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Rollups: Optimistic, ZK, and the Dencun Upgrade

Understand how rollups work, the difference between optimistic and ZK rollups, and how EIP-4844 (March 13, 2024) cut rollup fees by approximately 90 percent.

14 min · intermediate · part of Layer 2 Solutions & Scaling

What a Rollup Actually Is

A rollup is a Layer 2 system that executes transactions off the main chain, then posts compressed data and proofs back to the main chain. The key word is "rollup" — many transactions get rolled up into a single batch that the base chain treats as a single submission. The mechanics divide cleanly into four roles. A sequencer accepts user transactions, orders them, and produces blocks on the rollup. An execution layer runs those transactions against the rollup's state — typically using the same EVM (Ethereum Virtual Machine) Ethereum mainnet uses, so existing Ethereum smart contracts work without modification. A data-availability layer publishes the transaction data somewhere it can be reconstructed if needed; for an Ethereum rollup, this is Ethereum mainnet itself, posted via EIP-4844 blobs. A verification layer proves to Ethereum that the rollup's state transitions are valid — through a fraud-proof system in optimistic rollups or a validity proof (typically a SNARK or STARK) in ZK rollups. The result: hundreds or thousands of user transactions on the rollup correspond to a single batched commitment on Ethereum. Per-transaction cost is divided across the batch. Confirmation latency on the rollup is often sub-second. Withdrawal latency back to Ethereum varies — fast for ZK rollups, longer for optimistic rollups during the challenge window. Most importantly, security is inherited from Ethereum. The rollup does not have its own validator set deciding finality. The rollup commits to its data and proofs on Ethereum, and Ethereum's consensus and economic security back the result. This is the fundamental difference between rollups and sidechains: a sidechain has its own validator set and its own security; a rollup borrows Ethereum's. If the rollup operator goes rogue, users can prove it on Ethereum and force a rollback or exit. If a sidechain operator goes rogue, users have no such recourse without trusting the sidechain's own governance.

Also in this lesson

  • Optimistic Rollups vs ZK Rollups
  • EIP-4844 (Dencun): The 90 Percent Fee Reduction
  • Today's Rollup Ecosystem: Arbitrum, Base, Optimism, zkSync, StarkNet
  • Using Rollups: Bridging, Networks, and Direct On-Ramps

Key terms

Rollup
A Layer 2 that executes transactions off the main chain and posts compressed data and proofs back to the main chain. Inherits the base chain's security through these commitments.
Optimistic rollup
A rollup that assumes batches are valid and uses a fraud-proof challenge window (typically 7 days) to handle disputes. Arbitrum and Optimism are the leading examples.
ZK rollup (validity rollup)
A rollup that posts a cryptographic proof (SNARK or STARK) demonstrating correct execution of every batch. Withdrawals can finalize within hours rather than the 7-day optimistic window. zkSync Era and StarkNet are examples.
EIP-4844 (proto-danksharding)
Activated March 13, 2024 in the Dencun upgrade. Introduced blob-carrying transactions (max 6 blobs per block) for cheap, temporary rollup data storage. Cut rollup fees by approximately 90 percent.
Arbitrum One
The leading optimistic rollup, mainnet launched August 31, 2021 by Offchain Labs. ARB token launched March 23, 2023 (10 billion supply, 11.5 percent airdrop). Approximately 16 billion dollars TVL in late 2025.
Base
Coinbase's OP Stack rollup, mainnet launched August 9, 2023 ("Onchain Summer"). No native token. Approximately 15 billion dollars TVL in late 2025.
OP Stack / Superchain
Optimism's open-source rollup framework. Powers Base, World Chain, Mode, Ink, Unichain, opBNB, and others, with shared security primitives.
zkSync Era
Matter Labs' EVM-equivalent ZK rollup, mainnet launched March 24, 2023. The first production-grade zkEVM rollup at scale.
StarkNet
StarkWare's ZK rollup, mainnet alpha launched November 29, 2021. Uses Cairo language and STARK proofs. Co-founded by Eli Ben-Sasson.
Polygon zkEVM sunset
Polygon announced in June 2025 that the zkEVM sequencer would shut down July 1, 2026 due to sluggish adoption and reported annual losses of over 1 million dollars.

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