Rollups and Scalability
Rollups bundle many transactions, prove or assume their validity, and publish minimal data back to Layer 1. You'll compare optimistic and ZK rollups to see how each scales throughput.
Who Is This For?
- •Users curious why rollups lower gas costs
- •Builders deciding which L2 architecture to target
Learning Objectives
- 01Explain optimistic vs zero-knowledge rollups
- 02Identify trade-offs around speed, cost, and security
- 03Assess when a rollup is appropriate for your use case
Rollup Basics
Rollups process transactions off-chain but keep data on L1. This combination gives you L2 speed with L1 security—the best of both worlds.
The Rollup Flow
Try It: Transaction Batching
See how rollups compress many transactions into minimal L1 calldata. Click "Generate" then "Compress" to see the magic.
Sequencer
Orders transactions and submits batches. Usually centralized for now, with decentralization coming.
Data Availability
Transaction data posted to L1 so anyone can reconstruct state and verify correctness.
L1 Settlement
Final state roots anchored on Ethereum. Inherits L1's security guarantees.
Optimistic vs ZK Rollups
The key difference is how they prove transactions are valid. Optimistic rollups assume validity and allow challenges. ZK rollups prove it mathematically upfront.
🟡 Optimistic Rollups
Optimistic rollups assume all transactions are valid by default ("optimistic"). During a challenge period, anyone can submit a fraud proof if they detect invalid state transitions.
Simulation: Optimistic Flow
Advantages
- •Full EVM compatibility
- •Mature tooling ecosystem
- •Lower computational overhead
- •Battle-tested (Arbitrum, Optimism)
Trade-offs
- •7-day withdrawal period
- •Relies on active fraud watchers
- •Slower finality to L1
Side-by-Side Comparison
| Attribute | Optimistic | ZK |
|---|---|---|
| Proof Method | Fraud proofs (if challenged) | Validity proofs (always) |
| Withdrawal Time | ~7 days | Minutes to hours |
| EVM Compatibility | Full | Varies (zkEVM improving) |
| Proof Cost | Only if disputed | Every batch |
| Finality | After challenge period | Once proof verified |
| Maturity | More battle-tested | Rapidly evolving |
Developer & User Considerations
Choosing a rollup involves weighing fees, finality, and ecosystem maturity. Here's what to consider for your use case.
Fees
- •Lower than L1, but not zero
- •Depend on L1 calldata costs
- •Spike during L1 congestion
- •Compare across rollups
Finality
- •Optimistic: UX fast, L1 finality slow
- •ZK: L1 finality once proof posted
- •Consider your withdrawal needs
- •Fast bridges available (with fees)
Ecosystem
- •Check wallet support
- •Verify DEX liquidity
- •Test on testnets first
- •Review SDK documentation
Choose Optimistic Rollups When:
- •You need full EVM compatibility today
- •Deploying existing Solidity contracts
- •Ecosystem maturity matters (DeFi integrations)
- •7-day withdrawals are acceptable
Choose ZK Rollups When:
- •Fast withdrawals are critical
- •Building new applications from scratch
- •Privacy features are needed
- •Willing to adapt to zkEVM differences
Common Mistakes & Gotchas
These misconceptions about rollups can lead to unexpected costs or frustrating experiences.
Pro tip: Always test on testnet first. Get free testnet ETH from faucets, try bridging, swapping, and deploying contracts before committing real funds.
Knowledge Check
Let's see how well you understand rollups. Answer all 5 questions below.
What keeps rollup transactions verifiable?
How does an optimistic rollup handle fraud?
Name one ZK rollup project.
Why do ZK rollups have faster withdrawals than optimistic rollups?
What role does the sequencer play in a rollup?