Relay Chain

The relay chain is the main chain at the centre of the Polkadot network. It does not run applications or smart contracts itself. Instead it keeps the network's shared state, organises the validators who secure the system and checks the blocks produced by the specialised blockchains attached to it, called parachains. It also carries messages between those chains, so that tokens and data can move from one to another.

The design was set out in the Polkadot whitepaper written by Gavin Wood, a co-founder of Ethereum, in 2016, and the network launched in 2020. The native token, DOT, is used for staking, governance and paying for access to the relay chain, and is described under Polkadot (DOT). The relay chain is an example of a "shared security" model, in which many chains rely on one validator set instead of each recruiting its own.

Think of a busy airport. Each airline runs its own planes, timetables and ticket desks, but they all rely on the same control tower, runways and security checks. No airline has to build its own tower, and passengers can change from one airline to another inside the same terminal.

The relay chain is that control tower. Each parachain is an airline with its own rules and services, while the relay chain checks that everyone is following the safety rules and helps passengers, in this case tokens and messages, move between them.

Polkadot is often described as a sharded, multi-chain system. Its main parts are:

• The relay chain. A deliberately minimal chain that records staking, governance and the state of each connected chain, and finalises their blocks.
• Parachains. Independent blockchains, each with its own logic and often its own token, that connect to the relay chain and inherit its security. They can be built for specific uses such as DeFi, identity or smart contracts.
• Collators. Nodes run for each parachain that gather its transactions and produce candidate blocks with proofs for the relay chain's validators to check.
• Bridges. Connections to outside networks such as Ethereum or Bitcoin, built as parachains or dedicated modules.

Messages between parachains use a format called XCM (cross-consensus messaging), which lets one chain ask another to transfer assets or run an action. This supports a form of blockchain interoperability that relies on the relay chain rather than on separate bridge operators.

The relay chain uses nominated proof of stake. DOT holders, known as nominators, back validators with their stake, and the protocol selects an active validator set from the most-backed candidates. Block production uses a mechanism called BABE, and finality is provided separately by a gadget called GRANDPA, which can finalise many blocks at once. This split lets the chain keep producing blocks even when finality is delayed.

Validators are assigned in small, rotating groups to check parachain blocks, and further random checks catch any group that approves an invalid block. Validators who misbehave face slashing of their own and their nominators' stake. Because all parachains share one validator set, a smaller chain gets the same security as the network as a whole, rather than depending on its own token's value. The approach is a form of sharding, with each parachain acting like a shard that has its own rules.

At first, parachains won the right to connect through slot auctions, starting in late 2021, in which projects locked large amounts of DOT for up to two years, often raised from supporters through a crowdloan. The auctions were criticised as expensive and slow for new teams. From 2024 Polkadot moved to "agile coretime", in which projects buy blockspace on the relay chain in shorter periods or on demand, an approach compared to renting cloud computing instead of buying a server. The relay chain can be upgraded through on-chain governance without a hard fork, because the chain's rules are stored as code that DOT holders can vote to replace. Kusama, a separate network with faster governance, is used as a live testing ground for new features.

Critics point to the complexity of the design, the cost and lock-up demanded by the original auctions and relatively low usage of many parachains compared with the attention they received. Others note that Ethereum's rollup ecosystem and the Cosmos approach compete for the same developers. Cosmos originally let each chain run its own validators, then added a shared model called replicated security, which narrows the gap between the two designs. Supporters argue that the relay chain's single validator set gives stronger guarantees than bridges between independently secured chains, which have been a frequent source of hacks.

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• Polkadot Wiki