A rollup is a Layer 2 scaling solution that executes transactions off the main blockchain, compresses them into a batch, and posts the result back to the base layer for security. Rollups inherit the security of the underlying chain — typically Ethereum — while dramatically reducing gas fees and increasing transaction throughput.
How Rollups Work
Instead of processing each transaction on the expensive base layer, a rollup:
- Collects many user transactions off-chain into a sequencer.
- Executes those transactions and computes the resulting state.
- Compresses the transaction data (or just the proof) and posts it to Layer 1.
- Inherits security from Layer 1 by making it possible for anyone to detect or disprove fraud.
The base chain only needs to verify a small amount of data per batch rather than run every transaction, which is why rollups scale so effectively.
Types of Rollups
There are two main rollup architectures, differing in how they prove correctness:
Optimistic Rollups
ZK-Rollups
| Feature | Optimistic Rollup | ZK-Rollup |
|---|---|---|
| Proof type | Fraud proof (after the fact) | Validity proof (upfront) |
| Withdrawal delay | 7 days (challenge period) | Near-instant |
| Computation cost | Lower (no proof generation) | Higher (proof generation) |
| EVM compatibility | High (EVM-equivalent) | Improving (zkEVM) |
Data Availability
Rollups must post transaction data somewhere so anyone can reconstruct state and detect fraud. Most rollups post calldata or blobs to Ethereum. EIP-4844 (Dencun upgrade, 2024) introduced blob transactions that dramatically cut the cost of this data posting, reducing L2 fees by 80–95%.
Some rollups use alternative data availability layers (like Celestia or EigenDA) instead of Ethereum — these are sometimes called validiums or optimiums depending on the proof type.
History
- 2020 — Optimism and Arbitrum launch testnets, pioneering the optimistic rollup model.
- 2021 — Arbitrum One goes live on mainnet; Optimism follows. Both attract billions in TVL.
- 2021 — zkSync and StarkNet launch on mainnet, proving ZK-rollup viability.
- 2023 — Base (by Coinbase) launches on the OP Stack, validating rollup-as-a-service.
- 2024 — EIP-4844 (Dencun) ships, cutting blob data costs and enabling sub-cent fees across major rollups.
- 2024–2025 — Rollup explosion. Dozens of app-specific rollups (appchains) launch on the OP Stack and Arbitrum Orbit.
Why Rollups Matter
Ethereum processes ~15–30 transactions per second on L1. Rollups bring that to hundreds or thousands of TPS while keeping assets secured by Ethereum’s validator set. This makes rollups the dominant scaling path for Ethereum and the primary home of DeFi, NFTs, and gaming activity today.
Common Misconceptions
“Rollups are separate blockchains.”
Rollups are not independent — they settle to and inherit security from their base layer. If Ethereum finalizes, rollup state is final.
“ZK-rollups are always better.”
ZK-rollups have higher computational overhead for proof generation and have historically been harder to make EVM-compatible. Optimistic rollups remain widely used due to simpler developer tooling.
“All rollups are the same.”
Rollup designs vary significantly in sequencer decentralization, proof systems, upgrade keys, and data availability choices. The term “rollup” covers a wide spectrum.