- A hybrid approach to scaling, one that doesn't put calldata on-chain and instead takes some compromises in security in order to increase scalability.
- This approach is no longer considered a 'true L2' where the definition is a scaling network that inherits the same security guarantees of the network it is built on top of.
- Validiums publish zero-knowledge proofs to verify off-chain transactions on Ethereum. This prevents invalid state transitions and enhances the security guarantees of a validium chain.
- These "validity proofs" can come in the form of ZK-SNARKs (Zero-Knowledge Succinct Non-Interactive Argument of Knowledge) or ZK-STARKs (Zero-Knowledge Scalable Transparent ARgument of Knowledge). More on zero-knowledge proofs
- A validium is a type of scaling solution that utilizes validity proofs but has off-chain data availability. It compromises Ethereum security, however is still much more secure than a sidechain since the state transitions have verified validity through the use of STARKs/SNARKs.
- Currently, validium based solutions only work for specialized use cases and are not universally compatible with execution targets like the EVM or WASM, however with recent progress by teams like Starkware and zkSync, this will be possible in the near future.
- For a more in-depth comparison of zk-rollups and validium, read zkSync's comparison.
Volition is an architecture (pioneered by Starkware) that an L2 can adopt where the user can choose whether to use a validium or a zk-rollup on the L2 on a per transaction basis. This would allow the user to specify whether he wants to maximize decentralization and security or scalability within the same L2; this architecture is getting a lot of traction and is set to be an integral design decision for the Starknet and zkSync 2.0 L2s as well as other validity proof based solutions in the future.