Data Availability
The guarantee that the data behind a block or batch is published and retrievable, so anyone can verify or rebuild state.
A rollup can post a valid proof, but if it withholds the underlying transaction data, users cannot reconstruct their balances or exit. That failure mode is what data availability addresses. The question is not whether a state is correct, but whether everyone can actually see the data needed to check it.
Posting all data to Ethereum is secure but expensive. Cheaper options exist. Data availability committees vouch for the data off-chain, while dedicated layers like Celestia use sampling so light nodes can confirm availability without downloading everything. EIP-4844 added blob space in 2024 to cut this cost for rollups.
Related terms
A design that splits a blockchain's core jobs across separate specialized layers instead of handling them all in one chain.
RollupA scaling method that batches many transactions off-chain and posts a compressed proof or record to the base layer.
zk-RollupA rollup that posts a cryptographic validity proof for every batch, giving near-instant finality on the base chain.
NodeA computer running a blockchain's software, keeping a copy of the ledger and relaying transactions to peers.