Polkadot validate_block, PVF, and relay validators
Companion to Polkadot HRMP protocol (implementation guide).
What implementation.rs is
cumulus/pallets/parachain-system/src/validate_block/implementation.rs implements validate_block: take the PoV’s storage proof, build a sparse in-memory trie, override sp_io::storage (and related) host functions so validation runs inside the Wasm instead of calling the node’s DB, execute the parachain block, then return ValidationResult — head data, horizontal_messages, hrmp_watermark, upward messages, processed downward message count, optional new validation code.
The helper run_with_externalities_and_recorder wraps Substrate Externalities (Ext over proof + OverlayedChanges) so pallet storage reads/writes during execute_verified_block and the follow-up reads for the result see consistent state. That is production PVF behavior, not limited to tests.
PVF entry point
Parachain runtimes register validation with cumulus_pallet_parachain_system::register_validate_block!. In no_std builds, the proc-macro in cumulus/pallets/parachain-system/proc-macro/src/lib.rs emits a #[no_mangle] unsafe fn validate_block(...) that forwards into this pallet’s validate-block implementation. That symbol is part of the parachain validation Wasm registered on the relay chain — the PVF.
So implementation.rs is compiled into the parachain PVF, not into the Polkadot relay runtime (frame_executive).
Relay validators vs relay on-chain runtime
| Accurate | Misleading |
|---|---|
| Relay validators load the parachain’s PVF and run it in a sandboxed VM (Wasm / PolkaVM) as part of candidate validation. | This code runs on-chain as a relay-chain pallet in a relay block. |
PVF execution is validator node work, off-chain with respect to relay state transitions — but it is mandatory for security and is what backs committed parachain progress.
One-liner
validate_block in implementation.rs is the core logic inside the parachain PVF; relay validators execute that Wasm when validating candidates; it is not relay-chain runtime logic, but it is how the relay chain checks parachain blocks.