I think you're looking for transaction functions? https://docs.datomic.com/cloud/transactions/transaction-data-reference.html#Transaction%20Functions

> in particular they allow the data added in a transaction to be derived from the current value of the database.

Hi, the example queries the entity and then execute an update on that entity, which is not atomic (assume a race condition where the query verifies that the salary hasn't been increased yet). How can I execute a transaction like

update employees 
set salary = salary*1.1,
increased_at = getdate()
where experience > 3 and
increased_at < dateadd(year, -1, getdate());

perhaps :db/cas (https://docs.datomic.com/cloud/transactions/transaction-functions.html#db-cas) is what you're looking for?

I have looked into that, but it works for a single entity. I guess I can create multiple cas in a single tx, but it still means that I have to separate the query and the mutation

so you create a transaction function which queries the current employees’ salaries and experience and emits new salaries for those who have enough experience

how would you write it? I'm struggling with understand where does the tx start (datomic execution) and where my code does

for cloud, see https://docs.datomic.com/cloud/transactions/transaction-functions.html#creating

it is just a pure function that gets the current db and returns tx data

With a transaction function, the transactor executes the function with the current db value, and applies the tx-data returned from that function immediately after. The transactor is single threaded, so nothing else is applied to the database during this time. Basically, changes in transaction functions are atomic.

You can subscribe to the transactor log too e do this.

any idea where can I find some docs about it? couldn’t find

It's called the Transactor Report Queue. Here's a short blog post about it https://blog.datomic.com/2013/10/the-transaction-report-queue.html, and googling around for projects using it I found https://github.com/ivarref/yoltq/blob/e848610ac341db31b804644a7dfaaf98389469d5/src/com/github/ivarref/yoltq/report_queue.clj#L39

thanks! i’ll check that