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I am always interested in Datomic's approach. How can you build a simple thread-safe counter with it?

If you do it in a time-series db fashion, there will soon be too many facts to count.

You can define functions inside the database and then use them inside a transaction [1] so that you can get atomic update, if that's the question. You can also mark attributes as noHistory [2] if you don't care about the past state.

[1] http://docs.datomic.com/database-functions.html

[2] http://docs.datomic.com/schema.html

From the FAQ (http://www.datomic.com/faq.html):

When is Datomic not a good fit? Datomic is not a good fit if you need unlimited write scalability, or have data with a high update churn rate (e.g. counters). And, at present, Datomic does not have support for BLOBs.

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