try look up the entity, if got nothing, transact

Doesn’t that have an obvious race condition?

not obvious to me

Two parties check for presence simultaneously. Both see that it’s not there’ both transact

writes are serialized, two entities will be written, or one will has an error

both are expected behavior

Or imagine a counter. Two theads try to increment. Both read value 1. Both will transact to value 2

it will end up with 2, as expected.

if you are talking about atomicity, we have an issue for that

https://github.com/juji-io/datalevin/issues/110

We do miss a function that does both read and write atomically. So you are right. I wonder why datomic doesn’t have these?

sure, we can add such functionality.

or more generally, maybe expose the transaction explicitly, e.g. introduce something like (start-transaction), (end-transaction), so whatever you do inside, will be part of the transaction

Datomic has transaction functions for this I think

we have transaction functions too, but that’s not the same thing

at the KV db level, we need to expose transaction start and end, so as to ensure the correctness.

Some of these work is being done in the query branch, but maybe I should move them to the main branch.

this work does introduce a lot of changes, it would be messy to merge. I would wait for the finish of the rewrite.

On a second thought, the rewrite still has a lot more work remaining, I should move this part to the main branch first, since it is about correctness in multiple threads situation.

OK, I will work on this.

We will not change existing API, instead, we will introduce (start-tx …) and (end-tx …) for both KV and Datalog DB.

So people can do whatever they want in between and be considered as part of the same transaction, ensuring correctness.

so in your case, you will get 3, instead of 2, as desired.

That’s great

with-transaction-kv and with-transaction macro have been implemented to do this for both KV and Datalog DB. So this is resolved.