Substrate Inherents Sample

A Substrate runtime sample showing how to use inherent data in your custom modules.

In this sample, we get some data from a WebAPI and store it on-chain using Substrate inherents.

How to use inherents

Following are the steps involved in using Substrate inherents in runtime modules.

Initial setup and prerequisites

• Create a new node runtime using the substrate-node-new command.
• Add the dependency substrate-inherents in the cargo.toml inside the runtime directory. This can be just like the outer Substrate packages added in there.
• Make sure that your module is declared as public in the lib.rs. This is to make sure that the module is accessible to the outside code for provider registration. (see Register section below)
• Add the Inherent parameter to your custom module definition in the construct_runtime! macro (in lib.rs).

Define InherentIdentifier and InherentType

The InherentIdentifier is the unique identifier for your module's inherent data in the InherentData storage. This should be unique across the runtime.

The InherentType is the data type for the inherent data of your module.

pub const INHERENT_IDENTIFIER: InherentIdentifier = *b"tknusd00";
pub type InherentType = u64;

Define InherentDataProvider

Your module should define an InherentDataProvider type and it should implement the ProvideInherentData trait. This is how the consensus engine provides the inherent data to the runtime, which should be available at the block production time. The ProvideInherentData trait defines a function called provide_inherent_data where we get (or calculate) the inherent data and store it in the InherentData storage.

In this sample, we call a Web API to get some data and we store this data on-chain using Substrate inherents.

Rolling Median Approach: Assuming that 50% of the authorities are honest, a rolling median approach could be used to enhance seurity of the solution. In this sample, we collect the last 10 responses from the API in a Vec and then compare the most recent value with the median of those 10 values with a predefined drift value. If the difference between the median and the new value is more than the drift then we do not store that value. This could be further extended to punish the dishonest authorities.

Register InherentDataProvider

To let the Substrate runtime know that there is a new InherentDataProvider, we need to register it. In this sample, we are using substrate-node-template and the provider is registered in the /src/service.rs file.

let reg = service.config.custom.inherent_data_providers.register_provider(inherentsample::InherentDataProvider);

Implement ProvideInherent for your module

The final step is to implement the ProvideInherent trait for your module. The ProvideInherent trait allow the block author to store the inherent data in the module storage through an inherent extrinsics call, using the create_inherent function.

The ProvideInherent trait also has an optional function check_inherent which the other validators can call to verify or validate the inherent data submitted by the block author. For simplicity, we have not implemented this function in this sample.

Note

This sample is only for showing the process or steps involved in using Substrate inherents in runtime modules. This, by no means, is intended to be a tutorial for how to build a decentralized oracle. Please do not use this code in production applications without proper review and audits.