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Abstract

This proposal introduces workflow functionality in kubernetes. Workflows (aka DAG workflows since tasks are organized in a Directed Acyclic Graph) are ubiquitous in modern job schedulers, see for example:

Most of the job schedulers offer workflow functionality to some extent.

Use Cases

  • As a user, I want to create a JobB which depends upon JobA running to completion.
  • As a user, I want workflow composability. I want to create a JobA which will be triggered as soon as an already running workflow runs to completion.
  • As a user, I want to delete a workflow (eventually cascading to running tasks).
  • As a user, I want to debug a workflow (ability to track failure): in case a task didn't run user should have a way to backtrack the reason of the failure, understanding which dependency has not been satisified.

Implementation

In this proposal a new REST resource Workflow is introduced. A Workflow is represented as a graph, more specifically as a DAG. Vertices of the graph represent steps of the workflow. The workflow steps are represented via a WorkflowStep1 resource. The edges of the graph represent dependecies. To represent edges there is no explicit resource

  • rather they are stored as predecessors in each WorkflowStep (i.e. each node). The basic idea of this proposal consists in creation of each step postponing execution until all predecessors' steps run to completion.

Workflow

A new resource will be introduced in the API. A Workflow is a graph. In the simplest case it's a graph of Jobs but it can also be a graph of other entities (for example cross-cluster objects or other Workflows).

// Workflow is a DAG workflow
type Workflow struct {
  unversioned.TypeMeta `json:",inline"`

  // Standard object's metadata.
  // More info: http://releases.k8s.io/release-1.3/docs/devel/api-conventions.md#metadata.
  api.ObjectMeta `json:"metadata,omitempty"`

  // Spec defines the expected behavior of a Workflow. More info: http://releases.k8s.io/release-1.3/docs/devel/api-conventions.md#spec-and-status.
  Spec WorkflowSpec `json:"spec,omitempty"`

  // Status represents the current status of the Workflow. More info: http://releases.k8s.io/release-1.3/docs/devel/api-conventions.md#spec-and-status.
  Status WorkflowStatus `json:"status,omitempty"`
}

// WorkflowList implements list of Workflow.
type WorkflowList struct {
  unversioned.TypeMeta `json:",inline"`

  // Standard list metadata
  // More info: http://releases.k8s.io/release-1.3/docs/devel/api-conventions.md#metadata
  unversioned.ListMeta `json:"metadata,omitempty"`

  // Items is the list of Workflow
  Items []Workflow `json:"items"`
}

// WorkflowSpec contains Workflow specification
type WorkflowSpec struct {
  // Standard object's metadata.
  // More info: http://releases.k8s.io/release-1.3/docs/devel/api-conventions.md#metadata
  api.ObjectMeta `json:"metadata,omitempty"`

  //ActiveDealineSeconds contains
  ActiveDeadlineSeconds *int64 `json:"activeDeadlineSeconds,omitempty"`

  // Steps is a map containing the workflow steps. Key of the
  // map is a string which uniquely identifies the workflow step
  Steps map[string]WorkflowStep `json:"steps,omitempty"`

   // Selector for created jobs (if any)
   Selector *LabelSelector `json:"selector,omitempty"`
}

// WorkflowStep contains necessary information to identifiy the node of the workflow graph
type WorkflowStep struct {
  // JobTemplate contains the job specificaton that should be run in this Workflow.
  // Only one between externalRef and jobTemplate can be set.
  JobTemplate *JobTemplateSpec `json:"jobTemplate,omitempty"`

  // External contains a reference to another schedulable resource.
  // Only one between ExternalRef and JobTemplate can be set.
  ExternalRef *api.ObjectReference `json:"externalRef,omitempty"`

  // Dependecies represent dependecies of the current workflow step.
  // Irrilevant for ExteranlRef step
  Dependencies []string `json:"dependencies,omitempty"`
}

type WorkflowConditionType string

// These are valid conditions of a workflow.
const (
  // WorkflowComplete means the workflow has completed its execution.
  WorkflowComplete WorkflowConditionType = "Complete"
)

type WorkflowCondition struct {
  // Type of workflow condition, currently only Complete.
  Type WorkflowConditionType `json:"type"`

  // Status of the condition, one of True, False, Unknown.
  Status api.ConditionStatus `json:"status"`

  // Last time the condition was checked.
  LastProbeTime unversioned.Time `json:"lastProbeTime,omitempty"`

  // Last time the condition transited from one status to another.
  LastTransitionTime unversioned.Time `json:"lastTransitionTime,omitempty"`

  // (brief) reason for the condition's last transition.
  Reason string `json:"reason,omitempty"`

  // Human readable message indicating details about last transition.
  Message string `json:"message,omitempty"`
}

// WorkflowStatus represents the
type WorkflowStatus struct {
  // Conditions represent the latest available observations of an object's current state.
  Conditions []WorkflowCondition `json:"conditions,omitempty" patchStrategy:"merge" patchMergeKey:"type"`

  // StartTime represents time when the job was acknowledged by the Workflow controller
  // It is not guaranteed to be set in happens-before order across separate operations.
  // It is represented in RFC3339 form and is in UTC.
  // StartTime doesn't consider startime of `ExternalReference`
  StartTime *unversioned.Time `json:"startTime,omitempty"`

  // CompletionTime represents time when the workflow was completed. It is not guaranteed to
  // be set in happens-before order across separate operations.
  // It is represented in RFC3339 form and is in UTC.
  CompletionTime *unversioned.Time `json:"completionTime,omitempty"`

  // Statuses represent status of different steps
  Statuses map[string]WorkflowStepStatus `json:statuses`
}

// WorkflowStepStatus contains necessary information for the step status
type WorkflowStepStatus struct {
  //Complete is set to true when resource run to complete
  Complete bool `json:"complete"`

  // Reference of the step resource
  Reference api.ObjectReference `json:"reference"`
}

JobTemplateSpec is already introduced by ScheduledJob controller proposal](#11980). Reported for readability:

type JobTemplateSpec struct {
  // Standard object's metadata.
  // More info: http://releases.k8s.io/release-1.3/docs/devel/api-conventions.md#metadata
  api.ObjectMeta

  // Spec is a structure defining the expected behavior of a job.
  // More info: http://releases.k8s.io/release-1.3/docs/devel/api-conventions.md#spec-and-status
  Spec JobSpec
}

Controller

Workflow controller will watch Workflow objects and any Job objects created by the workflow. the Jobs objects created in each step. Each step can contain either another Workflow referenced via workflowStep.ExternalRef or a Job created via workflowStep.jobTemplate. For each non finished workflow (similarly to Job, Workflow completion is detected iterating over all the status.conditions condition) we check if deadline is not expired. If deadline expired the workfow is terminated. If deadline didn't expires the workflow controller iterates over all workflow steps:

  • If step has a status (retrieved via step name (map key) in the status.statuses map check whether the step is already completed.
  • If step is completed nothing is done.
  • If step is not completed two sub-cases should be analyzed:
    • Step containing workflow: check wether workflow terminated and eventually update the status.statuses[name].complete entry if applicable
    • Step containing job: check whether job needs to be started or is already started.
      • A step/job is started if it has no dependecies or all its dependencies are already terminated. Workflow controller adds some labels to the Job. This will permit to obtain the workflow each job belongs to (via spec.Selector). The step name is equally inserted as a label in each job.
      • If the job is already running, a completion check is performed. If the job terminated (checked via conditions job.status) the field status.statusues[name].complete is updated.
  • When all steps are complete: complete condition is added to status.condition and the status.completionTime is updated. At this point, workflow is finished.

Changing a Workflow

Updating

User can modify a workflow only if the steps under modification are not already running.

Deleting

Users can cancel a workflow by deleting it before it's completed. All running jobs will be deleted. Other workflows referenced in steps will not be deleted as they are not owned by the parent workflow.

Events

The events associated to Workflows will be:

  • WorkflowCreated
  • WorkflowStarted
  • WorkflowEnded
  • WorkflowDeleted

Kubectl

Kubectl will be modified to display workflows. More particularly the describe command will display all the steps with their status. Steps will be topologically sorted and each dependency will be decorated with its status (wether or not step is waitin for dependency).

Future evolution

In the future we may want to extend Workflow with other kinds of resources, modifying WorkflowStep to support a more general template to create other resources. One of the major functionalities missing here is the ability to set a recurring Workflow (cron-like), similar to the ScheduledJob #11980 for Job. If the scheduled job is able to support various resources Workflow will benefit from the schedule functionality of Job.

Relevant use cases out of scope of this proposal

  • As an admin I want to set quota on workflow resources #13567.
  • As an admin I want to re-assign a workflow resource to another namespace/user2.
  • As a user I want to set an action when a workflow ends/start #3585

1Something about naming: literature is full of different names, a commonly used name is task, but since we plan to compose Workflows (i.e. a task can execute another whole Workflow) we have chosen the more generic word Step.

2A very common feature in industrial strength workflow tools.

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