Substrate

Substrate is an open-source framework for building blockchains, written in the Rust programming language and developed by Parity Technologies, the company co-founded by Gavin Wood. Rather than writing a blockchain from scratch, developers use Substrate's ready-made components for networking, consensus, storage and transaction processing, and add only the features specific to their project. It is the technology underneath Polkadot, its sister network Kusama and the chains that connect to them.

Since 2023, Substrate has been maintained as part of the Polkadot SDK, a combined code base that also includes the software for Polkadot's relay chain and for parachains. The name is still widely used for the blockchain-building layer of that toolkit. Substrate is often compared with the Cosmos SDK, used to build chains in the Cosmos ecosystem, as one of the main frameworks for launching application-specific blockchains, or appchains.

Building a blockchain from nothing is like building a car by first designing an engine, wheels, brakes and wiring yourself. Substrate is more like a car platform offered by a manufacturer: the chassis, engine and safety systems already exist and have been tested, and you choose the body, the interior and the features that make your model distinctive.

A team that wants a chain for, say, digital identity or decentralized trading can take Substrate, keep the parts that move blocks between computers and agree on their order, and plug in modules for accounts, tokens and voting. They then write only the logic unique to their idea. Because the platform is shared, improvements made by one team can benefit everyone who builds on it.

A Substrate chain is split into two main parts. The client, sometimes called the node, handles networking through libp2p, stores the database, takes part in consensus and exposes interfaces for wallets and applications. The runtime contains the chain's business logic: the rules that decide what a valid transaction is and how it changes balances and other state.

• FRAME and pallets. The runtime is usually assembled with FRAME, a library of modules called pallets. Ready-made pallets cover balances, staking, governance, multi-signature accounts, smart contracts and more, and teams can write their own.
• WebAssembly runtime. The runtime is compiled to WebAssembly and stored on the chain itself. Nodes run whichever version is recorded on-chain.
• Pluggable consensus. Developers can choose among block production and finality mechanisms supplied with the framework, or write their own, as long as they fit Substrate's interfaces.

Because the runtime lives on the chain, a Substrate network can change its own rules by approving a new runtime through a transaction, usually after a vote under on-chain governance. Nodes switch to the new logic automatically at the agreed block, without operators having to install new software. This is often described as a forkless upgrade, in contrast with the coordinated software releases, and the risk of chain splits, involved in a hard fork on networks such as Bitcoin. Supporters see it as a way to evolve quickly; critics note that it puts considerable power in the hands of whoever controls the governance process.

Most chains in the Polkadot ecosystem are built with Substrate, including Polkadot's relay chain and the parachains that lease or buy capacity on it. Polkadot relies on nominators and validators to secure the network. Substrate has also been used for standalone networks that never connected to Polkadot, and for private or consortium chains. Tools around it include light clients that run in a browser, block explorers and development templates. Projects building on it range from DeFi platforms and privacy networks to identity systems and decentralized storage.

Substrate competes with frameworks such as the Cosmos SDK, which is written in Go and usually paired with the CometBFT consensus engine, formerly Tendermint, and with rollup toolkits that launch chains on top of Ethereum. Substrate gives developers a high degree of freedom, including in consensus and in how fees work, but its reliance on Rust and its large, complex code base give it a steep learning curve. Chains built with it can join Polkadot to share security, or run independently and secure themselves, a choice that shapes their cost and decentralization. Compared with deploying a smart contract on an existing chain, building a full chain demands more engineering and a community of validators, so Substrate is generally used by teams with specialized needs.

Developers have criticized Substrate's frequent breaking changes in its earlier years, its complexity and the time needed to become productive. The consolidation into the Polkadot SDK was partly meant to make releases more stable and coordinated. Because many chains share the same code base, a serious bug in a common component could affect a number of networks at once, though widespread use also means more eyes reviewing the code. Like other open-source blockchain frameworks, its future depends on continued funding of developers, which in Substrate's case has come largely from Parity and Polkadot's treasury.

• Polkadot Developer Documentation
• Parity Technologies