Pallet Coupling

Pallets can use each other as dependencies to access their specific functionalities. For example, we use pallet_balance to access our pallet's underlying balances and lock statuses.

This is referred to as pallet coupling, and they define how pallets can interact with each other on a software level. This is similar to object-oriented programming, where tight and loose coupling indicates the relationship between two data structures. In Rust, this refers to how various traits and associated types are used with one another so that they may call upon each other's respective methods and implementations.

Loose Coupling

Our pallet used loose coupling, as we merely utilized an exposed trait from the Balances pallet (Currency). We further stated that we wish for our notion of Currency also to contain the traits ReservableCurrency and LockableCurrency, which further opens up our configuration to potentially other pallets:

// Loosely including Currency in the configuration
/// Configure the pallet by specifying the parameters and types it depends on.
#[pallet::config]
pub trait Config: frame_system::Config {
    ...
    type Currency: ReservableCurrency<Self::AccountId>
        + LockableCurrency<Self::AccountId, Moment = Self::BlockNumber>;
    ...
}

Tight Coupling

Tight coupling a pallet involves explicitly declaring an entire, specific pallet as a dependency. This would also mean that this pallet has to be within your pallet's Cargo.toml file, precisely just that specific pallet's instance. An example of this is the Bounties pallet, which tightly couples the Treasury pallet to it:

/// Notice the use of the '+' operator to tightly couple the Config of pallet_treasury
pub trait Config<I: 'static = ()>: frame_system::Config + pallet_treasury::Config<I> {}

Tightly coupling two pallets in this manner is helpful when they share very similar goals and objectives and for pallets that are simple enough. Because the Bounties pallet solely depends on the Treasury pallet for payouts (and its subsequent types), it is sensible to couple it tightly.

Which to use?

Overall, for maintainability and flexibility, loosely coupling is preferred. The benefit of loosely coupling our pallet was to reduce the apparent need to bring in the entire pallet versus just utilizing the traits you need.