Rollups (Optimistic vs ZK) in Blockchain / Solidity - Visual Side-by-Side Comparison

Concept Flow - Rollups (Optimistic vs ZK)

User sends transaction

↓

Rollup collects transactions

↓

Optimistic Rollup: Assumes valid

↓

Publish batch on main chain

↓

Challenge period starts

↓

If fraud proof submitted

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Revert batch

↓

Correct batch

↓

If no fraud, batch final

↓

ZK Rollup: Generate proof

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Publish batch + ZK proof on main chain

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Main chain verifies proof instantly

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Batch final immediately

User transactions are collected by rollups. Optimistic rollups assume batches are valid and wait for challenges, while ZK rollups generate cryptographic proofs that are verified instantly on the main chain.

Execution Sample

Blockchain / Solidity

function optimisticRollup(batch) {
  publishBatch(batch);
  waitChallengePeriod();
  if (fraudProofSubmitted()) {
    revertBatch();
  } else {
    finalizeBatch();
  }
}

function zkRollup(batch) {
  const proof = generateZKProof(batch);
  publishBatchWithProof(batch, proof);
  verifyProofOnChain(proof);
  finalizeBatch();
}

Shows simplified functions for optimistic and ZK rollups handling batches of transactions.

Execution Table

Step	Rollup Type	Action	Condition	Result
1	Optimistic	Publish batch on main chain	Assumes valid	Batch accepted, challenge period starts
2	Optimistic	Wait for challenge period	Challenge period ongoing	No immediate finalization
3	Optimistic	Check for fraud proof	Fraud proof submitted?	If yes, revert batch
4	Optimistic	Finalize batch	No fraud proof	Batch finalized
5	ZK	Generate ZK proof	Batch ready	Proof created
6	ZK	Publish batch + proof	Proof generated	Batch and proof on main chain
7	ZK	Verify proof on chain	Proof valid?	If yes, batch finalized immediately
8	ZK	Finalize batch	Proof verified	Batch finalized instantly

💡 Optimistic rollup waits for challenge period; ZK rollup finalizes immediately after proof verification.

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	After Step 5	After Step 7	Final
batchStatus	pending	published	reverted or published	finalized or reverted	proofGenerated	proofVerified	finalized
challengePeriod	not started	started	ongoing or ended	ended	n/a	n/a	n/a
proof	none	none	none	none	created	verified	verified

Key Moments - 3 Insights

Why does the optimistic rollup wait before finalizing the batch?

Because it assumes batches are valid but allows a challenge period for fraud proofs, as shown in execution_table rows 2 and 3.

How does the ZK rollup finalize batches instantly?

It generates a cryptographic proof that the main chain verifies immediately, so no waiting is needed, as seen in execution_table rows 5 to 8.

What happens if a fraud proof is submitted in an optimistic rollup?

The batch is reverted and corrected, preventing invalid state changes, as shown in execution_table row 3.

Visual Quiz - 3 Questions

Look at the execution table, at which step does the optimistic rollup finalize the batch if no fraud proof is submitted?

A Step 2

B Step 3

C Step 4

D Step 1

💡 Hint

Check the 'Result' column for optimistic rollup finalization in row 4.

According to the variable tracker, what is the status of 'proof' after step 5?

A created

B verified

C none

D pending

💡 Hint

Look at the 'proof' row under 'After Step 5' in variable_tracker.

If a fraud proof is submitted during the optimistic rollup challenge period, what happens to the batch?

A It is finalized immediately

B It is reverted

C It waits longer

D It generates a ZK proof

💡 Hint

See execution_table row 3 under 'Result' for optimistic rollup.

Concept Snapshot

Rollups bundle many transactions off-chain to reduce main chain load.
Optimistic rollups assume batches are valid and wait for a challenge period to detect fraud.
ZK rollups generate cryptographic proofs that verify batch validity instantly.
Optimistic rollups have delay but simpler proofs; ZK rollups finalize faster but need complex proofs.
Both improve scalability but use different trust and verification methods.

Full Transcript

Rollups are methods to bundle many blockchain transactions off the main chain to improve speed and reduce costs. Optimistic rollups publish batches assuming they are valid and wait for a challenge period where anyone can submit fraud proofs to revert invalid batches. If no fraud proof appears, the batch finalizes. ZK rollups generate a zero-knowledge proof for each batch that the main chain verifies instantly, allowing immediate finalization without waiting. Variables like batchStatus and proof change as the rollup processes batches. Optimistic rollups have a delay due to the challenge period, while ZK rollups finalize faster but require generating and verifying complex proofs.