How does DiffOnly on the tempo servlet work?

[Tom McCluskey]

I'm working with the Tempo API and the servlet to get a bunch of information about worklogs. I'd like to minimize the amount of stuff that we pull, so being able to get just a diff seems ideal. The servlet documentation says about DiffOnly:

• External applications use diffOnly=true to get only changed records since last GetWorklogs
• diffOnly returns only records that have changed since last update (UpdateWorklogs) or are marked as external error (external_result is not OK)

In the data we've got, external_result is coming back as an empty element; does it need to literally be <external_result>OK</external_result> to not be marked as an external error? Does external_result get automatically set somewhere, or is that something we need to do? Also, does DiffOnly return things that have changed since the last GetWorklogs, or since the last UpdateWorklogs?

To be clear, what we'd like to do is pull info from the servlet once, and then the next time only get the data that has changed since last time we pulled, regardless of what time period that worklog is from.

Thanks for any help!