The ‘concurrence’ expression applies its child branches in parallel (well it makes a best effort to make them run in parallel).

  concurrence do
    alpha
    bravo
  end

attributes

The concurrence expression takes a number of attributes that allow for sophisticated control (especially at merge time).

:count

  concurrence :count => 1 do
    alpha
    bravo
  end

in that example, the concurrence will terminate as soon as 1 (count) of the branches replies. The other branch will get cancelled.

:remaining

As said for :count, the remaining branches get cancelled. By setting :remaining to :forget (or ‘forget’), the remaining branches will continue their execution, forgotten.

  concurrence :count => 1, :remaining => :forget do
    alpha
    bravo
  end

:merge

By default, the workitems override each others. By default, the first workitem to reply will win.

  sequence do
    concurrence do
      alpha
      bravo
    end
    charly
  end

In that example, if ‘alpha’ replied first, the workitem that reaches ‘charly’ once ‘bravo’ replied will have the payload as seen/modified by ‘alpha’.

The :merge attribute determines which branch wins the merge.

• first (default)
• last
• highest
• lowest

highest and lowest refer to the position in the list of branch. It‘s useful to set a fixed winner.

  concurrence :merge => :highest do
    alpha
    bravo
  end

makes sure that alpha‘s version of the workitem wins.

:merge_type

:override

By default, the merge type is set to ‘override’, which means that the ‘winning’ workitem‘s payload supplants all other workitems’ payloads.

:mix

Setting :merge_type to :mix, will actually attempt to merge field by field, making sure that the field value of the winner(s) are used.

:isolate

:isolate will rearrange the resulting workitem payload so that there is a new field for each branch. The name of each field is the index of the branch from ‘0’ to …

:stack

:stack will stack the workitems coming back from the concurrence branches in an array whose order is determined by the :merge attributes. The array is placed in the ‘stack’ field of the resulting workitem. Note that the :stack merge_type also creates a ‘stack_attributes’ field and populates it with the expanded attributes of the concurrence.

Thus

  sequence do
    concurrence :merge => :highest, :merge_type => :stack do
      reviewer1
      reviewer2
    end
    editor
  end

will see the ‘editor’ receive a workitem whose fields look like :

  { 'stack' => [{ ... reviewer1 fields ... }, { ... reviewer2 fields ... }],
    'stack_attributes' => { 'merge'=> 'highest', 'merge_type' => 'stack' } }

This could prove useful for participant having to deal with multiple merge strategy results.

:over_if (and :over_unless)

Like the :count attribute controls how many branches have to reply before a concurrence ends, the :over attribute is used to specify a condition upon which the concurrence will [prematurely] end.

  concurrence :over_if => '${f:over}'
    alpha
    bravo
    charly
  end

will end the concurrence as soon as one of the branches replies with a workitem whose field ‘over’ is set to true. (the remaining branches will get cancelled unless :remaining => :forget is set).

:over_unless needs no explanation.