Data Availability

Validity and availability are different guarantees, and conflating them is where the risk hides. A rollup can prove its state transitions were computed correctly and still leave users stranded, because a proof that a state is correct is useless if nobody can obtain the data needed to reconstruct that state and exit.

This is why where a chain publishes its data matters more than how it proves its work. Rollups posting to Ethereum originally paid for permanent calldata. EIP-4844, live with the Dencun upgrade in March 2024, introduced blobs, a cheaper channel that nodes prune after roughly eighteen days. The pruning is deliberate, since the window only has to be long enough for anyone who cares to download the data and verify it, not forever.

The weaker designs move that data off-chain entirely. Validiums and rollups backed by a data availability committee replace a public guarantee with a promise from a named set of parties. If that committee withholds data, honest users cannot construct the proofs they need to withdraw, and funds sit behind a contract that is working exactly as written.

The question worth asking of any chain holding your assets is narrow. Where does it publish its data, and can you exit without the operator's cooperation?