The problem with having a global counter is that using a single entity
for all writes will result in contention and limited throughput. He
recommended using a technique he called "write shards". In other
words, use numerous counter entities to increment the count. Then,
when you want to display the total count, you query all of the counter
entities and sum them up.
Hopefully, Brett's slides (with code examples!) and a video of his
presentation will be posted soon, but until then you can read this
blog post from one of the attendees that describes the technique:
http://blog.appenginefan.com/2008/06/efficient-global-counters.html
Tom
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Blog: http://offermann.us