Crypto glossary
Data availability
Data availability is the guarantee that the data behind a block has been published so anyone can check it, a key requirement for rollups and light clients.
A block header alone proves little; to verify transactions, or to challenge an invalid one, nodes need the full underlying data. Data availability means that data was actually published and can be downloaded by anyone. If a block producer withholds it, others cannot detect fraud or rebuild the state, even if the header looks valid.
The issue matters most for rollups, which execute transactions off the main chain but post their data back to a base layer so users can always exit. Ethereum added cheaper blob space for this in March 2024, while specialized data availability layers such as Celestia and EigenDA sell the same service. Techniques like data availability sampling let light nodes check availability by downloading only small random pieces.
Cheaper external data layers lower fees but add trust assumptions. If an L2's data sits off Ethereum and becomes unavailable, users may be unable to prove their balances and withdraw.
Example
A rollup posts a batch of 2,000 transactions as 120 KB of data to Ethereum; anyone can download it and recompute every balance, even if the rollup operator disappears.
