Module polkadot_sdk_docs::reference_docs::metadata

Learn about metadata, the main means through which an upgradeable runtime communicates its properties to the outside world.

Metadata

The existence of metadata in polkadot-sdk goes back to the (forkless) upgrade-ability of all Substrate-based blockchains, which is achieved through crate::reference_docs::wasm_meta_protocol. You can learn more about the details of how to deal with these upgrades in crate::reference_docs::frame_runtime_upgrades_and_migrations.

Another consequence of upgrade-ability is that as a UI, wallet, or generally an offchain entity, it is hard to know the types internal to the runtime, specifically in light of the fact that they can change at any point in time.

This is why all Substrate-based runtimes must expose a sp_api::Metadata api, which mandates the runtime to return a description of itself. The return type of this api is Vec<u8>, meaning that it is up to the runtime developer to decide on the format of this.

All crate::polkadot_sdk::frame_runtime based runtimes expose a specific metadata language, maintained in https://github.com/paritytech/frame-metadata which is adopted in the Polkadot ecosystem.

Metadata Explorers:

A few noteworthy tools that inspect the (FRAME-based) metadata of a chain:

• https://wiki.polkadot.network/docs/metadata
• https://paritytech.github.io/subxt-explorer/