Light Clients¶
Introduction¶
Light clients enable secure and efficient blockchain interaction without running a full node. They provide a trust-minimized alternative to JSON-RPC by verifying data through cryptographic proofs rather than blindly trusting remote nodes.
This guide covers:
- What light clients are and how they work
- Their advantages compared to full nodes and JSON-RPC
- Available implementations in the Polkadot ecosystem
- How to use light clients in your applications
Light clients are particularly valuable for resource-constrained environments and applications requiring secure, decentralized blockchain access without the overhead of maintaining full nodes.
Light node or light client?
The terms light node and light client are interchangeable. Both refer to a blockchain client that syncs without downloading the entire blockchain state. All nodes in a blockchain network are fundamentally clients, engaging in peer-to-peer communication.
Light Clients Workflow¶
Unlike JSON-RPC interfaces, where an application must maintain a list of providers or rely on a single node, light clients are not limited to or dependent on a single node. They use cryptographic proofs to verify the blockchain's state, ensuring it is up-to-date and accurate. By verifying only block headers, light clients avoid syncing the entire state, making them ideal for resource-constrained environments.
flowchart LR
DAPP([dApp])-- Query Account Info -->LC([Light Client])
LC -- Request --> FN(((Full Node)))
LC -- Response --> DAPP
FN -- Response (validated via Merkle proof) --> LCIn the diagram above, the decentralized application queries on-chain account information through the light client. The light client runs as part of the application and requires minimal memory and computational resources. It uses Merkle proofs to verify the state retrieved from a full node in a trust-minimized manner. Polkadot-compatible light clients utilize warp syncing, which downloads only block headers.
Light clients can quickly verify the blockchain's state, including GRANDPA finality justifications.
What does it mean to be trust-minimized?
Trust-minimized means that the light client does not need to fully trust the full node from which it retrieves the state. This is achieved through the use of Merkle proofs, which allow the light client to verify the correctness of the state by checking the Merkle tree root.
JSON-RPC and Light Client Comparison¶
Another common method of communication between a user interface (UI) and a node is through the JSON-RPC protocol. Generally, the UI retrieves information from the node, fetches network or pallet data, and interacts with the blockchain. This is typically done in one of two ways:
- User-controlled nodes - the UI connects to a node client installed on the user's machine
- These nodes are secure, but installation and maintenance can be inconvenient
- Publicly accessible nodes - the UI connects to a third-party-owned publicly accessible node client
- These nodes are convenient but centralized and less secure. Applications must maintain a list of backup nodes in case the primary node becomes unavailable
While light clients still communicate with full nodes, they offer significant advantages for applications requiring a secure alternative to running a full node:
| Full Node | Light Client |
|---|---|
| Fully verifies all blocks of the chain | Verifies only the authenticity of blocks |
| Stores previous block data and the chain's storage in a database | Does not require a database |
| Installation, maintenance, and execution are resource-intensive and require technical expertise | No installation is typically included as part of the application |
Using Light Clients¶
The smoldot client is the cornerstone of light client implementation for Polkadot SDK-based chains. It provides the primitives needed to build light clients and is also integrated into libraries such as PAPI.
PAPI Light Client Support¶
The Polkadot API (PAPI) library natively supports light client configurations powered by smoldot. This allows developers to connect to multiple chains simultaneously using a light client.
Substrate Connect - Browser Extension¶
The Substrate Connect browser extension enables end-users to interact with applications connected to multiple blockchains or to connect their own blockchains to supported applications.
Establishing a sufficient number of peers can be challenging due to browser limitations on WebSocket connections from HTTPS pages, as many nodes require TLS. The Substrate Connect browser extension addresses this limitation by keeping chains synced in the background, enabling faster application performance.
Substrate Connect automatically detects whether the user has the extension installed. If not, an in-page Wasm light client is created for them.
Resources¶
- What is a light client and why you should care?
- Introducing Substrate Connect: Browser-Based Light Clients for Connecting to Substrate Chains
- Substrate Connect GitHub Repository
- Light Clients - Polkadot Specification
| Created: May 13, 2025