Intantiable Pallets

Warning

This guide is based on FRAME v1 macros and will soon be deprecated.

Goal

Learn how to configure instantiable pallets.

Use Cases

A token chain hosts two independent cryptocurrencies.
Governance has two (or more) houses which act similarly internally.
A social network chain that maintains different treasuries.

Overview

Instantiable pallets provide separate storage items for a runtime that contains more than one reference to the same pallet. This is useful in cases where you want to reuse the logic provided by a single pallet. This procedure shows you how to create two instances of the same pallet and how to configure their capabilities.

Steps

1. Implement the `Instance` type

Instantiable pallets must call the decl_storage! macro so that the Instance type is created.

Add a generic type for Instance in the Config trait, Event type, and Store trait. Learn more about how generic types work in Rust here. The code snippet below shows the modifications required for our pallet's Config and Store traits:

pub trait Config<I: Instance>: frame_system::Config {
  type Event: From<Event<Self>> + Into<<Self as frame_system::Config>::Event>;
}
/// Include the I: Instance type parameter in storage declaration
decl_storage! {
  trait Store for Module<T: Config<I>, I: Instance> as MintToken {
/* --snip-- */

2. Configure your runtime

Include two instances of mint_token in runtime/src/lib.rs:

/* --snip-- */
// Instance1 of mint_token
impl mint_token::Config<mint_token::Instance1> for Runtime {
    type Event = Event;
}

// Instance2 of mint_token
impl mint_token::Config<mint_token::Instance2> for Runtime {
    type Event = Event;
}

/* --snip-- */
MintToken1: mint_token::<Instance1>::{Pallet, Call, Storage, Event<T>},
MintToken2: mint_token::<Instance2>::{Pallet, Call, Storage, Event<T>},
/* --snip-- */

Examples

Polkadot's runtime

Other

Rust book on generics