It's not an obvious thing when I should use Promises when developing SmartConract in NEAR Protocol. And when it's redundant.
For example, I have some logic that finalize some public methods:
#[private]
fn record_data(&mut self, key: &Vec<u8>) -> Balance {
let initial_storage = env::storage_usage();
assert!(
!self.used_events.contains(&key),
"Proof event cannot be reused. Proof already exist."
);
self.used_events.insert(&key);
let current_storage = env::storage_usage();
let attached_deposit = env::attached_deposit();
let required_deposit =
Balance::from(current_storage - initial_storage) * STORAGE_PRICE_PER_BYTE;
attached_deposit - required_deposit
}
And in the same contract I have a function:
#[payable]
fn deposit(&mut self, amount: Balance) {
// ...some logic
self.record_data(&data)
}
So with Promises, I can call record_data in async way.
But it's not clear when I must use that way (Promise call). And when I can call in a very simple manner:
self.record_data(&data)
It's an obvious fact, that cross-contract call always via Promises. But it the same contract call specific inner Contract functions -it's not clear when we should use Promises.
So I need good clarification when I must use Promises and when it will redundant.
You can make the call to record_data async, but it comes at a cost. You are paying a base fee to make another transaction + the gas cost to run the code + the gas to interact with the host calls (e.g. env::*). So the reason to make internal calls asynchronous is to take advantage of the concurrency: see the rust-sdk's merge sort
pub fn merge_sort(&self, arr: Vec<u8>) -> PromiseOrValue<Vec<u8>> {
if arr.len() <= 1 {
return PromiseOrValue::Value(arr);
}
let pivot = arr.len() / 2;
let arr0 = arr[..pivot].to_vec();
let arr1 = arr[pivot..].to_vec();
let prepaid_gas = env::prepaid_gas();
let account_id = env::current_account_id();
ext::merge_sort(arr0, &account_id, 0, prepaid_gas / 4)
.and(ext::merge_sort(arr1, &account_id, 0, prepaid_gas / 4))
.then(ext::merge(&account_id, 0, prepaid_gas / 4))
.into()
}