.(string))
    if err == nil {
        if forget {
            jm.queue.Forget(key)
        }
        return true
    }

    utilruntime.HandleError(fmt.Errorf("Error syncing job: %v", err))
    jm.queue.AddRateLimited(key)

    return true
}

• WaitForCacheSync等待jobController cache同步；
• 启动5个goruntine，每个协程分别执行worker，每个worker执行完后等待1s，继续执行，如此循环；
• worker负责从从queue中get job key，对每个job，调用syncJob进行同步，如果syncJob成功，则forget the job（其实就是让rate limiter 停止tracking it），否则将该key再次加入到queue中，等待下次sync。

syncJob

// syncJob will sync the job with the given key if it has had its expectations fulfilled, meaning
// it did not expect to see any more of its pods created or deleted. This function is not meant to be invoked
// concurrently with the same key.
func (jm *JobController) syncJob(key string) (bool, error) {
    startTime := time.Now()
    defer func() {
        glog.V(4).Infof("Finished syncing job %q (%v)", key, time.Now().Sub(startTime))
    }()

    ns, name, err := cache.SplitMetaNamespaceKey(key)
    if err != nil {
        return false, err
    }
    if len(ns) == 0 || len(name) == 0 {
        return false, fmt.Errorf("invalid job key %q: either namespace or name is missing", key)
    }
    sharedJob, err := jm.jobLister.Jobs(ns).Get(name)
    if err != nil {
        if errors.IsNotFound(err) {
            glog.V(4).Infof("Job has been deleted: %v", key)
            jm.expectations.DeleteExpectations(key)
            return true, nil
        }
        return false, err
    }
    job := *sharedJob

    // if job was finished previously, we don't want to redo the termination
    if IsJobFinished(&job) {
        return true, nil
    }

    // retrieve the previous number of retry
    previousRetry := jm.queue.NumRequeues(key)

    // Check the expectations of the job before counting active pods, otherwise a new pod can sneak in
    // and update the expectations after we've retrieved active pods from the store. If a new pod enters
    // the store after we've checked the expectation, the job sync is just deferred till the next relist.
    jobNeedsSync := jm.expectations.SatisfiedExpectations(key)

    pods, err := jm.getPodsForJob(&job)
    if err != nil {
        return false, err
    }

    activePods := controller.FilterActivePods(pods)
    active := int32(len(activePods))
    succeeded, failed := getStatus(pods)
    conditions := len(job.Status.Conditions)
    // job first start
    if job.Status.StartTime == nil {
        now := metav1.Now()
        job.Status.StartTime = &now
        // enqueue a sync to check if job past ActiveDeadlineSeconds
        if job.Spec.ActiveDeadlineSeconds != nil {
            glog.V(4).Infof("Job %s have ActiveDeadlineSeconds will sync after %d seconds",
                key, *job.Spec.ActiveDeadlineSeconds)
            jm.queue.AddAfter(key, time.Duration(*job.Spec.ActiveDeadlineSeconds)*time.Second)
        }
    }

    var manageJobErr error
    jobFailed := false
    var failureReason string
    var failureMessage string

    jobHaveNewFailure := failed > job.Status.Failed

    // check if the number of failed jobs increased since the last syncJob
    if jobHaveNewFailure && (int32(previousRetry)+1 > *job.Spec.BackoffLimit) {
        jobFailed = true
        failureReason = "BackoffLimitExceeded"
        failureMessage = "Job has reach the specified backoff limit"
    } else if pastActiveDeadline(&job) {
        jobFailed = true
        failureReason = "DeadlineExceeded"
        failureMessage = "Job was active longer than specified deadline"
    }

    if jobFailed {
        errCh := make(chan error, active)
        jm.deleteJobPods(&job, activePods, errCh)
        select {
        case manageJobErr = <-errCh:
            if manageJobErr != nil {
                break
            }
        default:
        }

        // update status values accordingly
        failed += active
        active = 0
        job.Status.Conditions = append(job.Status.Conditions, newCondition(batch.JobFailed, failureReason, failureMessage))
        jm.recorder.Event(&job, v1.EventTypeWarning, failureReason, failureMessage)
    } else {
        if jobNeedsSync && job.DeletionTimestamp == nil {
            active, manageJobErr = jm.manageJob(activePods, succeeded, &job)
        }
        completions := succeeded
        complete := false
        if job.Spec.Completions == nil {
            // This type of job is complete when any pod exits with success.
            // Each pod is capable of
            // determining whether or not the entire Job is done.  Subsequent pods are
            // not expected to fail, but if they do, the failure is ignored.  Once any
            // pod succeeds, the controller waits for remaining pods to finish, and
            // then the job is complete.
            if succeeded > 0 && active == 0 {
                complete = true
            }
        } else {
            // Job specifies a number of completions.  This type of job signals
            // success by having that number of successes.  Since we do not
            // start more pods than there are remaining completions, there should
            // not be any remaining active pods once this count is reached.
            if completions >= *job.Spec.Completions {
                complete = true
                if active > 0 {
                    jm.recorder.Event(&job, v1.EventTypeWarning, "TooManyActivePods", "Too many active pods running after completion count reached")
                }
                if completions > *job.Spec.Completions {
                    jm.recorder.Event(&job, v1.EventTypeWarning, "TooManySucceededPods", "Too many succeeded pods running after completion count reached")
                }
            }
        }
        if complete {
            job.Status.Conditions = append(job.Status.Conditions, newCondition(batch.JobComplete, "", ""))
            now := metav1.Now()
            job.Status.CompletionTime = &now
        }
    }

    forget := false
    // no need to update the job if the status hasn't changed since last time
    if job.Status.Active != active || job.Status.Succeeded != succeeded || job.Status.Failed != failed || len(job.Status.Conditions) != conditions {
        job.Status.Active = active
        job.Status.Succeeded = succeeded
        job.Status.Failed = failed

        if err := jm.updateHandler(&job); err != nil {
            return false, err
        }

        if jobHaveNewFailure && !IsJobFinished(&job) {
            // returning an error will re-enqueue Job after the backoff period
            return false, fmt.Errorf("failed pod(s) detected for job key %q", key)
        }

        forget = true
    }

    return forget, manageJobErr
}

• 从Indexer中查找指定的Job是否存在，如果不存在，则从expectations中删除该job，流程结束返回true。否则继续下面流程。
• 根据JobCondition Complete or Failed判断Job是否Finished，如果Finished，则流程结束返回true，否则继续下面流程。
• 调用SatisfiedExpectations，如果ControlleeExpectations中待add和del都<=0，或者expectations已经超过5分钟没更新过了，则返回jobNeedsSync=true，表示需要进行一次manageJob了。
• 对于那些第一次启动的jobs (StartTime==nil), 需要把设置StartTime，并且如果ActiveDeadlineSeconds不为空，则经过ActiveDeadlineSeconds后再次把该job加入到queue中进行sync。
• 获取该job管理的所有pods，过滤出activePods，计算出actived,successed,failed pods的数量。如果failed > job.Status.Failed，说明该job又有新failed Pods了，则jobHaveNewFailure为true。
• 如果jobHaveNewFailure，并且queue记录的该job retry次数加1，比job.Spec.BackoffLimit(默认为6)，则表示该job BackoffLimitExceeded，jobFailed。如果job StartTime到现在为止的历时>=ActiveDeadlineSeconds，则表示该job DeadlineExceeded，jobFailed。
• 如果jobFailed，则用sync.WaitGroup并发等待删除所有的前面过滤出来的activePods，删除成功，则failed += acitve, active = 0, 并设置Condition Failed为true。
• 如果job not failed, jobNeedSync为true，并且job的DeletionTimestamp为空（没有标记为删除），则调用manageJob对Job管理的pods根据复杂的策略进行add or del。
• 如果job not failed且job.Spec.Completions为nil，表示This type of job is complete when any pod exits with success。因此如果succeeded > 0 && active == 0，则表示job completed。
• 如果如果job not failed且job.Spec.Completions不为nil，表示This type of job signals success by having that number of successes。因此如果succeeded >= job.Spec.Completions，则表示job completed。
• 如果job completed，则更新其Conditions Complete为true，并设置CompletionTime。
• 接下来invoke updateJobStatus更新etcd中job状态，如果更新失败，则返回false，该job将再次加入queue。如果jobHaveNewFailure为true，并且Job Condition显示该Job not Finished，则返回false，该job将再次加入queue。

manageJob

// manageJob is the core method responsible for managing the number of running
// pods according to what is specified in the job.Spec.
// Does NOT modify .
func (jm *JobController) manageJob(activePods []*v1.Pod, succeeded int32, job *batch.Job) (int32, error) {
    var activeLock sync.Mutex
    active := int32(len(activePods))
    parallelism := *job.Spec.Parallelism
    jobKey, err := controller.KeyFunc(job)
    if err != nil {
        utilruntime.HandleError(fmt.Errorf("Couldn't get key for job %#v: %v", job, err))
        return 0, nil
    }

    var errCh chan error
    if active > parallelism {
        diff := active - parallelism
        errCh = make(chan error, diff)
        jm.expectations.ExpectDeletions(jobKey, int(diff))
        glog.V(4).Infof("Too many pods running job %q, need %d, deleting %d", jobKey, parallelism, diff)
        // Sort the pods in the order such that not-ready < ready, unscheduled
        // < scheduled, and pending < running. This ensures that we delete pods
        // in the earlier stages whenever possible.
        sort.Sort(controller.ActivePods(activePods))

        active -= diff
        wait := sync.WaitGroup{}
        wait.Add(int(diff))
        for i := int32(0); i < diff; i++ {
            go func(ix int32) {
                defer wait.Done()
                if err := jm.podControl.DeletePod(job.Namespace, activePods[ix].Name, job); err != nil {
                    defer utilruntime.HandleError(err)
                    // Decrement the expected number of deletes because the informer won't observe this deletion
                    glog.V(2).Infof("Failed to delete %v, decrementing expectations for job %q/%q", activePods[ix].Name, job.Namespace, job.Name)
                    jm.expectations.DeletionObserved(jobKey)
                    activeLock.Lock()
                    active++
                    activeLock.Unlock()
                    errCh <- err
                }
            }(i)
        }
        wait.Wait()

    } else if active < parallelism {
        wantActive := int32(0)
        if job.Spec.Completions == nil {
            // Job does not specify a number of completions.  Therefore, number active
            // should be equal to parallelism, unless the job has seen at least
            // once success, in which leave whatever is running, running.
            if succeeded > 0 {
                wantActive = active
            } else {
                wantActive = parallelism
            }
        } else {
            // Job specifies a specific number of completions.  Therefore, number
            // active should not ever exceed number of remaining completions.
            wantActive = *job.Spec.Completions - succeeded
            if wantActive > parallelism {
                wantActive = parallelism
            }
        }
        diff := wantActive - active
        if diff < 0 {
            utilruntime.HandleError(fmt.Errorf("More active than wanted: job %q, want %d, have %d", jobKey, wantActive, active))
            diff = 0
        }
        jm.expectations.ExpectCreations(jobKey, int(diff))
        errCh = make(chan error, diff)
        glog.V(4).Infof("Too few pods running job %q, need %d, creating %d", jobKey, wantActive, diff)

        active += diff
        wait := sync.WaitGroup{}

        // Batch the pod creates. Batch sizes start at SlowStartInitialBatchSize
        // and double with each successful iteration in a kind of "slow start".
        // This handles attempts to start large numbers of pods that would
        // likely all fail with the same error. For example a project with a
        // low quota that attempts to create a large number of pods will be
        // prevented from spamming the API service with the pod create requests
        // after one of its pods fails.  Conveniently, this also prevents the
        // event spam that those failures would generate.
        for batchSize := int32(integer.IntMin(int(diff), controller.SlowStartInitialBatchSize)); diff > 0; batchSize = integer.Int32Min(2*batchSize, diff) {
            errorCount := len(errCh)
            wait.Add(int(batchSize))
            for i := int32(0); i < batchSize; i++ {
                go func() {
                    defer wait.Done()
                    err := jm.podControl.CreatePodsWithControllerRef(job.Namespace, &job.Spec.Template, job, metav1.NewControllerRef(job, controllerKind))
                    if err != nil && errors.IsTimeout(err) {
                        // Pod is created but its initialization has timed out.
                        // If the initialization is successful eventually, the
                        // controller will observe the creation via the informer.
                        // If the initialization fails, or if the pod keeps
                        // uninitialized for a long time, the informer will not
                        // receive any update, and the controller will create a new
                        // pod when the expectation expires.
                        return
                    }
                    if err != nil {
                        defer utilruntime.HandleError(err)
                        // Decrement the expected number of creates because the informer won't observe this pod
                        glog.V(2).Infof("Failed creation, decrementing expectations for job %q/%q", job.Namespace, job.Name)
                        jm.expectations.CreationObserved(jobKey)
                        activeLock.Lock()
                        active--
                        activeLock.Unlock()
                        errCh <- err
                    }
                }()
            }
            wait.Wait()
            // any skipped pods that we never attempted to start shouldn't be expected.
            skippedPods := diff - batchSize
            if errorCount < len(errCh) && skippedPods > 0 {
                glog.V(2).Infof("Slow-start failure. Skipping creation of %d pods, decrementing expectations for job %q/%q", skippedPods, job.Namespace, job.Name)
                active -= skippedPods
                for i := int32(0); i < skippedPods; i++ {
                    // Decrement the expected number of creates because the informer won't observe this pod
                    jm.expectations.CreationObserved(jobKey)
                }
                // The skipped pods will be retried later. The next controller resync will
                // retry the slow start process.
                break
            }
            diff -= batchSize
        }
    }

    select {
    case err := <-errCh:
        // all errors have been reported before, we only need to inform the controller that there was an error and it should re-try this job once more next time.
        if err != nil {
            return active, err
        }
    default:
    }

    return active, nil
}

• 如果active > job.Spec.Parallelism, 表示要scale down：
  
  • 计算active与parallelism的差值diff，修改ControllerExpectations中该job的dels为diff，表示要删除diff这么多的pod。
  • 计算active与parallelism的差值diff，修改ControllerExpectations中该job的dels为diff，表示要删除diff这么多的pod。
  • 将activePods中的Pods按照not-ready < ready, unscheduled < scheduled, pending < running进行排序，确保先删除stage越早的pods。
  • 更新active (active减去diff），用sync.WaitGroup并发等待删除etcd中那些Pods。如果删除某个Pod失败，active要加1，expectations中dels要减1.
  • 返回active
• 如果active < job.Spec.Parallelism，
  表示要scale up：
  
  • 如果job.Spec.Completions为nil，且succeeded大于0，则diff设为0；如果job.Spec.Completions为nil，但successed = 0，则diff为 parallelism-active；如果job.Spec.Completions不为nil，则diff为max(job.Spec.Completions - succeeded，parallelim) - active；
  • 修改ControllerExpectations中该job的adds为diff，表示要新增diff这么多的pod。
  • 更新active (active加上diff），用sync.WaitGroup分批的创建Pods，第一批创建1个(代码写死SlowStartInitialBatchSize = 1)，第二批创建2，然后4,8,16…这样下去，但是每次不能超过diff的值。每一批创建pod后，注意更新diff的值（减去batchsize）。如果某一批创建过程Pods中存在失败情况，则更新active和expectations中adds，且不进行后续未启动的批量创建pods行为。
• 如果active == job.Spec.Parallelism，返回active。

总结

关于Job工作原理及配置，请直接阅读官方文档 jobs run to completion,那里有关于job配置 .spec.completions,.spec.parallelism,spec.activeDeadlineSeconds的使用说明，但是并没有把真正内部怎么工作的讲清楚，本博文就是希望能把这些东西讲清楚。