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Learn about Substrate, the main blockchain framework used in the Polkadot ecosystem.

§Substrate

Substrate is a Rust framework for building blockchains in a modular and extensible way. While in itself un-opinionated, it is the main engine behind the Polkadot ecosystem.

§Overview, Philosophy

Substrate approaches blockchain development with an acknowledgement of a few self-evident truths:

  1. Society and technology evolves.
  2. Humans are fallible.

This, makes the task of designing a correct, safe and long-lasting blockchain system hard.

Nonetheless, in strive towards achieve this goal, Substrate embraces the following:

  1. Use of Rust as a modern and safe programming language, which limits human error through various means, most notably memory and type safety.
  2. Substrate is written from the ground-up with a generic, modular and extensible design. This ensures that software components can be easily swapped and upgraded. Examples of this is multiple consensus mechanisms provided by Substrate, as listed below.
  3. Lastly, the final blockchain system created with the above properties needs to be upgradeable. In order to achieve this, Substrate is designed as a meta-protocol, whereby the application logic of the blockchain (called “Runtime”) is encoded as a WASM blob, and is stored in the state. The rest of the system (called “node”) acts as the executor of the WASM blob.

In essence, the meta-protocol of all Substrate based chains is the “Runtime as WASM blob” accord. This enables the Runtime to become inherently upgradeable, crucially without forks. The upgrade is merely a matter of the WASM blob being changed in the state, which is, in principle, same as updating an account’s balance. Learn more about this in detail in crate::reference_docs::wasm_meta_protocol.

A great analogy for substrate is the following: Substrate node is a gaming console, and a WASM runtime, possibly created with FRAME is the game being inserted into the console.

frame, Substrate’s default runtime development library, takes the above safety practices even further by embracing a declarative programming model whereby correctness is enhanced and the system is highly configurable through parameterization. Learn more about this in crate::reference_docs::trait_based_programming.

§How to Get Started

Substrate offers different options at the spectrum of technical freedom <-> development ease.

  • The easiest way to use Substrate is to use one of the templates (some of which listed at crate::polkadot_sdk::templates) and only tweak the parameters of the runtime or node. This allows you to launch a blockchain in minutes, but is limited in technical freedom.
  • Next, most developers wish to develop their custom runtime modules, for which the de-facto way is frame.
  • Finally, Substrate is highly configurable at the node side as well, but this is the most technically demanding.

A notable Substrate-based blockchain that has built both custom FRAME pallets and custom node-side components is https://github.com/Cardinal-Cryptography/aleph-node.

flowchart LR
	T[Using a Template] --> P[Writing Your Own FRAME-Based Pallet] --> C[Custom Node]

§Structure

Substrate contains a large number of crates, therefore it is useful to have an overview of what they are, and how they are organized. In broad terms, these crates are divided into three categories:

  • sc-* (short for Substrate-client) crates, located under ./client folder. These are all the crates that lead to the node software. Notable examples sc_network, various consensus crates, RPC (sc_rpc_api) and database (sc_client_db), all of which are expected to reside in the node side.
  • sp-* (short for substrate-primitives) crates, located under ./primitives folder. These are crates that facilitate both the node and the runtime, but are not opinionated about what framework is using for building the runtime. Notable examples are sp_api and sp_io, which form the communication bridge between the node and runtime.
  • pallet-* and frame-* crates, located under ./frame folder. These are the crates related to FRAME. See frame for more information.

§WASM Build

Many of the Substrate crates, such as entire sp-*, need to compile to both WASM (when a WASM runtime is being generated) and native (for example, when testing). To achieve this, Substrate follows the convention of the Rust community, and uses a feature = "std" to signify that a crate is being built with the standard library, and is built for native. Otherwise, it is built for no_std.

This can be summarized in #![cfg_attr(not(feature = "std"), no_std)], which you can often find in any Substrate-based runtime.

Substrate-based runtimes use substrate_wasm_builder in their build.rs to automatically build their WASM files as a part of normal build command (e.g. cargo build). Once built, the wasm file is placed in ./target/{debug|release}/wbuild/{runtime_name}.wasm.

§Binaries

Multiple binaries are shipped with substrate, the most important of which are located in the ./bin folder.

  • node_cli is an extensive substrate node that contains the superset of all runtime and node side features. The corresponding runtime, called [kitchensink_runtime] contains all of the modules that are provided with FRAME. This node and runtime is only used for testing and demonstration.
    • chain_spec_builder: Utility to build more detailed chain-specs for the aforementioned node. Other projects typically contain a build-spec subcommand that does the same.
  • node_template: a template node that contains a minimal set of features and can act as a starting point of a project.
  • subkey: Substrate’s key management utility.

§Anatomy of a Binary Crate

From the above, node_cli/[kitchensink_runtime] and node-template are essentially blueprints of a Substrate-based project, as the name of the latter is implying. Each Substrate-based project typically contains the following:

  • Under ./runtime, a ./runtime/src/lib.rs which is the top level runtime amalgamator file. This file typically contains the frame::runtime::prelude::construct_runtime and frame::runtime::prelude::impl_runtime_apis macro calls, which is the final definition of a runtime.

  • Under ./node, a main.rs, which is the starting point, and a ./service.rs, which contains all the node side components. Skimming this file yields an overview of the networking, database, consensus and similar node side components.

The above two are conventions, not rules.

See https://github.com/paritytech/polkadot-sdk/issues/5 for an update on how the node side components are being amalgamated.

§Parachain?

As noted above, Substrate is the main engine behind the Polkadot ecosystem. One of the ways through which Polkadot can be utilized is by building “parachains”, blockchains that are connected to Polkadot’s shared security.

To build a parachain, one could use Cumulus, the library on top of Substrate, empowering any substrate-based chain to be a Polkadot parachain.

§Where To Go Next?

Additional noteworthy crates within substrate: