Polkadot — An Early In-Depth Analysis — Part Two — How Consensus Works

Collators

Collators maintain a “full-node” for a particular parachain; meaning they retain all necessary information to be able to author new blocks and execute transactions. Under normal circumstances, they will collate and execute transactions to create an unsealed block and provide it, together with a proof of state transition, to one or more validators responsible for proposing a parachain block. Collators will also watch the progress of block-producing and consensus protocols in BABE (explained later) and build on what they think is the latest relay chain block that will be finalised. Collators do not directly participate in the consensus for the relay chain and therefore never stake DOT. Each parachain will likely have their own token with incentives provided to collators to run on their parachain.

Validators

Validators secure the Relay Chain by staking DOT, validating proofs from collators and participating in consensus with other validators.

Nominators

A nominator is a stake-holding party who contributes to the security bond of a validator. They have no additional role except to place risk capital and as such to signal that they trust a particular validator (or set thereof) to act responsibly in their maintenance of the network. They receive a pro-rata increase or reduction in their deposit according to the bond’s growth to which they contribute.

If a validator does not behave properly, they will get slashed and nominators will also lose DOT as well. For attacks this can result in 100% of the stake being slashed, meaning you could lose all of your DOT, so choose wisely.

https://wiki.polkadot.network/docs/en/learn-nominator

Fishermen

fishermen are not directly related to the block authoring process. Rather they are independent “bounty hunters” motivated by a large one-off reward. Fishermen are checking proposed blocks in every part of the network. Ideally, some fishermen focus on one parachain, another group tackles another parachain and a group of fishermen checks relay chain validators. If they find a compromised block, then the creator of the block gets slashed and the fisherman receives a portion of this slashed stake as reward. Fishermen are particularly important when there is a group of validators colluding and proposing a compromised parachain block. Fishermen are required to stake a small amount of DOT to prevent sybil attacks from wasting validators’ time and compute resources.

How Consensus works

Collators are selected by each parachain to produce the next block for the parachain. The collator will watch the progress of block-producing and consensus protocols in BABE and build on what they think is the latest relay chain block that will be finalised. They will collate and execute transactions for the parachain to create an unsealed block and provide it, together with a proof of state transition, to one or more validators responsible for proposing a parachain block.

  • The block (list of state transitions),
  • The values in the parachain’s database that the block modifies, and
  • The hashes of the unaffected points in the Merkle tree.
  • The parachain ID
  • The collator’s ID and signature
  • A hash of the parent block’s candidate receipt
  • A Merkle root of the block’s erasure coding
  • A Merkle root of any outgoing messages
  • A hash of the block
  • The state root of the parachain before executing the block
  • The state root of the parachain after executing the block
https://polkadot.network/the-path-of-a-parachain-block/

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