Data Availability

Data availability is the guarantee that the transaction data behind a block has actually been published, so anyone can download it and independently check the chain's state. It matters most for rollups: if a Layer 2 posts only a summary or a proof but withholds the underlying data, users may be unable to reconstruct their balances or exit without the operator's cooperation. Ethereum addresses this by making rollups publish data on the base chain, including in blobs, and by sampling techniques that let light nodes verify availability without downloading everything. Some networks instead use a separate data availability layer or committee, which is cheaper but shifts trust away from the base chain.