Token distribution to many users in substrate

Question

I'm planning to write a pallet to distribute token to many users in substrate blockchain (Ex: 500k-1m users). Should I do a for loop in a user list and transfer token to them? Do you have any example?

Answer 1

Both are easy to implement. But if it is an extrinsic call, you might need to pay more attention to the bandwidth/weight costs.

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One-time job in migration way.

Find your Executive type in the runtime lib.rs and add a TokenDistribution:

pub type Executive = frame_executive::Executive<
    Runtime,
    Block,
    ChainContext<Runtime>,
    Runtime,
    AllPalletsWithSystem,
+   TokenDistribution,
>;

Define the struct TokenDistribution and implement OnRuntimeUpgrade for it:

use frame_support::traits::OnRuntimeUpgrade;

pub struct TokenDistribution;
impl OnRuntimeUpgrade for TokenDistribution {
    fn on_runtime_upgrade() -> Weight {
        let fund_account = x;
        let distribute_amount = y;

        load_user_list().into_iter().for_each(|user: AccountId| {
            // `Balances` is the pallet name of your `pallet-balances` 
            // instance which defines in the runtime `lib.rs`           
            Balances::transfer(
                Origin::signed(fund_account),
                user.into(),
                distribute_amount
            );
        });
        
        // Or you could calculate a more precise weight here
        RuntimeBlockWeights::get().max_block
    }
}

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In the next runtime upgrade, the distribution will be applied.

I believe this is the cleanest way to do it.

And remember to remove these codes before the next-next runtime upgrade.