Customize a Chain Spec

After you have a Substrate node crafted, you want to start a network with many peers! This guide shows one method to create chain specification files uniformly and distribute them so other nodes can discover and peer with your network explicitly.

This guide illustrates:

  • How to generate a chain-spec.json and include it for other nodes to join a common network

  • How to modify an existing plain chain spec without editing the node source code

Create a chain specification

Plain chain specification file

  1. Start in your node's working directory, generate a plain chain spec with this command:

    ./target/release/node-template build-spec > chain-spec-plain.json

    We have just generated a plain chain spec file for the default network set in your file. This file can be passed to other nodes.

  2. Modify the plain chain specification (optional):

    This optional step we can leverage an existing plain chain specification for a network that otherwise would require modification of the source of the node to run on a new network. For example, this can be quite useful in the Prepare a local relay chain where we want to create a custom relay chain without customizing Polkadot's source.

    Here we use the same chain spec, but pass a flag to disable bootnodes, as we want a new network where these nodes will be different.

    ./target/release/node-template build-spec --chain chain-spec-plain.json --raw --disable-default-bootnode > no-bootnodes-chain-spec-plain.json

    This no-bootnodes-chain-spec-plain.json can be used to generate a SCALE storage encoded, distributable raw chain spec.

Raw chain specification file

  1. Generate the raw chain specification.

    With a plain spec available, you can generate a final raw chain spec by running:

    ./target/release/node-template build-spec --chain chain-spec-plain.json --raw > chain-spec.json

Raw chain specifications should always be used when passed into a node to execute with it.

Publish the raw chain specification

Because Rust builds that target WebAssembly are optimized, the binaries aren't deterministically reproducible. If each network participant were to generate the plain and/or raw chain specification, the differences in the resulting Wasm blob would break consensus.

It is conventional to include the chain specification files for your node within the source code itself so that anyone can build your node in the same way, whereby it becomes easy to check for non-determinism by comparing a genesis blob with another. Polkadot, Kusama, Rococo, and more network chain spec files are found in the source here along with a .gitignore file to ensure that you don't accidentally change these !/*.json files as you build further on your node's software and do runtime upgrades.

Start a new node

If you publish a node binary, or have users build their own and then they want to join your network, all then need is the same raw chain spec file and to run your binary with:

# binary named `node-template`
# `chain-spec.json` obtained from canonical common source
node-template --chain chain-spec.json

This can also simply be configured to be the default network. For reference, you can see how Polkadot implements a default command that uses the chain specs for various networks in the source here.


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