Original source: Offchain Labs
The debate about whether ZK Rollups represent the future of general-purpose smart contract systems has been exhaustively discussed. However, based on our practical experience operating hundreds of DApps with hundreds of thousands of users and millions of transactions on an open, secure, EVM-compatible Layer 2 solution, we've concluded that ZK Rollups won't become the standard for general smart contract systems. Here's why.
Key Takeaways
- Users demand trustless blockchains offering security, guaranteed progress, visibility, and fast finality - all while maintaining low costs and compatibility with existing tools.
- Optimistic Rollups deliver these features more effectively than ZK Rollups when examining implementation details.
- Optimistic Rollups provide desired functionality at lower costs because ZK proof generation carries significant off-chain expenses.
- ZK proofs' expense may necessitate specialized hardware and/or massive parallelism, potentially centralizing networks.
- ZK Rollup's purported advantages either exist in Optimistic Rollups or require sacrificing crucial security/availability features.
- Ultimately, Optimistic Rollups hold operational cost advantages since code execution proves significantly cheaper than complex cryptographic proofs.
Understanding Rollup Fundamentals
Ethereum transactions represent requests for smart contracts to perform actions - record information, move assets, etc. When transactions publish to Ethereum's blockchain, two critical events occur:
- Consensus forms around an ordered transaction set
- Ethereum executes these transactions to compute resulting state updates
Rollups dramatically reduce network load by executing transactions off-chain (Layer 2) while maintaining Ethereum's security guarantees through specialized proofs.
Proof Mechanisms: The Core Difference
Rollups differ primarily in their proof approaches:
- ZK Rollups use validity proofs requiring complex cryptographic operations to construct succinct proofs verified by Layer 1 contracts.
- Optimistic Rollups use fraud proofs, optimistically assuming correctness unless challenged. Disputes resolve via challenge protocols managed by Layer 1 contracts.
Cost Comparison: The Decisive Factor
The most critical distinction lies in cost structures:
- Optimistic Rollup: Nodes simply execute contract operations (e.g., addition)
- ZK Rollup: Requires generating expensive cryptographic proofs involving hundreds of elliptic curve operations per instruction
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While ZK advocates argue only one party must create proofs, practical blockchains require many nodes regardless of proof system. Optimistic Rollups leverage nodes' existing transactional work, while ZK Rollups add enormous proof-generation overhead.
EVM Compatibility: Developer Experience Matters
EVM represents blockchain's most robust toolset, offering:
- Immediate compatibility with existing codebases
- Access to Ethereum's developer/tool ecosystem
- Simplified migration paths for projects
Optimistic Rollups like Arbitrum achieve full EVM compatibility, whereas many ZK systems:
- Lack critical opcode support (ADDMOD, SMOD, etc.)
- Consider removing basic operations (XOR, AND, OR)
- Don't support standard transaction formats
- May limit contract call depths
Trustless Visibility & Compression
Optimistic Rollups provide trustless visibility - anyone can reconstruct complete chain history without centralized help. Some ZK systems misleadingly tout "compression" while actually withholding blockchain data, compromising visibility guarantees.
Trustless, Timely Finality
Effective rollups must offer trustless timely finality - submitted transactions become quickly and irrevocably settled. Optimistic Rollups achieve this by:
- Frequently publishing ordered transaction data to Layer 1 (~1 minute intervals)
- Separating transaction ordering from execution
- Maintaining deterministic execution
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ZK Rollups face impractical choices:
- Frequent proofs: Prohibitively expensive (500K-5M gas per proof)
- Infrequent proofs: Unacceptably slow finality (~1 hour)
Trustless Liveness
Optimistic Rollups let any node advance the chain by executing transactions and posting bonds. ZK Rollups require specialized hardware for proof generation, potentially centralizing progress.
Bridging Realities
While ZK Rollups enable slightly faster Layer 1 withdrawals, Optimistic Rollups match this via:
- Rapid bridging services
- Superior multichain support
- Established ecosystems (Arbitrum hosts hundreds of DeFi DApps)
Conclusion
After comprehensive comparison, Optimistic Rollups clearly outperform ZK Rollups by:
- Offering lower costs
- Delivering full EVM/tool compatibility
- Maintaining blockchain visibility
- Ensuring trustless progress
ZK Rollup's theoretical advantages often require sacrificing critical functionality. Furthermore, ZK's structural limitations seem fundamental - EVM-compatible ZK proofs will likely remain substantially more expensive than Optimistic execution indefinitely.
FAQ
Q: Can ZK Rollups eventually overcome their cost disadvantages?
A: The cryptographic overhead for ZK proofs appears fundamentally unavoidable for general computations, making significant cost parity unlikely.
Q: How does Arbitrum's Nitro upgrade improve Optimistic Rollups?
A: Nitro enhances onchain data compression and reduces transaction costs while maintaining all security guarantees.
Q: Are there any use cases where ZK Rollups excel?
A: Yes - specialized applications like private payments (Zcash) or simple token transfers can benefit from ZK's specific strengths.
Q: How do Optimistic Rollups handle malicious actors?
A: Fraud proofs and challenge protocols guarantee correct outcomes, with dishonest parties losing staked funds.
Q: What about ZK's "quantum resistance" advantage?
A: While theoretically valid, quantum computers capable of breaking current cryptography remain speculative, giving time for Optimistic Rollups to adapt if needed.
Q: Doesn't ZK's faster withdrawal matter for DeFi?
A: Modern bridging services make this difference negligible for practical applications, especially considering Arbitrum's thriving ecosystem.