A rollup is a layer 2 scaling solution that executes transactions outside a base blockchain such as Ethereum, "rolls up" hundreds or thousands of them into one batch, and posts that batch, compressed, to the base layer. Because the transaction data is published on Ethereum, anyone can reconstruct the rollup's state and verify it, and users can always get their funds back via Ethereum. That is what makes rollups more secure than sidechains, and why they are the main way Ethereum scales.
How does a rollup work?
- Sequencer: users send transactions to the rollup's sequencer, which orders and executes them and gives near-instant confirmations.
- Batching and compression: the sequencer bundles many transactions and compresses the data.
- Publishing to layer 1: the batch data is posted to Ethereum, since 2024 mostly as cheap "blobs", together with the new state root.
- Verification: the new state is either assumed correct and open to challenge (optimistic rollups) or proven correct with a cryptographic proof (ZK rollups).
- Withdrawals: users can withdraw to Ethereum via the rollup's bridge contract once the state is final.
Optimistic rollups vs. ZK rollups
- Optimistic rollups (Arbitrum, Optimism, Base) assume each batch is valid. During a challenge period of about seven days, anyone can submit a fraud proof showing an invalid state transition. They are simpler and fully EVM-compatible, but standard withdrawals take a week.
- ZK rollups (zkSync, Starknet, Scroll, Linea) submit a validity proof (a zero-knowledge proof) with each batch, which Ethereum verifies on-chain. Withdrawals are final once the proof is accepted, but generating proofs is computationally heavy and EVM compatibility varies.
Rollup trade-offs
Most rollups still run a single, centralised sequencer: it can't steal funds, but it can delay or reorder transactions. Good rollups therefore offer a way to force transactions in via Ethereum. Upgrade keys, proof systems that are not yet fully live, and fragmented liquidity across many rollups are other points to weigh. For applications, rollups offer much lower fees with Ethereum-level settlement, which is why most new DeFi and consumer apps launch on one. See our layer 2 development, Optimism and zkSync.
Frequently Asked Questions about Rollups
Is a rollup the same as a layer 2?
A rollup is the most common type of layer 2. Not every layer 2 is a rollup: validiums and state channels are other types, and sidechains are often loosely called layer 2 as well.
Why do optimistic rollup withdrawals take a week?
Because the challenge period must pass before a withdrawal is final, giving anyone time to prove fraud. Third-party bridges offer faster withdrawals by fronting the funds for a fee.
What is a sequencer?
The component of a rollup that receives, orders and executes transactions and posts batches to layer 1. Decentralising sequencers is one of the main open challenges for rollups.
Is this the same as Rollup, the JavaScript bundler?
No. Rollup.js is a tool that bundles JavaScript code for the web. A blockchain rollup is a scaling technology for Ethereum; the two only share the name.