Can you write a short bit about what you'd do with the API/the use case?

@Lydia_Pintscher Sure! I have just deployed a demonstration of this use case on EditGroups.

The goal is to index all Wikidata edit groups by the properties that they change (as statements or qualifiers, added or removed by the edit group). This can be useful to get an overview of the data imports that happened in a particular domain. For instance, the list of all edit groups using P50 (author) gives you an overview of the ongoing author disambiguation effort in Wikicite.

Some editing actions (such as wbeditentity-create or wbeditentity-update) allow users to perform big changes on items, and therefore do not expose the properties used in the edit summary. So, to make sure these edits are also indexed, we need to analyze their diffs. Hence the need for this API.

One alternative would be to retrieve the JSON representation of the entities for all revisions involved and compute the diff manually, client-side. This is potentially more scalable as we could retrieve 25 diffs in one API call (25*2 revisions). But implementing the diff logic is probably a bit more involved.

Relevant code:
https://github.com/Wikidata/editgroups/blob/master/tagging/diffinspector.py

Thanks a lot! That makes it clear.
@Addshore Care to chime in?

I just stumbled over a use case that woud perhaps also benefit from structured diffs.

I made this small tool to view the Wikidata edit stream:

https://tobias47n9e.gitlab.io/wikidata-stream/

From the edit event I now fetch the current and previous revision, and then fetch both items in order to compare the changes in a structured form. This is quick and easy enough for me to work.

I see issues with the structured diffs especially with the deeply nested statements. Plus the data-structure would need to handle additions, modifications and deletions, which would probably make the JSON big. This would further increase the size of the edit event JSON object.

At the moment I can't decide if this is needed or if most use cases anyway need to fetch both revisions and handle the structured diffs client-side.

Ad structured diffs, see also http://magnusmanske.de/wordpress/?p=617, esp. the second and third paragraph - thanks @Tobias1984 for the pointer to the article!