## Module transaction_extensions

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Learn about the transaction extensions that form a part of extrinsics. Transaction extensions are, briefly, a means for different chains to extend the “basic” extrinsic format with custom data that can be checked by the runtime.

## §FRAME provided transaction extensions

FRAME by default already provides the following transaction extensions:

- `CheckGenesis`: Ensures that a transaction was sent for the same network. Determined based on genesis.

- `CheckMortality`: Extends a transaction with a configurable mortality.

- `CheckNonZeroSender`: Ensures that the sender of a transaction is not the all zero account (all bytes of the accountid are zero).

- `CheckNonce`: Extends a transaction with a nonce to prevent replay of transactions and to provide ordering of transactions.

- `CheckSpecVersion`: Ensures that a transaction was built for the currently active runtime.

- `CheckTxVersion`: Ensures that the transaction signer used the correct encoding of the call.

- `CheckWeight`: Ensures that the transaction fits into the block before dispatching it.

- `ChargeTransactionPayment`: Charges transaction fees from the signer based on the weight of the call using the native token.

- `ChargeAssetTxPayment`: Charges transaction fees from the signer based on the weight of the call using any supported asset (including the native token).

- `ChargeAssetTxPayment`(using conversion): Charges transaction fees from the signer based on the weight of the call using any supported asset (including the native token). The asset is converted to the native token using a pool.

- `SkipCheckIfFeeless`: Allows transactions to be processed without paying any fee. This requires that the `call` that should be dispatched is augmented with the `feeless_if` attribute.

- `CheckMetadataHash`: Extends transactions to include the so-called metadata hash. This is required by chains to support the generic Ledger application and other similar offline wallets.

- `WeightReclaim`: A transaction extension for the relay chain that reclaims unused weight after executing a transaction.

- `StorageWeightReclaim`: A transaction extension for teyrchains that reclaims unused storage weight after executing a transaction.

For more information about these extensions, follow the link to the type documentation.

## §Building a custom transaction extension

Defining a couple of very simple transaction extensions looks like the following:

ⓘ

    pub mod transaction_extensions_example {
    	use codec::{Decode, DecodeWithMemTracking, Encode};
    	use pezsp_runtime::{
    		impl_tx_ext_default,
    		traits::{Dispatchable, TransactionExtension},
    		transaction_validity::TransactionValidityError,
    	};
    	use scale_info::TypeInfo;

    	// This doesn't actually check anything, but simply allows
    	// some arbitrary `u32` to be added to the extrinsic payload
    	#[derive(Debug, Encode, Decode, DecodeWithMemTracking, Clone, Eq, PartialEq, TypeInfo)]
    	pub struct AddToPayload(pub u32);

    	impl<Call: Dispatchable> TransactionExtension<Call> for AddToPayload {
    		const IDENTIFIER: &'static str = "AddToPayload";
    		type Implicit = ();
    		type Pre = ();
    		type Val = ();

    		impl_tx_ext_default!(Call; weight validate prepare);
    	}

    	// This is the opposite; nothing will be added to the extrinsic payload,
    	// but the Implicit type (`1234u32`) will be added to the
    	// payload to be signed.
    	#[derive(Debug, Encode, Decode, DecodeWithMemTracking, Clone, Eq, PartialEq, TypeInfo)]
    	pub struct AddToSignaturePayload;

    	impl<Call: Dispatchable> TransactionExtension<Call> for AddToSignaturePayload {
    		const IDENTIFIER: &'static str = "AddToSignaturePayload";
    		type Implicit = u32;

    		fn implicit(&self) -> Result<Self::Implicit, TransactionValidityError> {
    			Ok(1234)
    		}
    		type Pre = ();
    		type Val = ();

    		impl_tx_ext_default!(Call; weight validate prepare);
    	}
    }

## Modules§

transaction_extensions_example