Create a new pallet

In this workshop, you'll learn how to create a custom Substrate module-called a pallet-that's going to contain the code for your application-specific blockchain. Pallets are built using FRAME libraries and the Rust programming language. FRAME includes a lot of specialized macros that make it easy to compose the application logic in a reusable container. Let's get started by creating a new project and preparing a basic skeleton for the key parts of the Substrate collectibles application.

Create a new project

Because this workshop is all about demonstrating the full workflow for creating a new module—a new custom pallet—we won't start with the pallet_template. Instead, the first step is to create a new project for the collectibles pallet you'll be building.

To create a project:

  1. Open a new terminal, if needed.
  2. Change to the workshop-node-template/pallets directory in your workspace.
  3. Create a new Rust project for the collectibles pallet by running the following command:

    cargo new collectibles

    Note that cargo warns you that the new package isn't in the current workspace and there are a few different ways to fix the issue. For now, you can ignore the warning and continue preparing the new project.

  4. Change to the collectibles directory by running the following command:

    cd collectibles
  5. View the contents of the collectibles directory and and notice that it provides a default Cargo.toml and src/main.rs program:

    ├── Cargo.toml
    └── src
    └── main.rs

    In Rust, the Cargo.toml file for each package is called the package manifest and it defines configuration settings and dependencies that the package requires. The Cargo.toml file in the workspace-node-template/pallets/collectibles folder defines the dependencies for the collectibles package you are building.

    By convention, the source code for Rust projects in Substrate—including pallet modules—is typically in the src/lib.rs file. By default, Cargo creates a template src/main.rs file for new projects. For clarity in the workshop, let's rename the main source file for the new module.

  6. Rename src/main.rs source file by running the following command:

    mv src/main.rs src/lib.rs

    Now that your project files are in place, let's fix that issue that cargo warned you about by adding the new project to the current workspace.

Add the pallet to the workspace

As with other Rust programs, the node template has a Cargo.toml manifest file. In this case, the manifest file describes the workspace members. To add the new pallet to the workspace:

  1. Change to the workshop-node-template directory in your workspace.
  2. Open the Cargo.toml file in a text editor.
  3. Add the new pallets/collectibles pallet as a member of the workspace.

    [workspace]
    members = [
     "node",
     "pallets/collectibles",
     "pallets/template",
     "runtime",
    ]
  4. Save your changes and close the file.

Prepare the project manifest

Your new project is now included in the workspace and ready for you to start configuring. You already know that the project has a default Cargo.toml manifest file and that this file describes the package attributes and dependencies. Defining package attributes and dependencies is particularly important when developing a blockchain using Substrate and FRAME because the modular development environment enables you to import functionality from one part of code into another part of code and you can take advantage of this to reuse common functionality.

In this workshop, the collectibles module is going to be part of the Substrate runtime and its Cargo.toml file needs to define some modules it depends on. For example, two core packages the collectibles module requires are the frame_system and frame_support modules:

  • [frame_system] provides core functionality for working with common data structures and primitives so they are available to all of the pallets that need them, enabling new pallets to be easily integrated into a runtime and to interact with each other.
  • [frame_support] provides core support services for handling function calls that are dispatched to the runtime, defining storage structures, preparing events and errors and core utilities.

In addition to frame_system and frame_support, the collectibles module requires packages to support the type encoding and decoding required to minimize network traffic for the blockchain. To support encoding and decoding in the SCALE format, the collectibles module needs access to the codec and scale-info packages.

To update the manifest for the collectibles project:

  1. Open the default Cargo.toml file for the collectibles module in a text editor.
  2. Add frame-support and frame-system to the dependencies.

    [dependencies]
    frame-support = { default-features = false, version = "4.0.0-dev", git = "https://github.com/paritytech/substrate.git", branch = "polkadot-v0.9.31"}frame-system = { default-features = false, version = "4.0.0-dev", git = "https://github.com/paritytech/substrate.git", branch = "polkadot-v0.9.31" }
  3. Add codec and scale-info to the dependencies.

    codec = { package = "parity-scale-codec", version = "3.0.0", default-features = false, features = ["derive",] }
    scale-info = { version = "2.1.1", default-features = false, features = ["derive"] }
  4. Add the standard features for these packages to the Cargo.toml file to support the native binary target for the runtime.

    [features]
    default = ["std"]
    std = [
      "frame-support/std",
      "frame-system/std",
      "codec/std",
      "scale-info/std",
    ]

    Currently, the Substrate runtime is compiled to both a cross-platform WebAssembly target and platform-specific native binary target.

Prepare common code sections

You now have the bare minimum of package dependencies that your pallet requires specified in the Cargo.toml manifest. The next step is to prepare a set of common macros to serve as scaffolding for your new pallet.

  1. Open src/lib.rs in a text editor and delete the template main() function.

    You now have a clean slate for creating the Substrate collectibles pallet.

  2. Prepare the scaffolding for the Substrate collectibles pallet by adding the following common set of marco declarations to the src/lib.rs file:

    #![cfg_attr(not(feature = "std"), no_std)]
    
    pub use pallet::*;
    
    #[frame_support::pallet]
    pub mod pallet {
        use frame_support::pallet_prelude::*;
        use frame_system::pallet_prelude::*;
    
        #[pallet::pallet]
        #[pallet::generate_store(pub(super) trait Store)]
        pub struct Pallet<T>(_);
    
        #[pallet::config]
        pub trait Config: frame_system::Config {
        }
    }
  3. Save your changes and close the file.
  4. Verify that your program compiles by running the following command:

    cargo build --package collectibles

    You can ignore the compiler warnings about unused code for now.