Data Availability
The guarantee that the data behind a transaction batch is actually published and retrievable — a subtle but critical security pillar.
Data availability is the guarantee that the underlying data for a batch of transactions is genuinely published and accessible, so anyone can verify the chain’s state and, if needed, reconstruct it or challenge fraud. It sounds mundane but is a linchpin of Layer 2 security: a rollup that posts a valid proof but withholds the data leaves users unable to prove their balances or exit.
This is why Ethereum’s scaling roadmap focuses on cheap data availability — the EIP-4844 “blob” upgrade created dedicated, low-cost space for rollup data. “Data availability layers” (like Celestia) specialize in this function within modular designs. When evaluating a rollup’s security, where and how it guarantees data availability is as important as its proof system.