Data Availability
The guarantee that transaction data has been published and is accessible to anyone who needs to reconstruct or verify the blockchain state.
Data availability (DA) is the property that the data needed to verify and reconstruct a blockchain's state is publicly accessible. In rollup architecture, this is critical: if the sequencer posts a transaction batch to Ethereum but withholds the underlying transaction data, users cannot reconstruct the L2 state, prove fraud, or withdraw funds. DA ensures that data is available — not just that it was published.
Ethereum has been the primary DA layer for rollups, with transaction data posted as calldata (historically) or blobs (post EIP-4844). EIP-4844 introduced a dedicated data blob space that is cheaper than calldata and expires after ~18 days — since validators only need to verify DA for a finite period, not store it forever. This reduced L2 transaction costs dramatically.
Alternative DA layers — Celestia, EigenDA, Avail — provide cheaper data availability by specialising in storage and availability rather than consensus and execution. 'Validiums' use ZK proofs for computation validity but post data off-chain to a trusted committee, trading the DA guarantee for lower costs. The security trade-off between on-chain DA (Ethereum) and off-chain DA (Validium/committee) is a key design decision for rollup teams.