samza-core/src/main/java/org/apache/samza/container/grouper/task/GroupByContainerCount.java - samza - Git at Google

 /*
  * Licensed to the Apache Software Foundation (ASF) under one
  * or more contributor license agreements.  See the NOTICE file
  * distributed with this work for additional information
  * regarding copyright ownership.  The ASF licenses this file
  * to you under the Apache License, Version 2.0 (the
  * "License"); you may not use this file except in compliance
  * with the License.  You may obtain a copy of the License at
  *
  *   http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing,
  * software distributed under the License is distributed on an
  * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
  * KIND, either express or implied.  See the License for the
  * specific language governing permissions and limitations
  * under the License.
  */
 package org.apache.samza.container.grouper.task;

 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;

 import org.apache.samza.SamzaException;
 import org.apache.samza.container.LocalityManager;
 import org.apache.samza.container.TaskName;
 import org.apache.samza.job.model.ContainerModel;
 import org.apache.samza.job.model.TaskModel;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;


 /**
  * Group the SSP taskNames by dividing the number of taskNames into the number
  * of containers (n) and assigning n taskNames to each container as returned by
  * iterating over the keys in the map of taskNames (whatever that ordering
  * happens to be). No consideration is given towards locality, even distribution
  * of aggregate SSPs within a container, even distribution of the number of
  * taskNames between containers, etc.
  *
  * TODO: SAMZA-1197 - need to modify balance to work with processorId strings
  */
 public class GroupByContainerCount implements BalancingTaskNameGrouper {
   private static final Logger log = LoggerFactory.getLogger(GroupByContainerCount.class);
   private final int containerCount;

   public GroupByContainerCount(int containerCount) {
     if (containerCount <= 0) throw new IllegalArgumentException("Must have at least one container");
     this.containerCount = containerCount;
   }

   @Override
   public Set<ContainerModel> group(Set<TaskModel> tasks) {

     validateTasks(tasks);

     // Sort tasks by taskName.
     List<TaskModel> sortedTasks = new ArrayList<>(tasks);
     Collections.sort(sortedTasks);

     // Map every task to a container in round-robin fashion.
     Map<TaskName, TaskModel>[] taskGroups = new Map[containerCount];
     for (int i = 0; i < containerCount; i++) {
       taskGroups[i] = new HashMap<>();
     }
     for (int i = 0; i < sortedTasks.size(); i++) {
       TaskModel tm = sortedTasks.get(i);
       taskGroups[i % containerCount].put(tm.getTaskName(), tm);
     }

     // Convert to a Set of ContainerModel
     Set<ContainerModel> containerModels = new HashSet<>();
     for (int i = 0; i < containerCount; i++) {
       containerModels.add(new ContainerModel(String.valueOf(i), i, taskGroups[i]));
     }

     return Collections.unmodifiableSet(containerModels);
   }

   @Override
   public Set<ContainerModel> balance(Set<TaskModel> tasks, LocalityManager localityManager) {

     validateTasks(tasks);

     if (localityManager == null) {
       log.info("Locality manager is null. Cannot read or write task assignments. Invoking grouper.");
       return group(tasks);
     }

     TaskAssignmentManager taskAssignmentManager = localityManager.getTaskAssignmentManager();
     List<TaskGroup> containers = getPreviousContainers(taskAssignmentManager, tasks.size());
     if (containers == null || containers.size() == 1 || containerCount == 1) {
       log.info("Balancing does not apply. Invoking grouper.");
       Set<ContainerModel> models = group(tasks);
       saveTaskAssignments(models, taskAssignmentManager);
       return models;
     }

     int prevContainerCount = containers.size();
     int containerDelta = containerCount - prevContainerCount;
     if (containerDelta == 0) {
       log.info("Container count has not changed. Reusing previous container models.");
       return buildContainerModels(tasks, containers);
     }
     log.info("Container count changed from {} to {}. Balancing tasks.", prevContainerCount, containerCount);

     // Calculate the expected task count per container
     int[] expectedTaskCountPerContainer = calculateTaskCountPerContainer(tasks.size(), prevContainerCount, containerCount);

     // Collect excess tasks from over-assigned containers
     List<String> taskNamesToReassign = new LinkedList<>();
     for (int i = 0; i < prevContainerCount; i++) {
       TaskGroup taskGroup = containers.get(i);
       while (taskGroup.size() > expectedTaskCountPerContainer[i]) {
         taskNamesToReassign.add(taskGroup.removeTask());
       }
     }

     // Assign tasks to the under-assigned containers
     if (containerDelta > 0) {
       List<TaskGroup> newContainers = createContainers(prevContainerCount, containerCount);
       containers.addAll(newContainers);
     } else {
       containers = containers.subList(0, containerCount);
     }
     assignTasksToContainers(expectedTaskCountPerContainer, taskNamesToReassign, containers);

     // Transform containers to containerModel
     Set<ContainerModel> models = buildContainerModels(tasks, containers);

     // Save the results
     saveTaskAssignments(models, taskAssignmentManager);

     return models;
   }

   /**
    * Reads the task-container mapping from the provided {@link TaskAssignmentManager} and returns a
    * list of TaskGroups, ordered ascending by containerId.
    *
    * @param taskAssignmentManager the {@link TaskAssignmentManager} that will be used to retrieve the previous mapping.
    * @param taskCount             the number of tasks, for validation against the persisted tasks.
    * @return                      a list of TaskGroups, ordered ascending by containerId or {@code null}
    *                              if the previous mapping doesn't exist or isn't usable.
    */
   private List<TaskGroup> getPreviousContainers(TaskAssignmentManager taskAssignmentManager, int taskCount) {
     Map<String, String> taskToContainerId = taskAssignmentManager.readTaskAssignment();
     taskToContainerId.values().forEach(id -> {
         try {
           int intId = Integer.parseInt(id);
         } catch (NumberFormatException nfe) {
           throw new SamzaException("GroupByContainerCount cannot handle non-integer processorIds!", nfe);
         }
       });
     if (taskToContainerId.isEmpty()) {
       log.info("No task assignment map was saved.");
       return null;
     } else if (taskCount != taskToContainerId.size()) {
       log.warn(
           "Current task count {} does not match saved task count {}. Stateful jobs may observe misalignment of keys!",
           taskCount, taskToContainerId.size());
       // If the tasks changed, then the partition-task grouping is also likely changed and we can't handle that
       // without a much more complicated mapping. Further, the partition count may have changed, which means
       // input message keys are likely reshuffled w.r.t. partitions, so the local state may not contain necessary
       // data associated with the incoming keys. Warn the user and default to grouper
       // In this scenario the tasks may have been reduced, so we need to delete all the existing messages
       taskAssignmentManager.deleteTaskContainerMappings(taskToContainerId.keySet());
       return null;
     }

     List<TaskGroup> containers;
     try {
       containers = getOrderedContainers(taskToContainerId);
     } catch (Exception e) {
       log.error("Exception while parsing task mapping", e);
       return null;
     }
     return containers;
   }

   /**
    * Saves the task assignments specified by containers using the provided TaskAssignementManager.
    *
    * @param containers            the set of containers from which the task assignments will be saved.
    * @param taskAssignmentManager the {@link TaskAssignmentManager} that will be used to save the mappings.
    */
   private void saveTaskAssignments(Set<ContainerModel> containers, TaskAssignmentManager taskAssignmentManager) {
     for (ContainerModel container : containers) {
       for (TaskName taskName : container.getTasks().keySet()) {
         taskAssignmentManager.writeTaskContainerMapping(taskName.getTaskName(), container.getProcessorId());
       }
     }
   }

   /**
    * Verifies the input tasks argument and throws {@link IllegalArgumentException} if it is invalid.
    *
    * @param tasks the tasks to validate.
    */
   private void validateTasks(Set<TaskModel> tasks) {
     if (tasks.size() <= 0)
       throw new IllegalArgumentException("No tasks found. Likely due to no input partitions. Can't run a job with no tasks.");

     if (tasks.size() < containerCount)
       throw new IllegalArgumentException(String.format(
           "Your container count (%s) is larger than your task count (%s). Can't have containers with nothing to do, so aborting.",
           containerCount,
           tasks.size()));
   }

   /**
    * Creates a list of empty {@link TaskGroup} instances for a range of container id's
    * from the start(inclusive) to end(exclusive) container id.
    *
    * @param startContainerId  the first container id for which a TaskGroup is needed.
    * @param endContainerId    the first container id AFTER the last TaskGroup that is needed.
    * @return                  a set of empty TaskGroup instances corresponding to the range
    *                          [startContainerId, endContainerId)
    */
   private List<TaskGroup> createContainers(int startContainerId, int endContainerId) {
     List<TaskGroup> containers = new ArrayList<>(endContainerId - startContainerId);
     for (int i = startContainerId; i < endContainerId; i++) {
       TaskGroup taskGroup = new TaskGroup(String.valueOf(i), new ArrayList<String>());
       containers.add(taskGroup);
     }
     return containers;
   }

   /**
    * Assigns tasks from the specified list to containers that have fewer containers than indicated
    * in taskCountPerContainer.
    *
    * @param taskCountPerContainer the expected number of tasks for each container.
    * @param taskNamesToAssign     the list of tasks to assign to the containers.
    * @param containers            the containers (as {@link TaskGroup}) to which the tasks will be assigned.
    */
   // TODO: Change logic from using int arrays to a Map<String, Integer> (id -> taskCount)
   private void assignTasksToContainers(int[] taskCountPerContainer, List<String> taskNamesToAssign,
       List<TaskGroup> containers) {
     for (TaskGroup taskGroup : containers) {
       for (int j = taskGroup.size(); j < taskCountPerContainer[Integer.valueOf(taskGroup.getContainerId())]; j++) {
         String taskName = taskNamesToAssign.remove(0);
         taskGroup.addTaskName(taskName);
         log.info("Assigned task {} to container {}", taskName, taskGroup.getContainerId());
       }
     }
   }

   /**
    * Calculates the expected number of tasks for each container. The count is generated for
    * max(oldContainerCount, newContainerCount) s.t. if the container count has decreased,
    * the excess containers will have a count == 0, indicating that any tasks assigned to
    * them should be reassigned.
    *
    * @param taskCount             the number of tasks to divide among the containers.
    * @param prevContainerCount    the previous number of containers.
    * @param currentContainerCount the current number of containers.
    * @return                      the expected number of tasks for each container.
    */
   private int[] calculateTaskCountPerContainer(int taskCount, int prevContainerCount, int currentContainerCount) {
     int[] newTaskCountPerContainer = new int[Math.max(currentContainerCount, prevContainerCount)];
     Arrays.fill(newTaskCountPerContainer, 0);

     for (int i = 0; i < currentContainerCount; i++) {
       newTaskCountPerContainer[i] = taskCount / currentContainerCount;
       if (taskCount % currentContainerCount > i) {
         newTaskCountPerContainer[i]++;
       }
     }
     return newTaskCountPerContainer;
   }

   /**
    * Translates the list of TaskGroup instances to a set of ContainerModel instances, using the
    * set of TaskModel instances.
    *
    * @param tasks             the TaskModels to assign to the ContainerModels.
    * @param containerTasks    the TaskGroups defining how the tasks should be grouped.
    * @return                  a mutable set of ContainerModels.
    */
   private Set<ContainerModel> buildContainerModels(Set<TaskModel> tasks, List<TaskGroup> containerTasks) {
     // Map task names to models
     Map<String, TaskModel> taskNameToModel = new HashMap<>();
     for (TaskModel model : tasks) {
       taskNameToModel.put(model.getTaskName().getTaskName(), model);
     }

     // Build container models
     Set<ContainerModel> containerModels = new HashSet<>();
     for (TaskGroup container : containerTasks) {
       Map<TaskName, TaskModel> containerTaskModels = new HashMap<>();
       for (String taskName : container.taskNames) {
         TaskModel model = taskNameToModel.get(taskName);
         containerTaskModels.put(model.getTaskName(), model);
       }
       containerModels.add(
           new ContainerModel(container.containerId, Integer.valueOf(container.containerId), containerTaskModels));
     }
     return Collections.unmodifiableSet(containerModels);
   }

   /**
    * Converts the task->containerId map to an ordered list of {@link TaskGroup} instances.
    *
    * @param taskToContainerId a map from each task name to the containerId to which it is assigned.
    * @return                  a list of TaskGroups ordered ascending by containerId.
    */
   private List<TaskGroup> getOrderedContainers(Map<String, String> taskToContainerId) {
     log.debug("Got task to container map: {}", taskToContainerId);

     // Group tasks by container Id
     HashMap<String, List<String>> containerIdToTaskNames = new HashMap<>();
     for (Map.Entry<String, String> entry : taskToContainerId.entrySet()) {
       String taskName = entry.getKey();
       String containerId = entry.getValue();
       List<String> taskNames = containerIdToTaskNames.get(containerId);
       if (taskNames == null) {
         taskNames = new ArrayList<>();
         containerIdToTaskNames.put(containerId, taskNames);
       }
       taskNames.add(taskName);
     }

     // Build container tasks
     List<TaskGroup> containerTasks = new ArrayList<>(containerIdToTaskNames.size());
     for (int i = 0; i < containerIdToTaskNames.size(); i++) {
       if (containerIdToTaskNames.get(String.valueOf(i)) == null) throw new IllegalStateException("Task mapping is missing container: " + i);
       containerTasks.add(new TaskGroup(String.valueOf(i), containerIdToTaskNames.get(String.valueOf(i))));
     }

     return containerTasks;
   }

   /**
    * A mutable group of tasks and an associated container id.
    *
    * Used as a temporary mutable container until the final ContainerModel is known.
    */
   private static class TaskGroup {
     private final List<String> taskNames = new LinkedList<>();
     private final String containerId;

     private TaskGroup(String containerId, List<String> taskNames) {
       this.containerId = containerId;
       Collections.sort(taskNames);        // For consistency because the taskNames came from a Map
       this.taskNames.addAll(taskNames);
     }

     public String getContainerId() {
       return containerId;
     }

     public void addTaskName(String taskName) {
       taskNames.add(taskName);
     }

     public String removeTask() {
       return taskNames.remove(taskNames.size() - 1);
     }

     public int size() {
       return taskNames.size();
     }
   }
 }
	/*
	* Licensed to the Apache Software Foundation (ASF) under one
	* or more contributor license agreements. See the NOTICE file
	* distributed with this work for additional information
	* regarding copyright ownership. The ASF licenses this file
	* to you under the Apache License, Version 2.0 (the
	* "License"); you may not use this file except in compliance
	* with the License. You may obtain a copy of the License at
	*
	* http://www.apache.org/licenses/LICENSE-2.0
	*
	* Unless required by applicable law or agreed to in writing,
	* software distributed under the License is distributed on an
	* "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
	* KIND, either express or implied. See the License for the
	* specific language governing permissions and limitations
	* under the License.
	*/
	package org.apache.samza.container.grouper.task;

	import java.util.ArrayList;
	import java.util.Arrays;
	import java.util.Collections;
	import java.util.HashMap;
	import java.util.HashSet;
	import java.util.LinkedList;
	import java.util.List;
	import java.util.Map;
	import java.util.Set;

	import org.apache.samza.SamzaException;
	import org.apache.samza.container.LocalityManager;
	import org.apache.samza.container.TaskName;
	import org.apache.samza.job.model.ContainerModel;
	import org.apache.samza.job.model.TaskModel;
	import org.slf4j.Logger;
	import org.slf4j.LoggerFactory;


	/**
	* Group the SSP taskNames by dividing the number of taskNames into the number
	* of containers (n) and assigning n taskNames to each container as returned by
	* iterating over the keys in the map of taskNames (whatever that ordering
	* happens to be). No consideration is given towards locality, even distribution
	* of aggregate SSPs within a container, even distribution of the number of
	* taskNames between containers, etc.
	*
	* TODO: SAMZA-1197 - need to modify balance to work with processorId strings
	*/
	public class GroupByContainerCount implements BalancingTaskNameGrouper {
	private static final Logger log = LoggerFactory.getLogger(GroupByContainerCount.class);
	private final int containerCount;

	public GroupByContainerCount(int containerCount) {
	if (containerCount <= 0) throw new IllegalArgumentException("Must have at least one container");
	this.containerCount = containerCount;
	}

	@Override
	public Set<ContainerModel> group(Set<TaskModel> tasks) {

	validateTasks(tasks);

	// Sort tasks by taskName.
	List<TaskModel> sortedTasks = new ArrayList<>(tasks);
	Collections.sort(sortedTasks);

	// Map every task to a container in round-robin fashion.
	Map<TaskName, TaskModel>[] taskGroups = new Map[containerCount];
	for (int i = 0; i < containerCount; i++) {
	taskGroups[i] = new HashMap<>();
	}
	for (int i = 0; i < sortedTasks.size(); i++) {
	TaskModel tm = sortedTasks.get(i);
	taskGroups[i % containerCount].put(tm.getTaskName(), tm);
	}

	// Convert to a Set of ContainerModel
	Set<ContainerModel> containerModels = new HashSet<>();
	for (int i = 0; i < containerCount; i++) {
	containerModels.add(new ContainerModel(String.valueOf(i), i, taskGroups[i]));
	}

	return Collections.unmodifiableSet(containerModels);
	}

	@Override
	public Set<ContainerModel> balance(Set<TaskModel> tasks, LocalityManager localityManager) {

	validateTasks(tasks);

	if (localityManager == null) {
	log.info("Locality manager is null. Cannot read or write task assignments. Invoking grouper.");
	return group(tasks);
	}

	TaskAssignmentManager taskAssignmentManager = localityManager.getTaskAssignmentManager();
	List<TaskGroup> containers = getPreviousContainers(taskAssignmentManager, tasks.size());
	if (containers == null \|\| containers.size() == 1 \|\| containerCount == 1) {
	log.info("Balancing does not apply. Invoking grouper.");
	Set<ContainerModel> models = group(tasks);
	saveTaskAssignments(models, taskAssignmentManager);
	return models;
	}

	int prevContainerCount = containers.size();
	int containerDelta = containerCount - prevContainerCount;
	if (containerDelta == 0) {
	log.info("Container count has not changed. Reusing previous container models.");
	return buildContainerModels(tasks, containers);
	}
	log.info("Container count changed from {} to {}. Balancing tasks.", prevContainerCount, containerCount);

	// Calculate the expected task count per container
	int[] expectedTaskCountPerContainer = calculateTaskCountPerContainer(tasks.size(), prevContainerCount, containerCount);

	// Collect excess tasks from over-assigned containers
	List<String> taskNamesToReassign = new LinkedList<>();
	for (int i = 0; i < prevContainerCount; i++) {
	TaskGroup taskGroup = containers.get(i);
	while (taskGroup.size() > expectedTaskCountPerContainer[i]) {
	taskNamesToReassign.add(taskGroup.removeTask());
	}
	}

	// Assign tasks to the under-assigned containers
	if (containerDelta > 0) {
	List<TaskGroup> newContainers = createContainers(prevContainerCount, containerCount);
	containers.addAll(newContainers);
	} else {
	containers = containers.subList(0, containerCount);
	}
	assignTasksToContainers(expectedTaskCountPerContainer, taskNamesToReassign, containers);

	// Transform containers to containerModel
	Set<ContainerModel> models = buildContainerModels(tasks, containers);

	// Save the results
	saveTaskAssignments(models, taskAssignmentManager);

	return models;
	}

	/**
	* Reads the task-container mapping from the provided {@link TaskAssignmentManager} and returns a
	* list of TaskGroups, ordered ascending by containerId.
	*
	* @param taskAssignmentManager the {@link TaskAssignmentManager} that will be used to retrieve the previous mapping.
	* @param taskCount the number of tasks, for validation against the persisted tasks.
	* @return a list of TaskGroups, ordered ascending by containerId or {@code null}
	* if the previous mapping doesn't exist or isn't usable.
	*/
	private List<TaskGroup> getPreviousContainers(TaskAssignmentManager taskAssignmentManager, int taskCount) {
	Map<String, String> taskToContainerId = taskAssignmentManager.readTaskAssignment();
	taskToContainerId.values().forEach(id -> {
	try {
	int intId = Integer.parseInt(id);
	} catch (NumberFormatException nfe) {
	throw new SamzaException("GroupByContainerCount cannot handle non-integer processorIds!", nfe);
	}
	});
	if (taskToContainerId.isEmpty()) {
	log.info("No task assignment map was saved.");
	return null;
	} else if (taskCount != taskToContainerId.size()) {
	log.warn(
	"Current task count {} does not match saved task count {}. Stateful jobs may observe misalignment of keys!",
	taskCount, taskToContainerId.size());
	// If the tasks changed, then the partition-task grouping is also likely changed and we can't handle that
	// without a much more complicated mapping. Further, the partition count may have changed, which means
	// input message keys are likely reshuffled w.r.t. partitions, so the local state may not contain necessary
	// data associated with the incoming keys. Warn the user and default to grouper
	// In this scenario the tasks may have been reduced, so we need to delete all the existing messages
	taskAssignmentManager.deleteTaskContainerMappings(taskToContainerId.keySet());
	return null;
	}

	List<TaskGroup> containers;
	try {
	containers = getOrderedContainers(taskToContainerId);
	} catch (Exception e) {
	log.error("Exception while parsing task mapping", e);
	return null;
	}
	return containers;
	}

	/**
	* Saves the task assignments specified by containers using the provided TaskAssignementManager.
	*
	* @param containers the set of containers from which the task assignments will be saved.
	* @param taskAssignmentManager the {@link TaskAssignmentManager} that will be used to save the mappings.
	*/
	private void saveTaskAssignments(Set<ContainerModel> containers, TaskAssignmentManager taskAssignmentManager) {
	for (ContainerModel container : containers) {
	for (TaskName taskName : container.getTasks().keySet()) {
	taskAssignmentManager.writeTaskContainerMapping(taskName.getTaskName(), container.getProcessorId());
	}
	}
	}

	/**
	* Verifies the input tasks argument and throws {@link IllegalArgumentException} if it is invalid.
	*
	* @param tasks the tasks to validate.
	*/
	private void validateTasks(Set<TaskModel> tasks) {
	if (tasks.size() <= 0)
	throw new IllegalArgumentException("No tasks found. Likely due to no input partitions. Can't run a job with no tasks.");

	if (tasks.size() < containerCount)
	throw new IllegalArgumentException(String.format(
	"Your container count (%s) is larger than your task count (%s). Can't have containers with nothing to do, so aborting.",
	containerCount,
	tasks.size()));
	}

	/**
	* Creates a list of empty {@link TaskGroup} instances for a range of container id's
	* from the start(inclusive) to end(exclusive) container id.
	*
	* @param startContainerId the first container id for which a TaskGroup is needed.
	* @param endContainerId the first container id AFTER the last TaskGroup that is needed.
	* @return a set of empty TaskGroup instances corresponding to the range
	* [startContainerId, endContainerId)
	*/
	private List<TaskGroup> createContainers(int startContainerId, int endContainerId) {
	List<TaskGroup> containers = new ArrayList<>(endContainerId - startContainerId);
	for (int i = startContainerId; i < endContainerId; i++) {
	TaskGroup taskGroup = new TaskGroup(String.valueOf(i), new ArrayList<String>());
	containers.add(taskGroup);
	}
	return containers;
	}

	/**
	* Assigns tasks from the specified list to containers that have fewer containers than indicated
	* in taskCountPerContainer.
	*
	* @param taskCountPerContainer the expected number of tasks for each container.
	* @param taskNamesToAssign the list of tasks to assign to the containers.
	* @param containers the containers (as {@link TaskGroup}) to which the tasks will be assigned.
	*/
	// TODO: Change logic from using int arrays to a Map<String, Integer> (id -> taskCount)
	private void assignTasksToContainers(int[] taskCountPerContainer, List<String> taskNamesToAssign,
	List<TaskGroup> containers) {
	for (TaskGroup taskGroup : containers) {
	for (int j = taskGroup.size(); j < taskCountPerContainer[Integer.valueOf(taskGroup.getContainerId())]; j++) {
	String taskName = taskNamesToAssign.remove(0);
	taskGroup.addTaskName(taskName);
	log.info("Assigned task {} to container {}", taskName, taskGroup.getContainerId());
	}
	}
	}

	/**
	* Calculates the expected number of tasks for each container. The count is generated for
	* max(oldContainerCount, newContainerCount) s.t. if the container count has decreased,
	* the excess containers will have a count == 0, indicating that any tasks assigned to
	* them should be reassigned.
	*
	* @param taskCount the number of tasks to divide among the containers.
	* @param prevContainerCount the previous number of containers.
	* @param currentContainerCount the current number of containers.
	* @return the expected number of tasks for each container.
	*/
	private int[] calculateTaskCountPerContainer(int taskCount, int prevContainerCount, int currentContainerCount) {
	int[] newTaskCountPerContainer = new int[Math.max(currentContainerCount, prevContainerCount)];
	Arrays.fill(newTaskCountPerContainer, 0);

	for (int i = 0; i < currentContainerCount; i++) {
	newTaskCountPerContainer[i] = taskCount / currentContainerCount;
	if (taskCount % currentContainerCount > i) {
	newTaskCountPerContainer[i]++;
	}
	}
	return newTaskCountPerContainer;
	}

	/**
	* Translates the list of TaskGroup instances to a set of ContainerModel instances, using the
	* set of TaskModel instances.
	*
	* @param tasks the TaskModels to assign to the ContainerModels.
	* @param containerTasks the TaskGroups defining how the tasks should be grouped.
	* @return a mutable set of ContainerModels.
	*/
	private Set<ContainerModel> buildContainerModels(Set<TaskModel> tasks, List<TaskGroup> containerTasks) {
	// Map task names to models
	Map<String, TaskModel> taskNameToModel = new HashMap<>();
	for (TaskModel model : tasks) {
	taskNameToModel.put(model.getTaskName().getTaskName(), model);
	}

	// Build container models
	Set<ContainerModel> containerModels = new HashSet<>();
	for (TaskGroup container : containerTasks) {
	Map<TaskName, TaskModel> containerTaskModels = new HashMap<>();
	for (String taskName : container.taskNames) {
	TaskModel model = taskNameToModel.get(taskName);
	containerTaskModels.put(model.getTaskName(), model);
	}
	containerModels.add(
	new ContainerModel(container.containerId, Integer.valueOf(container.containerId), containerTaskModels));
	}
	return Collections.unmodifiableSet(containerModels);
	}

	/**
	* Converts the task->containerId map to an ordered list of {@link TaskGroup} instances.
	*
	* @param taskToContainerId a map from each task name to the containerId to which it is assigned.
	* @return a list of TaskGroups ordered ascending by containerId.
	*/
	private List<TaskGroup> getOrderedContainers(Map<String, String> taskToContainerId) {
	log.debug("Got task to container map: {}", taskToContainerId);

	// Group tasks by container Id
	HashMap<String, List<String>> containerIdToTaskNames = new HashMap<>();
	for (Map.Entry<String, String> entry : taskToContainerId.entrySet()) {
	String taskName = entry.getKey();
	String containerId = entry.getValue();
	List<String> taskNames = containerIdToTaskNames.get(containerId);
	if (taskNames == null) {
	taskNames = new ArrayList<>();
	containerIdToTaskNames.put(containerId, taskNames);
	}
	taskNames.add(taskName);
	}

	// Build container tasks
	List<TaskGroup> containerTasks = new ArrayList<>(containerIdToTaskNames.size());
	for (int i = 0; i < containerIdToTaskNames.size(); i++) {
	if (containerIdToTaskNames.get(String.valueOf(i)) == null) throw new IllegalStateException("Task mapping is missing container: " + i);
	containerTasks.add(new TaskGroup(String.valueOf(i), containerIdToTaskNames.get(String.valueOf(i))));
	}

	return containerTasks;
	}

	/**
	* A mutable group of tasks and an associated container id.
	*
	* Used as a temporary mutable container until the final ContainerModel is known.
	*/
	private static class TaskGroup {
	private final List<String> taskNames = new LinkedList<>();
	private final String containerId;

	private TaskGroup(String containerId, List<String> taskNames) {
	this.containerId = containerId;
	Collections.sort(taskNames); // For consistency because the taskNames came from a Map
	this.taskNames.addAll(taskNames);
	}

	public String getContainerId() {
	return containerId;
	}

	public void addTaskName(String taskName) {
	taskNames.add(taskName);
	}

	public String removeTask() {
	return taskNames.remove(taskNames.size() - 1);
	}

	public int size() {
	return taskNames.size();
	}
	}
	}