Rollups are the leading scalability solution for Ethereum and other blockchains. They process transactions off-chain while posting minimal data on-chain — reducing fees and improving throughput without sacrificing security.
There are two main types of rollups: Optimistic Rollups and Zero-Knowledge (ZK) Rollups. Both aim to scale Ethereum, but they differ in how they prove transactions are valid.
📦 What Is a Rollup?
A rollup “rolls up” many transactions into a single compressed batch, then submits that batch to Ethereum for final settlement. This drastically reduces gas usage per transaction while keeping the base layer as the ultimate source of truth.
In simple terms:
Rollups inherit Ethereum’s security while offering near-L2 speeds and lower fees.
⚙️ How Rollups Work
- Users send transactions to the rollup chain.
- The rollup processes them off-chain and periodically posts a batch summary on Ethereum.
- Depending on the rollup type, either fraud proofs or validity proofs are used to verify correctness.
🔍 Optimistic Rollups Explained
Optimistic Rollups assume all transactions are valid by default (“optimistically”) and only verify them if someone challenges a batch during a set dispute window (typically 7 days).
Fraud Proofs
- If a user suspects fraud, they can submit a fraud proof showing the invalid transaction.
- If the challenge succeeds, the rollup reverts the invalid batch and penalizes the dishonest sequencer.
This system prioritizes simplicity and Ethereum compatibility but delays finality because of the challenge period.
| Optimistic Rollup | Examples |
|---|---|
| Arbitrum | Ethereum L2 using multi-round fraud proofs |
| Optimism | Simpler fraud proofs; retroactive public goods funding |
| Base | Coinbase’s L2 built with the OP Stack |
🧮 ZK-Rollups Explained
ZK-Rollups use advanced cryptography to generate validity proofs (zero-knowledge proofs) that confirm every transaction in a batch is valid. These proofs are posted on-chain, giving Ethereum instant mathematical assurance of correctness.
Validity Proofs
- Each batch includes a succinct proof verified directly by Ethereum’s smart contract.
- No waiting period — transactions finalize almost instantly once the proof is verified.
The trade-off: ZK systems are more complex and computationally expensive to build, but they offer faster finality and stronger security guarantees.
| ZK-Rollup | Examples |
|---|---|
| zkSync Era | Ethereum-compatible ZK-rollup focused on UX and speed |
| Starknet | Uses STARK proofs for scalable, censorship-resistant computation |
| Polygon zkEVM | Fully EVM-equivalent ZK-rollup with instant proof verification |
⚖️ Rollups Compared
| Feature | Optimistic Rollups | ZK-Rollups |
|---|---|---|
| Security Model | Fraud proofs (dispute-based) | Validity proofs (mathematical verification) |
| Finality | ~7 days (challenge window) | Instant after proof verification |
| Cost | Lower computation, higher withdrawal delay | Higher computation, faster settlement |
| UX | Slower withdrawals to L1 | Faster and more seamless bridging |
| EVM Compatibility | Full (easy dApp migration) | Improving (depends on zkEVM support) |
🚀 The Future of Rollups
Both rollup types are evolving quickly. Optimistic rollups dominate today due to simplicity and maturity, but ZK-rollups are catching up fast with breakthroughs in zkEVMs and proving efficiency.
- Optimism’s OP Stack is powering multiple rollups (Base, World Chain).
- ZK-rollups are moving toward universal compatibility with existing smart contracts.
- Hybrid models may combine optimistic and ZK verification for best performance.
Rollups are Ethereum’s path to scalability — where security meets speed, and every transaction stays trustless.
Written by BitBlog — helping you understand the technologies scaling Ethereum for the next billion users.