Is it possible to specify parachains in the Polkadot chain specification for local testing?

Question

I noticed that the chain specification for Polkadot contains a paras field that is empty. I have not been able to find any documentation on what it is used for or if I can specify parachains there to avoid having to reserve a spot after the chain has been launched. I am aiming to run a local relay chain and parachains for testing purposes.

Will the paras field allow me to add parachains as part of the genesis state and is there anywhere I can find documentation on its usage?

Answer 1

You can add the parachains to the paras field in your chain-spec. For doing this you can follow this steps:

1. Create the chain-spec in plain text `polkadot build-spec --chain rococo-local --disable-default-bootnode

   rococo-local-plain.json` (I will use rococo-local as chain)

2. Edit this file and add your para into paras field, with this format

 paras: [
   [ <para_id>, [<para_head>,<para_wasm>, true]
 ]

Where: para_id : is your parachain id.

para_head : you can get it running polkadot-parachain export-genesis-state

para_wasm : you can get it running polkadot-parachain export-genesis-wasm And the last param (true) is to set the as parachain.

3. You need then to get the raw file from your edited plain file

   polkadot build-spec --chain ./rococo-local-plain.json --disable-default-bootnode --raw > ./rococo-local-raw.json

4. Then you can start your relay-chain nodes using this chain-spec (rococo-local-raw.json).

5. You need to start a collator of your parachain, using this chain-spec for the relay-chain and you can use one of the dev accounts for the collator. For example --alice

You can follow this tutorial to get the step-by-step instructions.

Also, this could be handled easily by zombienet and get your network running in one of the available providers (native, podman or kubernetes). Please, let me know if you have questions about zombienet and I can help you.

Answer 2

If you want to setup a Relay chain with parachain(s) locally I recommend you start here:

• https://docs.substrate.io/tutorials/build-a-parachain

Once you are comfortable with how to setup a test network manually, you can then use Zombienet (which automates a lot of the manual work) to spin up a test network quickly:

• https://docs.substrate.io/test/simulate-parachains

Regarding your other question, chain spec fields are documented here:

• https://paritytech.github.io/substrate/master/sc_service/trait.ChainSpec.html

The paras field you are referencing:

paras: Default::default(),

• https://github.com/paritytech/polkadot/blob/dbae30efe080a1d41fe54ef4da8af47614c9ca93/node/service/src/chain_spec.rs#L387

This is part of the Polkadot GenesisConfig.

This is referring to the GenesisConfig of the paras pallet found in Polkadot's runtime:

Paras: parachains_paras::{Pallet, Call, Storage, Event, Config, ValidateUnsigned} = 56,

You can find the paras GenisisConfig here:

• https://github.com/paritytech/polkadot/blob/dbae30efe080a1d41fe54ef4da8af47614c9ca93/runtime/parachains/src/paras/mod.rs#L764-L788

#[pallet::genesis_config]
pub struct GenesisConfig {
    pub paras: Vec<(ParaId, ParaGenesisArgs)>,
}

#[cfg(feature = "std")]
impl Default for GenesisConfig {
    fn default() -> Self {
        GenesisConfig { paras: Default::default() }
    }
}

#[pallet::genesis_build]
impl<T: Config> GenesisBuild<T> for GenesisConfig {
    fn build(&self) {
        let mut parachains = ParachainsCache::new();
        for (id, genesis_args) in &self.paras {
            if genesis_args.validation_code.0.is_empty() {
                panic!("empty validation code is not allowed in genesis");
            }
            Pallet::<T>::initialize_para_now(&mut parachains, *id, genesis_args);
        }
        // parachains are flushed on drop
    }
}