Let’s say you want to find out about changes made to a table in Dataverse (Microsoft Dynamics, Microsoft Power Apps, etc.), perhaps because you’d like to synchronize those changes to an external system.

A primitive way of pulling this off is to query the table of interest, selecting only those records whose modifiedon value is greater than or equal to the last time you checked for changes:

GET {{webApiUrl}}contacts?$filter=modifiedon ge 2021-04-28T15:19:30Z
(include the "always include" headers)

This approach has several potential downsides, including:

• Deletes aren’t included—If a record was removed from the specified table, the results to a “GET filtered on modifiedon” query won’t return anything indicating the deletion.
• Missed modifications—Normally, modifiedon is automatically set to the moment the record was last changed in Dataverse; however, this behavior can be overridden. Since modifiedon isn’t an absolute indicator of whether or when changes occurred, filtering by it isn’t guaranteed to find all changes actually made in Dataverse after a point in time. (Think of a data import or an inbound data sync scenario: inserts/updates might be backdated so modifiedon matches when the record was last changed in the external system, which could be hours or days prior to when the data is brought into Dataverse.)

Dataverse’s Web API provides a better option for finding changes—an option built specifically for this purpose. When fetching entities from a table, if the table has change tracking enabled, you can ask Dataverse to provide you with a delta link. Later, you can use that link to fetch the delta of changes that occurred since your initial GET.