Status

Current state: "Under Discussion"

Discussion thread:

JIRA:  Unable to render Jira issues macro, execution error.

Released:

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

In order to support the reactive mode (FLIP-159) we need a different type of scheduler which first announces the required resources and only after having received the resources decides on the actual parallelism with which to execute the job. This has the benefit that this scheduler can schedule jobs if not all required resources are fulfilled. Moreover, it allows to continue executing jobs even after TaskManagers have been lost. The declarative scheduler builds upon the declarative resource management (FLIP-138).

Proposed Changes

The declarative scheduler will first work for streaming jobs only. This will simplify things considerably because we always have to schedule all operators. Moreover, by treating every failure as a global failover which restarts the whole topology, we can further simplify the scheduler. This failover behaviour is the default for many streaming topologies anyways if they don't consist of disjunct graphs. Given these assumptions we want to develop the following scheduler:

The scheduler takes the JobGraph for which it will first calculate the desired resources. After declaring these resources, the scheduler will wait until the available resources have stabilised. Once the resources are stabilised the scheduler should be able to decide on the actual parallelism of the job. Once the parallelism is decided and the executions are matched with the available slots, the scheduler deploys the executions.

Whenever a fault occurs, we will fail the whole job and try to restart it. Restarting works by cancelling all deployed tasks and then restarting the scheduling of the JobGraph following the same code paths as the initial scheduling operation.

An obvious regression of this implementation over the existing pipelined region scheduler is that we are always restarting the whole topology. For embarrassingly parallel jobs this might not be necessary since the running tasks don’t need to be reset to the latest checkpoint. Supporting partial failover would be the first extension of the proposed scheduler. One way to support partial failovers is to introduce a distinction between global and local failovers.

• Global failover: Restart of the whole topology which allows to change the parallelism of the job
• Local failover: Restart of a subset of the executions which does not change the parallelism of the operator

If the system cannot recover from a local failover because it does not have enough slots available, it must be escalated which makes it a global failover. A global failover will allow the system to rescale the whole job.

State machine of the scheduler

Given the description above we propose the following state machine to model the behaviour of the declarative scheduler:

The states have the following semantics:

• Created: Initial state of the scheduler
• Waiting for resources: The required resources are declared. The scheduler waits until either the requirements are fulfilled or the set of resources has stabilised.
• Executing: The set of resources is stable and the scheduler could decide on the parallelism with which to execute the job. The ExecutionGraph is created and the execution of the job has started.
• Restarting: A recoverable fault has occurred. The scheduler stops the ExecutionGraph by canceling it.
• Canceling: The job has been canceled by the user. The scheduler stops the ExecutionGraph by canceling it.
• Failing: An unrecoverable fault has occurred. The scheduler stops the ExecutionGraph by canceling it.
• Finished: The job execution has been completed.

In the states “Created” and “Waiting for resources” there does not exist an ExecutionGraph. Only after we have acquired enough resources to run the job, the ExecutionGraph can be instantiated. Hence, all operations which require the ExecutionGraph will be ignored until we are in a state where an ExecutionGraph exists.

Since we have a couple of asynchronous operations (resource timeout in "Waiting for resources" state, restart delay in Restarting) which only work if no other state change has happened, we need to introduce a state version which can be used to filter out outdated operations.

Stable set of resources

The "Waiting for resources" state has the purpose to wait for the required resources. Since the cluster might not be able to provide all of the declared resources, the system needs to handle this situation as well. Hence, this state waits until either all required resources have arrived or until the set of available resources has stabilised. A set of resources has stabilised if the system expects that it won't change anymore. One possible solution approach sets an upper limit for the waiting time. This is also the approach we want to implement in the first version of the scheduler. Consequently, whenever the scheduler enters the "Waiting for resources" state, it registers a timeout after which it will try to go into the Executing state. If the job cannot be executed with the available resources, then the scheduler will fail it.

In the future we might take a look at Kafka's consumer protocol and how consumer changes are handled there and how to decide on a stable set of consumers/resources.

Automatic scaling

In order to support automatic scaling, we ask a ScaleUpController whenever new slots arrive and the scheduler is in state Executing whether the job can be scaled up. If this is the case, then the scheduler transitions into the Restarting state which triggers a global failover and a restart which will make use of the available resources. It is important to note that scale down actions will be triggered by failures of tasks whose slots have been removed.

Components of the scheduler

The scheduler consists of the following services to accomplish its job. These services are used by the different states to decide on state transitions and to perform certain operations

SlotAllocator

The SlotAllocator is the component responsible for determining the resource requirements and mapping a JobGraph and its contained vertices to slots.

This consists of 2 parts:

1. Calculating the resources required for scheduling a JobGraph / set of vertices
2. Calculating a mapping of vertices to be scheduled to free slots, and optionally rescaling vertices.

The interface will look like this:

/** Component for calculating the slot requirements and mapping of vertices to slots. */
public interface SlotAllocator<T extends VertexAssignment> {

    /**
     * Calculates the total resources required for scheduling the given vertices.
     *
     * @param vertices vertices to schedule
     * @return required resources
     */
    ResourceCounter calculateRequiredSlots(Iterable<JobInformation.VertexInformation> vertices);

    /**
     * Determines the parallelism at which the vertices could run given the collection of slots.
     *
     * @param jobInformation information about the job graph
     * @param slots Slots to consider for determining the parallelism
     * @return parallelism of each vertex along with implementation specific information, if the job
     *     could be run with the given slots
     */
    Optional<T> determineParallelism(
            JobInformation jobInformation, Collection<? extends SlotInfo> slots);

    /**
     * Assigns vertices to the given slots.
     *
     * @param jobInformation information about the job graph
     * @param freeSlots currently free slots
     * @param assignment information on how slots should be assigned to the slots
     * @return parallelism of each vertex and mapping slots to vertices
     */
    ParallelismAndResourceAssignments assignResources(
            JobInformation jobInformation,
            Collection<SlotInfoWithUtilization> freeSlots,
            T assignment);
}

/** Base container for assignments of vertices to slots. */
public interface VertexAssignment {
    Map<JobVertexID, Integer> getMaxParallelismForVertices();
}

/** Information about the job. */
public interface JobInformation {
    Collection<SlotSharingGroup> getSlotSharingGroups();

    VertexInformation getVertexInformation(JobVertexID jobVertexId);

    /** Information about a single vertex. */
    interface VertexInformation {
        JobVertexID getJobVertexID();

        int getParallelism();

        SlotSharingGroup getSlotSharingGroup();
    }
}

/** Assignment of slots to execution vertices. */
public final class ParallelismAndResourceAssignments {
    private final Map<ExecutionVertexID, ? extends LogicalSlot> assignedSlots;

    private final Map<JobVertexID, Integer> parallelismPerJobVertex;
}

• calculateDesiredSlots returns a ResourceCounter that describes the ideal amount of resources for the job.
• determineParallelism accepts a JobInformation and a collection of free slots, and attempts to find the optimal parallelism for every operator given the available set of slots. If no such mapping could be found an empty Optional is returned to signal the Scheduler that this topology cannot be scheduled at this time. This method may be called by the scheduler irrespective of whether it has received the desired slots.
• assignResources assigns the available resources to the ExecutionVertices and reserves the slots according to the provided assignments parameter.

The first implementation of the SlotAllocator interface will support slot sharing w/o respecting previous allocations and input preferences. The SlotAllocator implementation will respect the configured parallelism and never decide on a parallelism which exceeds the configured maxParallelism of an operator.

FailureHandler

In order to handle failures, the declarative scheduler will support the same RestartBackoffTimeStrategy as used by the pipelined region scheduler. Hence all currently RestartBackoffTimeStrategies will be supported. The failure handling procedure is the following:

1. Check whether the failure is recoverable. If not, then go to Failing state
2. Ask the configured RestartBackoffTimeStrategy whether we can restart. If not, then go to Failing state
3. Ask the configured RestartBackoffTimeStrategy for the backoff time between failure and restart
4. Go into the Restarting state with the returned backoff time

ScaleUpController

Whenever the scheduler is in the Executing state and receives new slots, the scheduler checks whether the job can be run with an increased parallelism. If this is the case, then the scheduler will ask the ScaleUpController given the old and new cumulative parallelism of all operators whether it should scale up or not.

/**
 * Simple controller for controlling the scale up behavior of the {@link DeclarativeScheduler}.
 */
public interface ScaleUpController {

    /**
     * This method gets called whenever new resources are available to the scheduler to scale up.
     *
     * @param currentCumulativeParallelism Cumulative parallelism of the currently running job
     *     graph.
     * @param newCumulativeParallelism Potential new cumulative parallelism with the additional
     *     resources.
     * @return true if the policy decided to scale up based on the provided information.
     */
    boolean canScaleUp(int currentCumulativeParallelism, int newCumulativeParallelism);
}

A basic default implementation will only scale up if newCumulativeParallelism - currentCumulativeParallelism >= increaseThreshold.

How to distinguish streaming jobs

Since we can not execute batch jobs with the declarative scheduler, we need to be able to detect whether a job is a batch or a streaming job. For this purpose, we are introducing a new enum field in the JobGraph, called JobType. The default JobType of a JobGraph will be BATCH.

For batch jobs (from the DataSet API), setting this field is trivial (in the JobGraphGenerator).

For streaming jobs the situation is more complicated, since FLIP-134 introduced support for bounded (batch) jobs in the DataStream API. For the DataStream API, we rely on the result of StreamGraphGenerator#shouldExecuteInBatchMode, which checks if the DataStream program has unbounded sources.

Lastly, the Blink Table API / SQL Planner also generates StreamGraph instances, which contain batch jobs. We are tagging the StreamGraph as a batch job in the ExecutorUtils.setBatchProperties() method.

If we detect that the declarative scheduler has been configured for a batch job, we will fall back to another scheduler supporting batch jobs (currently the pipelined region scheduler).

Compatibility, Deprecation, and Migration Plan

The declarative scheduler will be a beta feature which the user has to activate explicitly by setting the config option jobmanager.scheduler: declarative. It will only be chosen if the user submitted a streaming job.

Limitations & future improvements

The first version of the declarative scheduler will come with a handful of limitations in order to reduce the scope of it.

Streaming jobs only

The declarative scheduler runs with streaming jobs only. When submitting a batch job, then the default scheduler will be used.

No support for local recovery

In the first version of the scheduler we don't intend to support local recovery. Adding support for it should be possible and we intend to add support for it as a follow up.

No support for local failovers

Supporting local failovers is another feature which we want to add as a follow up. Adding support for it allows to not having to restart the whole job. One idea could be to extend the existing state machine by a new state "Restarting locally":

No integration with Flink's web UI

The declarative scheduler allows that a job's parallelism can change over its lifetime. This means that we have to extend the web UI to be able to display different forms of a job. One idea would be to have a timeline which allows to pick a time for which the web UI displays the current job. This will require changes on the backend as well as frontend side.

No support for fine grained resource specifications

For the sake of simplicity and narrowing down the scope, the declarative scheduler will ignore any resource specifications. In the future when having different resource profiles to fulfil, it will be the task of the ResourceManager to make sure that different resource requirements are fulfilled equally well.

Non-zero downtime rescaling

Rescaling happens through restarting the job, thus jobs with large state might need a lot of resources and time to rescale. Rescaling a job causes downtime of your job, but no data loss.

Test Plan

The new scheduler needs extensive unit, IT and end-to-end testing because it is a crucial component which is at the heart of Flink.

Rejected Alternatives

We also tried to find a design for a declarative scheduler which supports batch and streaming jobs at the same time. This design has turned out to be a bit too complex and therefore we rejected it. The details for this design can be found here.