1. A computer-implemented method of processing non-comparable items associated with a workflow, comprising: accessing, by one or more computers, partial ordering data that describes, for each workflow item, one or more primary input items that is required under normal conditions to produce the workflow item; accessing, by the one or more computers, extension data comprising one or more alternative input items to replace at least one of the one or more primary input items when there is insufficient quantity of the at least one of the one or more primary input items to meet a demand in producing the workflow item, the extension data specifies one or more extensions to the partial ordering data; creating, by the one or more computers, for each of the workflow items, a first list comprising the one or more primary input items that are used to produce the workflow item according to the partial ordering data, wherein the one or more primary input items are not added to the list when the one or more primary input items are the same as the workflow item; creating, by the one or more computers, for each of the workflow items, a second list comprising the one or more alternative input items that are used directly to produce the workflow item according to the extension data, wherein the one or more alternative input items are not added to the list when the one or more alternative input items are the same as the workflow item; performing, by the one or more computers, a downstream transitive closure process, for each of the workflow items, by copying any items in the first list of the one or more primary input items of the workflow item to the first list of the workflow item and copying any items in the first list of the one or more primary input items of the one or more primary input items to the first list of the workflow item, wherein an item is not copied to the first list of the workflow item if the item is the same as the workflow item; performing, by the one or more computers, an extension process, for each of the workflow items, by: copying all items in the second list of the workflow item to a third list of the workflow item; emptying all items from the second list of the workflow item; for each item in the first list of the workflow item, add all items from the second list of each item in the first list of the workflow item to the third list of the workflow item; and for each third list workflow item, adding the third list workflow item and any items in a first list of the third list workflow item to the second list of the workflow item, when the item is not the same as the workflow item and the workflow item is not in the first list of the third list workflow item, adding each item from a second list of the third list workflow item to the third list of the workflow item, and, for each item in the first list of the third list workflow item, adding any items in a second list of any items in the first list of the third list workflow item to the third list of the workflow item; calculating, by the one or more computers, for each of the workflow items, a sum of a quantity of the items in the first list of the workflow item and a quantity of the items in the second list of the workflow item; generating, by the one or more computers, an ordering of the calculated workflow items based on the sum of the quantity of the items in the first list of the workflow item and the quantity of the items in the second list of the workflow item such that: (a) each workflow item that has one or more primary input items is placed ahead of its primary input items; and (b) each primary input item that has one or more alternative input items is placed ahead of its alternative input items provided doing so is not inconsistent with the ordering of items ordered according to condition (a); and when at least a first item and a second item are non-comparable with respect to the ordering of the calculated workflow items, processing the at least the first and second non-comparable items simultaneously using a different processor of the one or more computers.