Consensus vs Byzantine Broadcast in Sui

1. Two Approaches to Transaction Validation

Sui doesn’t use one-size-fits-all consensus. Instead, it chooses between:

Byzantine Consistent Broadcast (BCB): For simple owned-object transactions.

Consensus (Narwhal + Bullshark): For shared-object transactions.

This hybrid system minimizes overhead while preserving safety.

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2. Byzantine Consistent Broadcast (Fast Path)

Applicable when no shared state is involved.

Validators verify transaction validity and signatures.

No need for global ordering — transaction finalizes instantly.

Example: Alice sends Bob an NFT. No other user depends on that object, so consensus is unnecessary.

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3. Narwhal + Bullshark Consensus (Safe Path)

Used when multiple parties interact with shared state.

Narwhal organizes transactions in a DAG, ensuring data availability.

Bullshark orders transactions consistently across validators.

Example: Bob deposits tokens into a DeFi liquidity pool. Because multiple users can interact with the pool, ordering is crucial.

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4. Benefits of the Hybrid Approach

Speed: Owned-object transfers bypass consensus.

Scalability: Shared-object transactions still remain efficient due to DAG-based consensus.

Flexibility: Developers can design apps knowing both models exist.

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5. Implications

Payments and NFT transfers are lightning-fast.

Complex DeFi or gaming apps retain safety guarantees.

Users enjoy both performance and consistency.