Make a Custom Pallet

Introduction

Framework for Runtime Aggregation of Modular Entities (FRAME) provides a powerful set of tools for blockchain development through modular components called pallets. These Rust-based runtime modules allow you to build custom blockchain functionality with precision and flexibility. While FRAME includes a library of pre-built pallets, its true strength lies in the ability to create custom pallets tailored to your specific needs.

In this guide, you'll learn how to build a custom counter pallet from scratch that demonstrates core pallet development concepts. The pallet you'll create includes:

• User-triggered increment and decrement operations
• Root-only counter value setting
• Event emission for state changes
• Custom error handling
• Storage management
• User interaction tracking
• Genesis configuration for initial state

Prerequisites

Before you begin, ensure you have:

• Polkadot SDK dependencies installed
• A Polkadot SDK Parchain Template set up locally
• Basic familiarity with FRAME concepts

Core Pallet Components

As you build your custom pallet, you'll work with these key sections:

• Imports and dependencies: Bring in necessary FRAME libraries and external modules.
• Runtime configuration trait: Specify types and constants for pallet-runtime interaction.
• Runtime events: Define signals that communicate state changes.
• Runtime errors: Define error types returned from dispatchable calls.
• Runtime storage: Declare on-chain storage items for your pallet's state.
• Genesis configuration: Set initial blockchain state.
• Dispatchable functions (extrinsics): Create callable functions for user interactions.

For additional macros beyond those covered here, refer to the pallet_macros section of the Polkadot SDK Docs.

Create the Pallet Project

Begin by creating a new Rust library project for your custom pallet within the Polkadot SDK Parachain Template:

1. Navigate to the root directory of your parachain template:

   cd polkadot-sdk-parachain-template
   

2. Navigate to the pallets directory:

   cd pallets
   

3. Create a new Rust library project:

   cargo new --lib pallet-custom
   

4. Enter the new project directory:

   cd pallet-custom
   

5. Verify the project structure. It should look like:

   pallet-custom/
   ├── Cargo.toml
   └── src/
       └── lib.rs
   

Configure Dependencies

To integrate your custom pallet into the Polkadot SDK-based runtime, configure the Cargo.toml file with the required dependencies. Since your pallet exists within the parachain template workspace, you'll use workspace inheritance to maintain version consistency.

1. Open Cargo.toml and replace its contents with:

   pallet-custom/Cargo.toml

   [package]
   name = "pallet-custom"
   description = "A custom counter pallet for demonstration purposes."
   version = "0.1.0"
   license = "Unlicense"
   authors.workspace = true
   homepage.workspace = true
   repository.workspace = true
   edition.workspace = true
   publish = false
   
   [package.metadata.docs.rs]
   targets = ["x86_64-unknown-linux-gnu"]
   
   [dependencies]
   codec = { features = ["derive"], workspace = true }
   scale-info = { features = ["derive"], workspace = true }
   frame = { features = ["experimental", "runtime"], workspace = true }
   
   [features]
   default = ["std"]
   std = [
       "codec/std",
       "scale-info/std",
       "frame/std",
   ]
   

   Version Management

   The parachain template uses workspace inheritance to maintain consistent dependency versions across all packages. The actual versions are defined in the root Cargo.toml file, ensuring compatibility throughout the project. By using workspace = true, your pallet automatically inherits the correct versions.

2. The parachain template already includes pallets/* in the workspace members, so your new pallet is automatically recognized. Verify this by checking the root Cargo.toml:

   Cargo.toml

   [workspace.members]
   members = [
       "node",
       "pallets/*",
       "runtime",
   ]
   

Initialize the Pallet Structure

With dependencies configured, set up the basic scaffold that will hold your pallet's logic:

1. Open src/lib.rs and delete all existing content.

2. Add the initial scaffold structure using the unified frame dependency:

   src/lib.rs

   #![cfg_attr(not(feature = "std"), no_std)]
   
   pub use pallet::*;
   
   #[frame::pallet]
   pub mod pallet {
       use frame::prelude::*;
   
       #[pallet::pallet]
       pub struct Pallet<T>(_);
   
       #[pallet::config]
       pub trait Config: frame_system::Config {
           // Configuration will be added here
       }
   
       #[pallet::storage]
       pub type CounterValue<T> = StorageValue<_, u32, ValueQuery>;
   
       #[pallet::call]
       impl<T: Config> Pallet<T> {
           // Dispatchable functions will be added here
       }
   }
   

   Note

   This setup starts with a minimal scaffold without events and errors. These will be added in the following sections after the Config trait is properly configured with the required RuntimeEvent type.

3. Verify it compiles:

   cargo build --package pallet-custom
   

Configure the Pallet

The Config trait exposes configurable options and links your pallet to the runtime. All types and constants the pallet depends on must be declared here. These types are defined generically and become concrete when the pallet is instantiated at runtime.

Replace the #[pallet::config] section with:

src/lib.rs

#[pallet::config]
pub trait Config: frame_system::Config {
    /// The overarching runtime event type.
    type RuntimeEvent: From<Event<Self>> + IsType<<Self as frame_system::Config>::RuntimeEvent>;

    /// Maximum value the counter can reach.
    #[pallet::constant]
    type CounterMaxValue: Get<u32>;
}

Key configuration elements:

• RuntimeEvent: Required for the pallet to emit events that the runtime can process.
• CounterMaxValue: A constant that sets an upper limit on counter values, configurable per runtime.

Define Events

Events inform external entities (dApps, explorers, users) about significant runtime changes. Event details are included in the node's metadata, making them accessible to external tools.

The #[pallet::generate_deposit] macro automatically generates a deposit_event function that converts your pallet's events into the RuntimeEvent type and deposits them via frame_system::Pallet::deposit_event.

Add the #[pallet::event] section after the Config trait:

src/lib.rs

#[pallet::event]
#[pallet::generate_deposit(pub(super) fn deposit_event)]
pub enum Event<T: Config> {
    /// Counter value was explicitly set. [new_value]
    CounterValueSet { 
        new_value: u32 
    },
    /// Counter was incremented. [new_value, who, amount]
    CounterIncremented {
        new_value: u32,
        who: T::AccountId,
        amount: u32,
    },
    /// Counter was decremented. [new_value, who, amount]
    CounterDecremented {
        new_value: u32,
        who: T::AccountId,
        amount: u32,
    },
}

Define Errors

Errors indicate when and why a call fails. Use informative names and descriptions, as error documentation is included in the node's metadata.

Error types must implement the TypeInfo trait, and runtime errors can be up to 4 bytes in size.

Add the #[pallet::error] section after the events:

src/lib.rs

#[pallet::error]
pub enum Error<T> {
    /// The counter value has not been set yet.
    NoneValue,
    /// Arithmetic operation would cause overflow.
    Overflow,
    /// Arithmetic operation would cause underflow.
    Underflow,
    /// The counter value would exceed the maximum allowed value.
    CounterMaxValueExceeded,
}

Add Storage Items

Storage items persist state on-chain. This pallet uses two storage items:

• CounterValue: Stores the current counter value.
• UserInteractions: Tracks interaction counts per user account.

The initial scaffold already includes the CounterValue storage item. Now add the UserInteractions storage map after it:

src/lib.rs

/// Tracks the number of interactions per user.
#[pallet::storage]
pub type UserInteractions<T: Config> = StorageMap<_, Blake2_128Concat, T::AccountId, u32, ValueQuery>;

Your storage section should now look like this:

src/lib.rs

/// The current value of the counter.
#[pallet::storage]
pub type CounterValue<T> = StorageValue<_, u32, ValueQuery>;

/// Tracks the number of interactions per user.
#[pallet::storage]
pub type UserInteractions<T: Config> = StorageMap<_, Blake2_128Concat, T::AccountId, u32, ValueQuery>;

For more storage types and patterns, explore the Polkadot SDK storage documentation.

Configure Genesis State

Genesis configuration allows you to set the initial state of your pallet when the blockchain first starts. This is essential for both production networks and testing environments. It is particularly useful for:

• Setting initial parameter values.
• Pre-allocating resources or accounts.
• Establishing starting conditions for testing.
• Configuring network-specific initial state.

Add the #[pallet::genesis_config] and #[pallet::genesis_build] sections after your storage items:

src/lib.rs

#[pallet::genesis_config]
#[derive(frame_support::DefaultNoBound)]
pub struct GenesisConfig<T: Config> {
    /// Initial value for the counter
    pub initial_counter_value: u32,
    /// Pre-populated user interactions
    pub initial_user_interactions: Vec<(T::AccountId, u32)>,
}

#[pallet::genesis_build]
impl<T: Config> BuildGenesisConfig for GenesisConfig<T> {
    fn build(&self) {
        // Set the initial counter value
        CounterValue::<T>::put(self.initial_counter_value);

        // Set initial user interactions
        for (account, count) in &self.initial_user_interactions {
            UserInteractions::<T>::insert(account, count);
        }
    }
}

Genesis configuration components:

• GenesisConfig struct: Defines what can be configured at genesis.
• #[derive(DefaultNoBound)]: Provides sensible defaults (empty vec and 0 for the counter).
• BuildGenesisConfig implementation: Executes the logic to set initial storage values.
• build() method: Called once when the blockchain initializes.

Implement Dispatchable Functions

Dispatchable functions (extrinsics) allow users to interact with your pallet and trigger state changes. Each function must:

• Return a DispatchResult.
• Be annotated with a weight (computational cost).
• Have an explicit call index for backward compatibility.

Replace the #[pallet::call] section with:

src/lib.rs

#[pallet::call]
impl<T: Config> Pallet<T> {
    /// Set the counter to a specific value. Root origin only.
    #[pallet::call_index(0)]
    #[pallet::weight(0)]
    pub fn set_counter_value(origin: OriginFor<T>, new_value: u32) -> DispatchResult {
        // Ensure the caller is root
        ensure_root(origin)?;

        // Validate the new value doesn't exceed the maximum
        ensure!(
            new_value <= T::CounterMaxValue::get(),
            Error::<T>::CounterMaxValueExceeded
        );

        // Update storage
        CounterValue::<T>::put(new_value);

        // Emit event
        Self::deposit_event(Event::CounterValueSet { new_value });

        Ok(())
    }

    /// Increment the counter by a specified amount.
    #[pallet::call_index(1)]
    #[pallet::weight(0)]
    pub fn increment(origin: OriginFor<T>, amount: u32) -> DispatchResult {
        // Ensure the caller is signed
        let who = ensure_signed(origin)?;

        // Get current counter value
        let current_value = CounterValue::<T>::get();

        // Check for overflow
        let new_value = current_value
            .checked_add(amount)
            .ok_or(Error::<T>::Overflow)?;

        // Ensure new value doesn't exceed maximum
        ensure!(
            new_value <= T::CounterMaxValue::get(),
            Error::<T>::CounterMaxValueExceeded
        );

        // Update counter storage
        CounterValue::<T>::put(new_value);

        // Track user interaction
        UserInteractions::<T>::mutate(&who, |count| {
            *count = count.saturating_add(1);
        });

        // Emit event
        Self::deposit_event(Event::CounterIncremented {
            new_value,
            who,
            amount,
        });

        Ok(())
    }

    /// Decrement the counter by a specified amount.
    #[pallet::call_index(2)]
    #[pallet::weight(0)]
    pub fn decrement(origin: OriginFor<T>, amount: u32) -> DispatchResult {
        // Ensure the caller is signed
        let who = ensure_signed(origin)?;

        // Get current counter value
        let current_value = CounterValue::<T>::get();

        // Check for underflow
        let new_value = current_value
            .checked_sub(amount)
            .ok_or(Error::<T>::Underflow)?;

        // Update counter storage
        CounterValue::<T>::put(new_value);

        // Track user interaction
        UserInteractions::<T>::mutate(&who, |count| {
            *count = count.saturating_add(1);
        });

        // Emit event
        Self::deposit_event(Event::CounterDecremented {
            new_value,
            who,
            amount,
        });

        Ok(())
    }
}

Dispatchable Function Details

set_counter_value

• Access: Root origin only (privileged operations)
• Purpose: Set counter to a specific value
• Validations: New value must not exceed CounterMaxValue
• State changes: Updates CounterValue storage
• Events: Emits CounterValueSet

increment

• Access: Any signed account
• Purpose: Increase counter by specified amount
• Validations: Checks for overflow and max value compliance
• State changes: Updates CounterValue and UserInteractions
• Events: Emits CounterIncremented

decrement

• Access: Any signed account
• Purpose: Decrease counter by specified amount
• Validations: Checks for underflow
• State changes: Updates CounterValue and UserInteractions
• Events: Emits CounterDecremented

Verify Pallet Compilation

Before proceeding, ensure your pallet compiles without errors:

cargo build --package pallet-custom

If you encounter errors, carefully review the code against this guide. Once the build completes successfully, your custom pallet is ready for integration.

Complete Pallet Implementation

src/lib.rs

#![cfg_attr(not(feature = "std"), no_std)]

pub use pallet::*;

#[frame::pallet]
pub mod pallet {
    use frame::prelude::*;

    #[pallet::pallet]
    pub struct Pallet<T>(_);

    #[pallet::config]
    pub trait Config: frame_system::Config {
        type RuntimeEvent: From<Event<Self>> + IsType<<Self as frame_system::Config>::RuntimeEvent>;

        #[pallet::constant]
        type CounterMaxValue: Get<u32>;
    }

    #[pallet::event]
    #[pallet::generate_deposit(pub(super) fn deposit_event)]
    pub enum Event<T: Config> {
        CounterValueSet { new_value: u32 },
        CounterIncremented { new_value: u32, who: T::AccountId, amount: u32 },
        CounterDecremented { new_value: u32, who: T::AccountId, amount: u32 },
    }

    #[pallet::error]
    pub enum Error<T> {
        NoneValue,
        Overflow,
        Underflow,
        CounterMaxValueExceeded,
    }

    #[pallet::storage]
    pub type CounterValue<T> = StorageValue<_, u32, ValueQuery>;

    #[pallet::storage]
    pub type UserInteractions<T: Config> = StorageMap<_, Blake2_128Concat, T::AccountId, u32, ValueQuery>;

    #[pallet::genesis_config]
    #[derive(frame_support::DefaultNoBound)]
    pub struct GenesisConfig<T: Config> {
        pub initial_counter_value: u32,
        pub initial_user_interactions: Vec<(T::AccountId, u32)>,
    }

    #[pallet::genesis_build]
    impl<T: Config> BuildGenesisConfig for GenesisConfig<T> {
        fn build(&self) {
            CounterValue::<T>::put(self.initial_counter_value);
            for (account, count) in &self.initial_user_interactions {
                UserInteractions::<T>::insert(account, count);
            }
        }
    }

    #[pallet::call]
    impl<T: Config> Pallet<T> {
        #[pallet::call_index(0)]
        #[pallet::weight(0)]
        pub fn set_counter_value(origin: OriginFor<T>, new_value: u32) -> DispatchResult {
            ensure_root(origin)?;
            ensure!(new_value <= T::CounterMaxValue::get(), Error::<T>::CounterMaxValueExceeded);
            CounterValue::<T>::put(new_value);
            Self::deposit_event(Event::CounterValueSet { new_value });
            Ok(())
        }

        #[pallet::call_index(1)]
        #[pallet::weight(0)]
        pub fn increment(origin: OriginFor<T>, amount: u32) -> DispatchResult {
            let who = ensure_signed(origin)?;
            let current_value = CounterValue::<T>::get();
            let new_value = current_value.checked_add(amount).ok_or(Error::<T>::Overflow)?;
            ensure!(new_value <= T::CounterMaxValue::get(), Error::<T>::CounterMaxValueExceeded);
            CounterValue::<T>::put(new_value);
            UserInteractions::<T>::mutate(&who, |count| *count = count.saturating_add(1));
            Self::deposit_event(Event::CounterIncremented { new_value, who, amount });
            Ok(())
        }

        #[pallet::call_index(2)]
        #[pallet::weight(0)]
        pub fn decrement(origin: OriginFor<T>, amount: u32) -> DispatchResult {
            let who = ensure_signed(origin)?;
            let current_value = CounterValue::<T>::get();
            let new_value = current_value.checked_sub(amount).ok_or(Error::<T>::Underflow)?;
            CounterValue::<T>::put(new_value);
            UserInteractions::<T>::mutate(&who, |count| *count = count.saturating_add(1));
            Self::deposit_event(Event::CounterDecremented { new_value, who, amount });
            Ok(())
        }
    }
}

Add the Pallet to Your Runtime

Now that your custom pallet is complete, integrate it into the parachain runtime.

Add Runtime Dependency

1. In the runtime/Cargo.toml, add your custom pallet to the [dependencies] section:

   runtime/Cargo.toml

   [dependencies]
   # Local dependencies
   pallet-custom = { path = "../pallets/pallet-custom", default-features = false }
   
   # ... other dependencies
   

2. Enable the std feature by adding it to the [features] section:

   runtime/Cargo.toml

   [features]
   default = ["std"]
   std = [
       "codec/std",
       "pallet-custom/std",
       # ... other features
   ]
   

Implement the Config Trait

At the end of the runtime/src/configs/mod.rs file, add the implementation:

runtime/src/configs/mod.rs

/// Configure the custom counter pallet
impl pallet_custom::Config for Runtime {
    type RuntimeEvent = RuntimeEvent;
    type CounterMaxValue = ConstU32<1000>;
}

This configuration:

• Links the pallet's events to the runtime's event system
• Sets a maximum counter value of 1000 using ConstU32

Add to Runtime Construct

In the runtime/src/lib.rs file, locate the #[frame_support::runtime] section and add your pallet with a unique pallet_index:

runtime/src/lib.rs

#[frame_support::runtime]
mod runtime {
    #[runtime::runtime]
    #[runtime::derive(
        RuntimeCall,
        RuntimeEvent,
        RuntimeError,
        RuntimeOrigin,
        RuntimeTask,
        RuntimeFreezeReason,
        RuntimeHoldReason,
        RuntimeSlashReason,
        RuntimeLockId,
        RuntimeViewFunction
    )]
    pub struct Runtime;

    #[runtime::pallet_index(0)]
    pub type System = frame_system;

    // ... other pallets

    #[runtime::pallet_index(51)]
    pub type CustomPallet = pallet_custom;
}

Warning

Each pallet must have a unique index. Duplicate indices will cause compilation errors. Choose an index that doesn't conflict with existing pallets.

Configure Genesis for Your Runtime

To set initial values for your pallet when the chain starts, you'll need to configure the genesis in your chain specification. This is typically done in the node/src/chain_spec.rs file or when generating the chain specification.

For development and testing, you can use the default values provided by the #[derive(DefaultNoBound)] macro. For production networks, you'll want to set these values in your chain specification explicitly.

Verify Runtime Compilation

Compile the runtime to ensure everything is configured correctly:

cargo build --release

This command validates all pallet configurations and prepares the build for deployment.

Run Your Chain Locally

Launch your parachain locally to test the new pallet functionality using the Polkadot Omni Node.

Generate a Chain Specification

Create a chain specification file with the updated runtime:

chain-spec-builder create -t development \
--relay-chain paseo \
--para-id 1000 \
--runtime ./target/release/wbuild/parachain-template-runtime/parachain_template_runtime.compact.compressed.wasm \
named-preset development

This command generates a chain_spec.json that includes your custom pallet.

Start the Parachain Node

Launch the parachain:

polkadot-omni-node --chain ./chain_spec.json --dev

Verify the node starts successfully and begins producing blocks.

Interact with Your Pallet

Use the Polkadot.js Apps interface to test your pallet:

1. Navigate to Polkadot.js Apps.

2. Ensure you're connected to your local node at ws://127.0.0.1:9944.

3. Go to Developer > Extrinsics.

4. Locate customPallet in the pallet dropdown.

5. You should see the available extrinsics:

   • increment(amount): Increase the counter by a specified amount.
   • decrement(amount): Decrease the counter by a specified amount.
   • setCounterValue(newValue): Set counter to a specific value (requires sudo/root).

Key Takeaways

You've successfully created and integrated a custom pallet into a Polkadot SDK-based runtime. You learned:

• Configuration: Defined runtime-specific types and constants via the Config trait.
• Storage: Implemented on-chain state using StorageValue and StorageMap.
• Events: Created signals to communicate state changes to external systems.
• Errors: Established clear error handling with descriptive error types.
• Genesis: Configured initial blockchain state for both production and testing.
• Dispatchables: Built callable functions with proper validation and access control.
• Integration: Successfully added the pallet to a runtime and tested it locally.

These components form the foundation for developing sophisticated blockchain logic in Polkadot SDK-based chains.

Where to Go Next

• Guide Mock Your Runtime

  ---

  Learn to create a mock runtime environment for testing your pallet in isolation before integration.

  Continue

Last update: October 30, 2025
| Created: October 18, 2024