I know the concept of existential deposit (ED) means that if the account balance is below this value, it will be wiped out. I tried to figure out how it works. The AccountData is defined in the runtime while stored in the frame-system pallet. try_mutate_exists in the frame_system pallet used to mutate AccountData storage, but how and where does the substrate system wipe the account whose balance below ED? I find OnKilledAccount type in the frame_system::Config, but it's more like a callback after the account has been killed.

impl<T: Config> StoredMap<T::AccountId, T::AccountData> for Pallet<T> {
    fn get(k: &T::AccountId) -> T::AccountData {

    fn try_mutate_exists<R, E: From<DispatchError>>(
        k: &T::AccountId,
        f: impl FnOnce(&mut Option<T::AccountData>) -> Result<R, E>,
    ) -> Result<R, E> {
        let account = Account::<T>::get(k);
        let was_providing = is_providing(&account.data);
        let mut some_data = if was_providing {
        } else {
        let result = f(&mut some_data)?;
        let is_providing = some_data.is_some();
        if !was_providing && is_providing {
        } else if was_providing && !is_providing {
            match Self::dec_providers(k)? { // This may trigger account wipe
                DecRefStatus::Reaped => return Ok(result),
                DecRefStatus::Exists => {
                    // Update value as normal...
        } else if !was_providing && !is_providing {
            return Ok(result);
        Account::<T>::mutate(k, |a| a.data = some_data.unwrap_or_default());

1 Answer 1


Through the DustCleaner.

Take transfer as an example.

When you call the transfer it will call try_mutate_account_with_dust internally.

Let's go back to the transfer. The closure which pass to the try_mutate_account_with_dust will return a result after the allow_death check.

Then if the result is Ok(()). A DustCleaner instance will be created. Just a Rust tip: if the result is Err(..) then the result.map(..) does nothing.

Finally, when the DustCleaner instance gets dropped somewhere, the Drop will be triggered. Then the account gets killed/recycled.

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