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Substrate - powering Web3

Substrate is a modular framework that enables you to create domain-specific blockchains by composing custom or pre-built components. It is part of the Polkadot SDK, and is responsible for providing the tools needed to build blockchain runtimes.

Substrate is part of the Polkadot SDK (monorepo)

As of August, 2023, the Polkadot, Substrate, and Cumulus codebases were merged into one monorepo. Now referred to as the Polkadot SDK, this monorepo represents all the primary tools needed to build blockchains using Substrate.

The Framework for Runtime Aggregation of Modularized Entities (FRAME) is a set of modules and support libraries that simplify runtime development within Substrate. In Substrate, these modules are called pallets, each hosting domain-specific logic to include in a chain's runtime.

Substrate may be utilized to create a solo chain or a parachain for a relay chain like Polkadot or Kusama. At its core, it is a set of Rust libraries that provide the most basic primitives and protocols to implement a distributed, peer-to-peer state machine, such as a blockchain.

An analogy: React, the library for web and native user interfaces

Another way to think about Substrate is similar to how a web developer may use/think about React. Where React provides core libraries for manipulating the elements of the DOM via developer-defined components, Substrate also provides the base libraries and primitives needed to create a blockchain. It was designed to not be opinionated via a highly generic codebase, enabling high customizability.

Substrate's Libraries

Substrate, as mentioned, is a collection of Rust crates that define a generic way to implement a blockchain. Although you will only use a few of these libraries directly in this course, know that everything you use is built upon them.

You will encounter two primary prefixes when developing with Substrate crates: sc- (Substrate Client) and sp- (Substrate Primitive). Crates prefixed with sc- usually refer to network-related functionalities, such as peer-to-peer networking or consensus. The sp- prefix usually indicates that the library provides primitives for a particular aspect of Substrate, such as runtime-related primitives.

A few significant crates that you will encounter are:

  • sp_runtime - Runtime modules shared primitive types
  • sp_core - Shareable Substrate types
  • sp_io - I/O host interface for substrate runtime. Substrate runtime standard library as compiled when linked with Rust’s standard library.

These, along with many more, all pertain to building certain aspects of distributed systems, including but not limited to:

  • Consensus mechanisms
  • Peer-to-peer networking
  • Fork-choice rules
  • Block authoring
  • State management

A Brief Introduction to FRAME (Framework for Runtime Aggregation of Modularized Entities)

A common question is: If Substrate exists, what is FRAME for? Why is it mentioned so often?.

FRAME, or Framework for Runtime Aggregation of Modularized Entities, is another abstraction on top of Substrate. It introduces a set of conventions and structures for building a blockchain with Substrate through numerous Rust macros. These macros enable the powerful and trivial to use a system of pallets to construct runtimes using Substrate. A runtime, also called a state transition function (STF), defines the state transitions for a particular blockchain.


As we advance, terminologies such as pallets, and custom bundles of business logic that make up a runtime will be expected. If a term here is unfamiliar and undefined, reference the glossary.

Remember, a state machine is the core concept of a blockchain. FRAME facilitates the way for state to propagate and change in a more developer-friendly way.

The outcome of developing on Substrate, particularly FRAME, is almost always a runtime (compiled to WebAssembly, or Wasm), also called a state transition function (STF). This runtime defines the core logic that determines how state propagates and changes in a blockchain. In the coming lessons, you will learn more about how a runtime works and eventually dive deeper into its structure.