"Atomic-transaction" in NEAR protocol

Question

i'm trying to understand deeper in Near protocol and how the transaction process.

I know that txn in near not atomic and if contract call contract, one may fail and one mail success. but i want to ask if contract A call contract B(txn-1), then contract B call back to contract A(txn-2), is this '2-txn' happened continuously? i mean there won't be no others txn between 2 of them(??), which mean the state after txn-1 is what txn-2 begin with to process.

I want to know this because i want to make sure the state of contract in first call and the call back txn is trustable!

Thanks!

Answer 1

A contract call is atomic, but can result in a list of receipts for future contract calls, each of which are atomic.

-0-> A -1-> B -2-> A

In your example, contract A would execute and create the receipts ( call B; then call A). Any changes to contract A's state in this initial call would be atomic. That is if a separate new call to A from a different account were execute before the final call to A (-2-> A), it would see the updated state.

On the EVM an error at the end of a cross contract chain will cancel the transaction and any state changes. This is different on NEAR because if the last -2-> A fails the state changes in -0-> A will remain. This is why you any failure in -2-> A ( or it detects a failure in B), needs to roll back any state changes. It's tricky to get right but much more flexible than the EVM.