Introduction to Library

In working with the node template, you don’t need to know anything about the underlying architecture or libraries being used because the basic components are already assembled and ready to use. However, if you want to design and build a custom blockchain, you might want to be familiar with the libraries available and know what these different libraries do.

In Architecture, you learned about the core components of a Substrate node and how different parts of the node take on different responsibilities. On a more technical level, the separation of duties between different layers of a node are reflected in the core libraries used to build a Substrate-based blockchain. The following diagram illustrates how the libraries mirror the outer node and runtime responsibilities and how a library of primitives provides the communication layer between the two.

Core node libraries for the outer node and runtime

Core node libraries

The libraries that enable a Substrate node to handle its network responsibilities, including consensus and block execution are Rust crates that use the sc_ prefix in the crate name. For example, the sc_service library is responsible for building the networking layer for Substrate blockchains, managing the communication between the network participants and the transaction pool.

The libraries that provide the communication layer between the outer node and the runtime are Rust crates that use the sp_ prefix in the crate name. These libraries orchestrate the activities that require the outer node and runtime to interact. For example, thesp_std library takes useful primitives from Rust's standard library and makes them usable with any code that depends on the runtime.

The libraries that enable you to build the runtime logic and to encode and decode the information passed into and out of the runtime are Rust crates that use the frame_ prefix in the crate name. The frame_* libraries provide the infrastructure for the runtime. For example, the frame_system library provides a basic set of functions for interacting with other Substrate components andframe_support enables you to declare runtime storage items, errors, and events.

In addition to the infrastructure provided by the frame_* libraries, the runtime can include one or more pallet_* libraries. Each Rust crate that uses the pallet_ prefix represents a single FRAME module. In most cases, you use the pallet_* libraries to assemble the functionality you want to incorporate in the blockchain to suit your project.

You can build a Substrate runtime without using the frame_* or pallet_* libraries using the primitives exposed by the sp_* core libraries. However, the frame_* or pallet_* libraries provide the most efficient path to composing a Substrate runtime.

Modular architecture

The separation of the core libraries provides a flexible and modular architecture for writing the blockchain logic. The primitives library provides a foundation that both the outer node and the runtime can build on without communicating directly with each other. Primitive types and traits are exposed in their own separate crates, so they are available to the outer node and runtime components without introducing cyclic dependency issues.

Front-end libraries

In addition to the core libraries that enable you to build a Substrate-based blockchain, there are client libraries that you can use to interact with Substrate nodes. You can use the client libraries to build application-specific front-ends. In general, the capabilities that the client libraries expose are implemented on top of Substrate remote procedure call (RPC) APIs. For more information about using metadata and front-end libraries to build applications, see Application development.

Where to go next

Now that you are familiar with the libraries used to build and interact with Substrate nodes, you might want to explore the libraries in more depth. To learn more about the technical details for any library, you should review the Rust API documentation for that library.