Data availability

The guarantee that all data behind a block was actually published, so anyone can verify it independently and recover their funds.

• A rollup operator could post a valid-looking summary while withholding the transactions, leaving users unable to check it or exit.
• With 'sampling', data is encoded so any half rebuilds the whole; nodes fetch a few random pieces, and if all arrive, withholding is extremely unlikely.
• Ethereum added 'blobs' in 2024: cheap data space kept for about 18 days, long enough for anyone to check or copy it.
• Separate data networks such as Celestia are cheaper, but their security then rests on their own, usually smaller, validator set.