Pausing Transactions¶

Introduction¶

The Transaction Pause module is one of the built-in modules included in the Polkadot SDK, and it is available in any Tanssi appchain based on the official templates version 400 or higher.

This module allows an appchain governor to temporarily avoid executing a set of hand-picked transactions while the rest of the transactions carry on as usual. This feature is helpful in several scenarios, such as disabling functionality in which a security threat has been discovered, enabling seasonal functionality only when needed, and enabling a set of transactions exactly on a launch date.

In an emergency scenario, when a critical exploit is discovered, this module allows the appchain to isolate and stop only the affected functionality, effectively minimizing the overall impact.

Checking Prerequisites¶

For the examples in this guide, you will need to have the following:

• A Tanssi appchain (Snap or Dedicated) featuring the Transaction Pause module. Any new appchain deployment based on one of the templates will do; otherwise, make sure to include the module in your custom-made appchain runtime
• Your appchain's Sudo account connected to your appchain's Polkadot.js Apps. You can refer to the Managing Sudo guide for instructions on injecting your Sudo account into Polkadot.js Apps

If you're unsure what your Tanssi appchain's Sudo account is, you can find it in your Tanssi Dashboard underneath the Properties section.

Module and Transaction Names¶

The Transaction Pause module works by filtering the execution of specific transactions contained in the modules included in the appchain runtime. To do so, it keeps an internal list of the banned transactions identified by module and transaction name. This list is case-sensitive and works only when there is an exact match between one item in the paused transactions list and the transaction being processed. Therefore, using the exact names of the modules and the transactions is crucial.

To find out the names of the modules available in your runtime, you need to read the construct_runtime!() section in the lib.rs file of your appchain runtime in your project repository. If your appchain is based on one of the official templates, you'll find the file in the Tanssi repository:

• For appchains based on the EVM template: the lib.rs file
• For appchains based on the Substrate template: the lib.rs file

The following snippet is an example of how the construct_runtime!() section looks like. The module names are those located to the left of the colon.

construct_runtime!(
    pub enum Runtime
    {
        ...
        Migrations: pallet_migrations = 7,
        MaintenanceMode: pallet_maintenance_mode = 8,
        TxPause: pallet_tx_pause = 9,
        Balances: pallet_balances = 10,
        Multisig: pallet_multisig = 16,      
        ...
   }

To identify the transaction names included in a module, you need to refer to its source code. Modules built in Substrate identify their transactions using a macro #[pallet::call_index(INDEX)], where INDEX is a number. In the case of a built-in module, the code is located within the FRAME folder of the Polkadot-SDK repository. For example, if you want to know about the transaction names in the Balances module, refer to its lib.rs file and look for the function names below the #[pallet::call_index(INDEX)] macros. The following snippet is the transaction transfer_allow_death of the module Balances, which is the one used as an example in this guide:

#[pallet::call_index(0)]
pub fn transfer_allow_death(
    origin: OriginFor<T>,
    dest: AccountIdLookupOf<T>,
    #[pallet::compact] value: T::Balance,
) -> DispatchResult {
    // Code
    Ok(())
}

Some Frequently Used Modules and Transactions¶

When using any of the built-in Substrate modules, the name with which it's referenced within the runtime is entirely up to the developer, but the transaction names aren't customizable. Here is a list of some of the most commonly used modules with the most commonly used transactions they contain. Those are the transaction names to be used in this Transaction Pause module.

Pausing Transactions¶

As you know, the Sudo account can perform privileged actions, such as appchain upgrades, minting new tokens, and, in this case, pausing and unpausing transactions.

To pause a transaction, navigate to the Developer tab of Polkadot.js Apps for your Tanssi appchain and click on Sudo. If you do not see Sudo in this menu, you have not associated the Sudo account with Polkadot.js Apps. Make sure that your Sudo account is injected by your wallet and connected to Polkadot.js Apps. Then, take the following steps:

1. Select the txPause module
2. Select the pause method
3. Insert the module name that contains the transaction that will be paused
4. Insert the transaction name that will be paused
5. Press Submit Sudo and confirm the transaction in the resulting pop-up

In this example, the transaction paused is transfer_allow_death from the Balances module:

To verify that the transaction has been effectively paused, try executing it. You should get an error.

Unpausing Transactions¶

To unpause a transaction and return it to normal operation, navigate to the Developer tab of Polkadot.js Apps for your Tanssi appchain and click on Sudo. If you do not see Sudo in this menu, you have not associated the Sudo account with Polkadot.js Apps. Make sure that your Sudo account is injected by your wallet and connected to Polkadot.js Apps. Then, take the following steps:

1. Select the txPause module
2. Select the unpause method
3. Insert the module name that contains the transaction that will be unpaused
4. Insert the transaction name that will be unpaused
5. Press Submit Sudo and confirm the transaction in the resulting pop-up

In this example, the transaction to unpause is transfer_allow_death from the Balances module:

The unpause transaction executes successfully only if the module and transaction parameters have been previously paused; otherwise, it fails. After the successful unpausing, the transaction can be called and executed again.

And that's it! The Developer Portal section has plenty more guides on how to manage your Tanssi appchain.