Disponibilité des données et validiums

You have studied rollups and bridges. A subtler topic decides how secure a Layer 2 really is: data availability (DA). It is the guarantee that the data needed to reconstruct L2 state is actually published and retrievable.

Rollups post transaction data so anyone can verify or rebuild the chain. But where is that data stored, and can users trust it is available?

If data is withheld, users cannot prove their balances or exit, even if the computation was correct.

A standard rollup publishes compressed transaction data to Ethereum L1 as calldata or blobs. Because L1 stores it, DA is inherited from Ethereum's strong guarantees. This is the most secure but also the most expensive option.

Posting data to L1 is the biggest cost component of a rollup. EIP-4844 introduced blobs, a cheaper, temporary data space, to reduce this. Still, some systems want even lower costs and move DA off-chain.

A Validium uses validity (ZK) proofs for correctness like a ZK-rollup, but keeps transaction data off-chain. Only the proof and state root go to L1. This slashes cost but introduces a data availability assumption.

Because data is off-chain, if the off-chain provider withholds it, users may be unable to compute their state to exit, even though the proofs guarantee no invalid state transition occurred.

• Funds cannot be stolen (proofs prevent that)
• But funds could be frozen if data is withheld

Many Validiums rely on a Data Availability Committee (DAC): a set of known parties that sign attestations that they hold the data. Security depends on at least one honest committee member providing data on request.

A Volition lets each user choose per transaction whether data goes on-chain (rollup mode, higher security, higher cost) or off-chain (Validium mode, lower cost). Users pick their trust-cost balance.

Newer DA layers use sampling: light nodes download small random pieces of the data. With erasure coding, if enough samples succeed, the full data is almost certainly available, without anyone downloading all of it.

Projects like Celestia and EigenDA provide DA as a standalone service. Rollups post data there instead of Ethereum L1 to cut costs while keeping stronger guarantees than a small committee. This is the basis of many modular blockchain designs.

For high-value DeFi, full on-chain DA (true rollup) is safest. For high-throughput, low-value apps like gaming, Validiums or dedicated DA layers may be acceptable. Always understand who guarantees your data is available.

Test your understanding of data availability.

You learned about data availability:

• Rollups post data on-chain for strong DA at higher cost
• Validiums move data off-chain via committees, cheaper but with a DA assumption
• Volitions let users choose per transaction
• Sampling and dedicated DA layers offer modular tradeoffs

DA is a key dimension of Layer 2 security.