Implementing efficient audit trail of record changes in Google App Engine - design patterns?

If I would have to choose I would go for option 1. The reads are as (if not more) performant for the other options. And all other options only have speed advantages under specific circumstances (small or very large sets of changes).

It will also get you lots of flexibility (with more ease) like purging history after x days or query history across different model types. Make sure you create the history entities as a child of the changed entity in the same transaction to guarantee consistency. You could end up with one of these.

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