Validium
A Layer 2 scaling design that uses validity proofs for execution correctness but keeps transaction data off-chain, trading some data-availability guarantees for lower cost.
A validium is a scaling architecture that, like a zk-rollup (see Kaavon's "Optimistic Rollup" and "zk-SNARK" glossary definitions for related rollup concepts), uses validity proofs to cryptographically guarantee that its off-chain computation was executed correctly. The key difference is where the underlying transaction data lives: a zk-rollup posts that data to the base layer (ensuring anyone can reconstruct account balances), while a validium keeps it off-chain, typically held by a separate data-availability committee or system.
This design gives a validium meaningfully lower costs and higher throughput than a comparable rollup, since it isn't paying to publish data on the base chain — but it introduces a data-availability risk a rollup doesn't have: if the entity holding the off-chain data disappears or withholds it, users could be unable to prove their own balances or exit the system, even though the validity proofs confirm no fraudulent state transition occurred. Some systems offer a hybrid "volition" mode, letting users choose per-transaction between rollup-style on-chain data and validium-style off-chain data based on their own risk and cost tradeoff.
Examples
- StarkEx-based applications have offered a validium mode for use cases like gaming or high-frequency trading, where lower fees matter more than rollup-grade data-availability guarantees.
- A validium's validity proofs rule out invalid state transitions, but they can't by themselves rule out an off-chain data-availability failure.