Collator
Info
This page provides a general overview of the role of collators in the Polkadot ecosystem. For more detailed information, read the Parachain Protocol Overview.
Collators' Role¶
Rollup Maintainers¶
Collators maintain rollup chains (parachains) by collecting user transactions and producing state transition proofs for Polkadot's validators. In other words, collators aggregate transactions into parachain block candidates and produce state transition proofs (Proof-of-Validity, PoV) for validators.
Collators maintain a full node for the relay chain and a full node for their particular parachain, meaning they retain all necessary information to author new parablocks and execute transactions like miners do on PoW blockchains. Under normal circumstances, they will collate and execute transactions to create an unsealed block and provide it with a PoV to one or more validators on a Polkadot core.
Collators are similar to validators on any other blockchain. Still, they do not need to provide security guarantees because the relay chain provides those as a service via the ELVES protocol. If a parachain block is invalid, validators will reject it.
Rollup Liveness and Censorship¶
Rollup safety (i.e., the insurance that invalid transactions are not included on the rollup) is achieved through validators on the relay chain implementing the ELVES protocol. But the onus of liveness (i.e., new blocks are eventually produced) and censorship resistance on the rollup is the responsibility of the collators. Hence, a sufficiently decentralised collator set ensures liveness and mitigates censorship resistance. Theoretically, a single honest collator can ensure liveness but with reduced throughput.
Cross-consensus Messaging¶
Collators are a key element of the XCM (Cross-Consensus Message Passing Format). By being full nodes of the relay chain, they are all aware of each other as peers. This makes it possible for them to send messages from parachain A to parachain B.
Collators in the Wild¶
Blockchains built using Substrate cannot hook onto the relay chain independently. The Parity team built the Cumulus library to address this. Collators are being used on the Paseo testnet, and you can learn more about how they are used with Cumulus via the Cumulus Rust documentation. More information can be found under the Cumulus section on the build parachain page.