How to use transaction dependency tags (provides/requires)?

Question

I could not find much information/documentation on how to use the provides/requires tags (TransactionTag) Can someone please provide an example? The tag is a vector of bytes. Should I assume these are user-defined strings? Where/how do you specify them when creating a transaction?

Answer 1

AFAIK the most common example cited for explaining this where the requires/provides tag is used to provide nonce information for the current transaction in order to be properly considered a valid tx. The provides tag for a tx with nonce N would be the requires for a tx with nonce N+1.

Here's an example which constructs the provides and requires tag for a the checking correct nonce of a transaction:

fn validate(
        &self,
        who: &Self::AccountId,
        _call: &Self::Call,
        _info: &DispatchInfoOf<Self::Call>,
        _len: usize,
    ) -> TransactionValidity {
        // check index
        let account = crate::Account::<T>::get(who);
        if self.0 < account.nonce {
            return InvalidTransaction::Stale.into()
        }

        let provides = vec![Encode::encode(&(who, self.0))];
        let requires = if account.nonce < self.0 {
            vec![Encode::encode(&(who, self.0 - One::one()))]
        } else {
            vec![]
        };

        Ok(ValidTransaction {
            priority: 0,
            requires,
            provides,
            longevity: TransactionLongevity::max_value(),
            propagate: true,
        })
    }