Rollup

A rollup is a layer-2 scaling design that executes transactions off the main blockchain, then "rolls up" hundreds or thousands of them into a single compressed batch posted to the base chain, which stores the data and enforces correctness. Users get much cheaper, faster transactions, while the base layer — usually Ethereum — remains the ultimate arbiter of who owns what.

The defining property is that a rollup publishes enough data on the layer 1 for anyone to reconstruct its state. That is what separates rollups from sidechains: even if every rollup operator disappeared or turned malicious, users could still prove their balances and withdraw using the on-chain data. Correctness is enforced one of two ways. Optimistic rollups (Arbitrum, Optimism, Base) post batches assumed valid, with a challenge window during which anyone can submit fraud proofs. ZK-rollups (zkSync, Starknet) attach a cryptographic validity proof to every batch, so invalid state transitions are rejected immediately. For example, a swap on Arbitrum executes on the rollup in seconds for cents, then is included in a batch whose data lands on Ethereum, inheriting its security. Ethereum's 2024 Dencun upgrade added "blobs" — cheap, temporary data space designed specifically to cut rollup posting costs, which reduced layer-2 fees substantially.

A common misconception is that rollups are merely faster copies of Ethereum with their own trust assumptions; a properly built rollup derives its safety from the base chain — though in practice many still rely on centralized sequencers and multisig upgrade keys, so decentralization varies by project.