Think of the history of a document as a series of changes. In Google documents, all edits boil down to three basic types of changes: inserting text, deleting text, and applying styles to a range of text. We save your document as a revision log consisting of a list of these changes. When someone edits a document, they’re not modifying the underlying characters that represents the document. Instead they are appending their change to the end of the revision log. To display a document, we replay the revision log from the beginning.
To see what these changes look like, suppose that a document edited by John and Luiz initially reads; EASY AS 123. If John (represented by green) changes the document to EASY AS ABC, then he is making four changes:
Collaboration is not quite as simple as sending these changes to the other editors because people get out of sync. Suppose as John is typing, Luiz (represented by yellow) begins to change his document to IT'S EASY AS 123. He first inserts the I and the T at the beginning of the document:
Suppose Luiz naively applies John’s first change {DeleteText @9-11}:
He deleted the wrong characters! Luiz had two characters at the beginning of the doc that John was never aware of. So the location of John’s change was wrong relative to Luiz’s version of the document. To avoid this problem, Luiz must transform John’s changes and make them relative to his local document. In this case, when Luiz receives changes from John he needs to know to shift the changes over by two characters to adjust for the IT that Luiz added. Once he does this transformation and applies John’s first change, he gets:
Much better. The algorithm that we use to handle these shifts is called operational transformation (OT). If OT is implemented correctly, it guarantees that once all editors have received all changes, everyone will be looking at the same version of the document.
The OT logic in documents must handle all of the different ways that InsertText, DeleteText, and ApplyStyle changes can be paired and transformed against each other. The example above showed DeleteText being transformed against InsertText. To get a feel for how this works, here are a couple more examples of simple transformations:
- Style ranges expand when they are transformed against text insertions: {ApplyStyle bold @10-20} transformed against {InsertText 'ABC' @15} results in {ApplyStyle Bold @10-23}.
- Sometimes changes don’t conflict and there’s no need to transform anything. For example when a style change is transformed against a different type of style change, there is no conflict: {ApplyStyle italic @10-20} transformed against {ApplyStyle font-color=red @0-30} results in the same {ApplyStyle italic @10-20} because the range of text can be both red and italic simultaneously.
