public List<UpgradeGroupHolder> createSequence(UpgradePack upgradePack,
      UpgradeContext context) throws AmbariException {

    Cluster cluster = context.getCluster();
    MasterHostResolver mhr = context.getResolver();
    Map<String, AddComponentTask> addedComponentsDuringUpgrade = upgradePack.getAddComponentTasks();

    // Note, only a Rolling Upgrade uses processing tasks.
    Map<String, Map<String, ProcessingComponent>> allTasks = upgradePack.getTasks();
    List<UpgradeGroupHolder> groups = new ArrayList<>();

    UpgradeGroupHolder previousGroupHolder = null;
    for (Grouping group : upgradePack.getGroups(context.getDirection())) {

      // !!! grouping is not scoped to context
      if (!context.isScoped(group.scope)) {
        continue;
      }

      // if there is a condition on the group, evaluate it and skip scheduling
      // of this group if the condition has not been satisfied
      if (null != group.condition && !group.condition.isSatisfied(context)) {
        LOG.info("Skipping {} while building upgrade orchestration due to {}", group, group.condition );
        continue;
      }

      UpgradeGroupHolder groupHolder = new UpgradeGroupHolder();
      groupHolder.name = group.name;
      groupHolder.title = group.title;
      groupHolder.groupClass = group.getClass();
      groupHolder.skippable = group.skippable;
      groupHolder.supportsAutoSkipOnFailure = group.supportsAutoSkipOnFailure;
      groupHolder.allowRetry = group.allowRetry;
      groupHolder.processingGroup = group.isProcessingGroup();

      // !!! all downgrades are skippable
      if (context.getDirection().isDowngrade()) {
        groupHolder.skippable = true;
      }

      // Attempt to get the function of the group, during a NonRolling Upgrade
      Task.Type functionName = null;
      if (group instanceof UpgradeFunction) {
        functionName = ((UpgradeFunction) group).getFunction();
      }

      // NonRolling defaults to not performing service checks on a group.
      // Of course, a Service Check Group does indeed run them.
      if (upgradePack.getType() == UpgradeType.NON_ROLLING) {
        group.performServiceCheck = false;
      }

      StageWrapperBuilder builder = group.getBuilder();

      List<UpgradePack.OrderService> services = group.services;

      // Rolling Downgrade must reverse the order of services.
      if (upgradePack.getType() == UpgradeType.ROLLING) {
        if (context.getDirection().isDowngrade() && !services.isEmpty()) {
          List<UpgradePack.OrderService> reverse = new ArrayList<>(services);
          Collections.reverse(reverse);
          services = reverse;
        }
      }

      // !!! cluster and service checks are empty here
      for (UpgradePack.OrderService service : services) {

        if (!context.isServiceSupported(service.serviceName)) {
          continue;
        }

        if (upgradePack.getType() == UpgradeType.ROLLING && !allTasks.containsKey(service.serviceName)) {
          continue;
        }

        for (String component : service.components) {
          // Rolling Upgrade has exactly one task for a Component.
          // NonRolling Upgrade has several tasks for the same component, since it must first call Stop, perform several
          // other tasks, and then Start on that Component.
          if (upgradePack.getType() == UpgradeType.ROLLING && !allTasks.get(service.serviceName).containsKey(component)) {
            continue;
          }

          // if a function name is present, build the tasks dynamically;
          // otherwise use the tasks defined in the upgrade pack processing
          ProcessingComponent pc = null;
          if (null == functionName) {
            pc = allTasks.get(service.serviceName).get(component);
          } else {
            // Construct a processing task on-the-fly if it is a "stop" group.
            if (functionName == Type.STOP) {
              pc = new ProcessingComponent();
              pc.name = component;
              pc.tasks = new ArrayList<>();
              pc.tasks.add(new StopTask());
            } else {
              // For Start and Restart, make a best attempt at finding
              // Processing Components.
              // If they don't exist, make one on the fly.
              if (allTasks.containsKey(service.serviceName)
                  && allTasks.get(service.serviceName).containsKey(component)) {
                pc = allTasks.get(service.serviceName).get(component);
              } else {
                // Construct a processing task on-the-fly so that the Upgrade
                // Pack is less verbose.
                pc = new ProcessingComponent();
                pc.name = component;
                pc.tasks = new ArrayList<>();

                if (functionName == Type.START) {
                  pc.tasks.add(new StartTask());
                }
                if (functionName == Type.RESTART) {
                  pc.tasks.add(new RestartTask());
                }
              }
            }
          }

          if (pc == null) {
            LOG.error(MessageFormat.format("Couldn't create a processing component for service {0} and component {1}.", service.serviceName, component));
            continue;
          }

          HostsType hostsType = mhr.getMasterAndHosts(service.serviceName, component);

          // only worry about adding future commands if this is a start/restart task
          boolean taskIsRestartOrStart = functionName == null || functionName == Type.START
              || functionName == Type.RESTART;

          // see if this component has an add component task which will indicate
          // we need to dynamically schedule some more tasks by predicting where
          // the components will be installed
          String serviceAndComponentHash = service.serviceName + "/" + component;
          if (taskIsRestartOrStart && addedComponentsDuringUpgrade.containsKey(serviceAndComponentHash)) {
            AddComponentTask task = addedComponentsDuringUpgrade.get(serviceAndComponentHash);

            Collection<Host> candidateHosts = MasterHostResolver.getCandidateHosts(cluster,
                task.hosts, task.hostService, task.hostComponent);

            // if we have candidate hosts, then we can create a structure to be
            // scheduled
            if (!candidateHosts.isEmpty()) {
              if (null == hostsType) {
                // a null hosts type usually means that the component is not
                // installed in the cluster - but it's possible that it's going
                // to be added as part of the upgrade. If this is the case, then
                // we need to schedule tasks assuming the add works
                hostsType = HostsType.normal(
                    candidateHosts.stream().map(host -> host.getHostName()).collect(
                        Collectors.toCollection(LinkedHashSet::new)));
              } else {
                // it's possible that we're adding components that may already
                // exist in the cluster - in this case, we must append the
                // structure instead of creating a new one
                Set<String> hostsForTask = hostsType.getHosts();
                for (Host host : candidateHosts) {
                  hostsForTask.add(host.getHostName());
                }
              }
            }
          }

          // if we still have no hosts, then truly skip this component
          if (null == hostsType) {
            continue;
          }

          if (!hostsType.unhealthy.isEmpty()) {
            context.addUnhealthy(hostsType.unhealthy);
          }

          Service svc = cluster.getService(service.serviceName);

          setDisplayNames(context, service.serviceName, component);

          // Special case for NAMENODE when there are multiple
          if (service.serviceName.equalsIgnoreCase("HDFS") && component.equalsIgnoreCase("NAMENODE")) {

            // Rolling Upgrade requires first upgrading the Standby, then the Active NameNode.
            // Whereas NonRolling needs to do the following:
            //   NameNode HA:  Pick one to the be active, and the other the standby.
            //   Non-NameNode HA: Upgrade first the SECONDARY, then the primary NAMENODE
            switch (upgradePack.getType()) {
              case ROLLING:
                if (!hostsType.getHosts().isEmpty() && hostsType.hasMastersAndSecondaries()) {
                  // The order is important, first do the standby, then the active namenode.
                  hostsType.arrangeHostSecondariesFirst();
                  builder.add(context, hostsType, service.serviceName,
                      svc.isClientOnlyService(), pc, null);
                } else {
                  LOG.warn("Could not orchestrate NameNode.  Hosts could not be resolved: hosts={}, active={}, standby={}",
                      StringUtils.join(hostsType.getHosts(), ','), hostsType.getMasters(), hostsType.getSecondaries());
                }
                break;
              case NON_ROLLING:
                boolean isNameNodeHA = mhr.isNameNodeHA();
                if (isNameNodeHA && hostsType.hasMastersAndSecondaries()) {
                  // This could be any order, but the NameNodes have to know what role they are going to take.
                  // So need to make 2 stages, and add different parameters to each one.
                  builder.add(context, HostsType.normal(hostsType.getMasters()), service.serviceName,
                      svc.isClientOnlyService(), pc, nameNodeRole("active"));

                  builder.add(context, HostsType.normal(hostsType.getSecondaries()), service.serviceName,
                      svc.isClientOnlyService(), pc, nameNodeRole("standby"));
                } else {
                  // If no NameNode HA, then don't need to change hostsType.hosts since there should be exactly one.
                  builder.add(context, hostsType, service.serviceName,
                      svc.isClientOnlyService(), pc, null);
                }

                break;
            }
          } else {
            builder.add(context, hostsType, service.serviceName,
                svc.isClientOnlyService(), pc, null);
          }
        }
      }

      List<StageWrapper> proxies = builder.build(context);

      if (CollectionUtils.isNotEmpty(proxies)) {

        groupHolder.items = proxies;
        postProcess(context, groupHolder);

        // !!! prevent service checks from running twice.  merge the stage wrappers
        if (ServiceCheckGrouping.class.isInstance(group)) {
          if (null != previousGroupHolder && ServiceCheckGrouping.class.equals(previousGroupHolder.groupClass)) {
            mergeServiceChecks(groupHolder, previousGroupHolder);
          } else {
            groups.add(groupHolder);
          }
        } else {
          groups.add(groupHolder);
        }

        previousGroupHolder = groupHolder;
      }
    }

    if (LOG.isDebugEnabled()) {
      for (UpgradeGroupHolder group : groups) {
        LOG.debug(group.name);

        int i = 0;
        for (StageWrapper proxy : group.items) {
          LOG.debug("  Stage {}", Integer.valueOf(i++));
          int j = 0;

          for (TaskWrapper task : proxy.getTasks()) {
            LOG.debug("    Task {} {}", Integer.valueOf(j++), task);
          }
        }
      }
    }

    // !!! strip off the first service check if nothing has been processed
    Iterator<UpgradeGroupHolder> iterator = groups.iterator();
    boolean canServiceCheck = false;
    while (iterator.hasNext()) {
      UpgradeGroupHolder holder = iterator.next();

      if (ServiceCheckGrouping.class.equals(holder.groupClass) && !canServiceCheck) {
        iterator.remove();
      }

      canServiceCheck |= holder.processingGroup;
    }

    return groups;
  }
Generates a list of UpgradeGroupHolder items that are used to execute either
an upgrade or a downgrade.

@param upgradePack
         the upgrade pack
@param context
         the context that wraps key fields required to perform an upgrade
@return the list of holders
ambari-server/src/main/java/org/apache/ambari/server/stack/upgrade/orchestrate/UpgradeHelper.java