Basic instantiable pallets

There's gotta be a better way than re-writing the same pallet twice...

note

This guide is based on FRAME v1 macros.

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

The premise of instantiable pallets is to provide separate storage items for a runtime that contains more than one reference to the same pallet. This can be useful in cases where you may need to reuse the logic provided by a single pallet. This recipe guides you through 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:

```rust

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>::{Module, Call, Storage, Event<T>},

MintToken2: mint_token::<Instance2>::{Module, Call, Storage, Event<T>},

/* --snip-- */

Examples

• reward-coin pallet
• Polkadot's runtime

Related material