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Chain Specification

The chain specification describes any Substrate-based network. Located in node/src/chain_spec.rs, the chain specification describes the initial state of the network via the ChainSpec struct, which is usually exported as a JSON file using the substrate build-spec command.

When we start our chain in developmental mode, we have a set of pre-defined, funded accounts (endowed_accounts). These accounts are inside chain_spec.rs where default accounts, the chain's name, id, sudo accounts, and more are defined.

chain_spec.rs Overview​

The chain_spec.rs file describes a Substrate-based chain. It includes numerous functions, each defining a particular aspect of the configuration. The most notable functions are:

  • fn testnet_genesis() -> Defines the initial pallet state, including the Wasm runtime.
  • fn development_config() -> Defines the developmental instance of the node. Uses testnet_genesis() to configure the state of the development state.
Why does my chain progress in development mode with only one node?

If you have run cargo run --release on its own, and the chain doesn't progress, that's because it requires at least two authorities to start authoring blocks. The developmental configuration only requires a single validator, Alice.

Modifying Your Chain Spec​

It is possible to modify these functions to define your chain's initial state. New authorities for validation could be added, fund other accounts other than the "well-known" accounts, and any registered pallets' initial state can be modified in general.

Example: Changing the name of your chain​

Navigate to development_config() and find the following lines:

pub fn development_config() -> Result<ChainSpec, String> {
    let wasm_binary = WASM_BINARY.ok_or_else(|| "Development wasm not available".to_string())?;

    Ok(ChainSpec::from_genesis(
        // Name
        "Development",
...

Go ahead and change the name to whatever you like. By the end, it should look like this:

pub fn development_config() -> Result<ChainSpec, String> {
    let wasm_binary = WASM_BINARY.ok_or_else(|| "Development wasm not available".to_string())?;

    Ok(ChainSpec::from_genesis(
        // Name
        "TechEd Development Chain",
...

If you run your node and navigate to Polkadot.js, it should show in the upper left:

Development Chain Name PolkadotJs