[alkby]

I think their statement about ACID is too bold.

How does somebody do read-"modify" style of transactions ?

Say I want to bump some counter. So I delete old fact and I establish new fact. But new fact needs to be exactly 1 + old value of counter. With transactions as simple "add this and remove that" you seemingly cannot do that. So it's not ACID. Right?

[richhickey]

Transactions are not limited to add/retract. There are also things we call data functions, which are arbitrary, user-written, expansion functions that are passed the current value of the db (within the transaction) and any arbitrary args (passed in the call), and that emit a list of adds/retracts and/or other data function calls. This result gets spliced in place of the data function call. This expansion continues until the resulting transaction is only asserts/retracts, then gets applied. With this, increments, CAS and much more are possible.

We are still finalizing the API for installing your own data functions. The :db.fn/retractEntity call in the tutorial is an example of a data function. (retractEntity is built-in).

This call:

    [:db.fn/retractEntity entity-id]

must find all the in- and out-bound attributes relating to that entity-id (and does so via a query) and emit retracts for them. You will be able to write data functions of similar power. Sorry for the confusion, more and better docs are coming.

[danieljomphe]

From what I remember, compare-and-swap semantics are in place for that kind of case.

If that was not the case, you could still model such an order-dependent update as the fact that the counter has seen one more hit. Let the final query reduce that to the final count, and let the local cache implementation optimize that cost away for all but the first query, and then incrementally optimize the further queries when they are to see an increased count.

That said, I'm pretty sure I've seen the simpler CAS semantics support. (The CAS-successful update, if CAS is really supported, is still implemented as an "upsert", which means old counter values remain accessible if you query the past of the DB.)

[danieljomphe]

Forget my last paragraph. Anyways, richhickey answered. :)