Optimistic Rollup vs ZK Rollup

The main difference between rollups and other scaling solutions is the presence of complete historical transaction data on the L1: users are always able to reconstruct the state of the rollup from Ethereum data alone.

There are two main rollup categories: optimistic and zero-knowledge. We opted to build the latter, a zero-knowledge rollup, as it is a better fit for Term Structure's long term vision for the following reasons:

Cost

Optimistic rollups have the advantage of even lower transaction fees than ZK rollups, as they do not require generating and verifying zero-knowledge proofs. Currently, verifying proofs onchain is expensive, and generating ZKPs is very slow and costly. We do however expect these costs to gradually decrease, as newer ZKP primitives continue being developed.

Validity

Rollups guarantee the validity of their state either via a fraud proof or a validity proof.

Optimistic rollups assume the published state to be correct, and use fraud proofs both to correct an invalid published state and as a measure to deter malicious actors from submitting invalid states. Conversely, ZK rollups use ZKPs to directly prove the validity of the rollup state: once such proof is verified onchain, the state of the rollup is final.

UX

Optimistic rollups have longer finality times, as the challenge period where fraud proofs can be presented takes several days. The lack of such challenge periods in ZK rollups presents a great UX advantage, as users do not have to wait as long for their transactions to achieve finality, or for their withdrawals to be processed.