What is head data and how do I get it?

Question

When registering a parachain on a relay-chain you need to provide something called head data (or also referred as genesis state) and wasm.

What is this head data and how do I get that?

Answer 1

Parachains can be seen as a function h' = f(h), where:

• f is the wasm blob referred to as PVF (parachain validation function).
• h is the current head data.
• h' is the new head data.

"Head data" is just a small payload of data. For example, Cumulus stores a Substrate block header in head data.

Upon the registration of a parachain, h₀ needs to be provided. This is a very first head data and in case of Cumulus that will be the block header of the genesis block, the block 0.

In cumulus one can obtain head data using the export-genesis-state subcommand of cumulus-based collator. Similarly, genesis wasm can be obtain using export-genesis-wasm.

More information can be found in this tutorial.

Answer 2

For the curious, parachain heads are maintained in the relay chain storage using a map, defined as

// The head-data of every registered para.
    #[pallet::storage]
    #[pallet::getter(fn para_head)]
    pub(super) type Heads<T: Config> = StorageMap<_, Twox64Concat, ParaId, HeadData>; 

Source from polkadot code base: https://github.com/paritytech/polkadot/blob/aa4b5fc5df538fe281af8379fba45bc1937a2dd9/runtime/parachains/src/paras/mod.rs#L550-L553