Why does building the parachain template build the polkadot and kusama runtime?

Question

This is on my screen as I compile the substrate parachain template:

Building: kusama-runtime(build), polkadot-runtime(build), parachain-template-runtime(build)                                                      

Why does it need to build the kusama and polkadot runtime for a parachain?

Answer 1

You could figure out this yourself.

Just check the lock file.

Let's take parachain-template as an example.

1. search polkadot-runtime
2. found polkadot-service depends on polkadot-runtime
3. search polkadot-service
4. found cumulus-relay-chain-inprocess-interface depends on polkadot-service
5. search cumulus-relay-chain-inprocess-interface
6. found parachain-template-node depends on cumulus-relay-chain-inprocess-interface

That's it.

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https://github.com/substrate-developer-hub/substrate-parachain-template/blob/677a1a238a9592d0c8f1f0a4b7477b031da76c20/Cargo.lock#L6158

Answer 2

From the protocol perspective; due to polkadot's shared security the parachain's finalized state is guaranteed on the relay chain. In other words, information regarding a parachain block, whether it is included, reverted or finalized, is acquired from the relay chain. In order to obtain and verify the latest information about blocks and finality a parachain needs to be in sync with the relay chain (networking, consensus, finality, etc). For this reason a parachain node also consists of a relay chain client (e.g. polkadot or kusama).