When you submit a transaction on a shard, a validator will be responsible for adding your transaction to a shard block. Validators are algorithmically chosen by Ethereum: The Beacon Chain to propose new blocks.

  1. Attestation

    If a validator isn’t chosen to propose a new shard block, they’ll have to attest to another validator’s proposal and confirm that everything looks as it should. It’s the attestation that is recorded in the beacon chain rather than the transaction itself.

    • At least 128 validators are required to attest to each shard block – this is known as a “committee.”
    • The committee has a time-frame in which to propose and validate a shard block. This is known as a “slot.”

    Only one valid block is created per slot, and there are 32 slots in an “epoch.” After each epoch, the committee is disbanded and reformed with different, random participants. This helps keep shards safe from committees of bad actors.

  2. Crosslinks

    Once a new shard block proposal has enough attestations, a “crosslink” is created which confirms the inclusion of the block and your transaction in the beacon chain.

    Once there’s a crosslink, the validator who proposed the block gets their reward.

  3. Finality

    In distributed networks, a transaction has “finality” when it’s part of a block that can’t change.

    To do this in proof-of-stake, Casper, a finality protocol, gets validators to agree on the state of a block at certain checkpoints. So long as 2/3 of the validators agree, the block is finalised. Validators will lose their entire stake if they try and revert this later on via a 51% attack.