Parallel Optimistic Execution

Designed for performance and scalability, Nibiru leverages parallel optimistic execution to achieve high transaction throughput and reduced latency. This technique overlaps consensus and execution, optimizing for a superior user experience.

1 - Introduction

As blockchain networks scale, the need for higher throughput and lower latency becomes critical. Traditional execution models process transactions sequentially, causing delays in block finalization. Parallel optimistic execution (OE) addresses this inefficiency by allowing transactions to be executed speculatively before consensus is reached. If the proposed block is accepted, the precomputed results are committed; otherwise, they are discarded. This approach improves performance without compromising security.

2 - Background: Traditional Execution Flow

In a conventional execution model, block processing follows a strictly sequential order:

1. A block proposal is received.
2. The consensus engine (CE) ensures the block gains sufficient validator votes.
3. Only after finalization does the execution engine (EE) process the block.
4. Execution results in a new state and state hash.

This structure enforces correctness but introduces latency, as execution cannot begin until consensus is reached.

3 - The Mechanics of Parallel Optimistic Execution

Parallel optimistic execution enables the execution engine to process transactions in parallel before consensus finalization. This speculative execution allows the network to compute state transitions while consensus mechanisms operate concurrently.

Execution Flow with OE

1. Block Proposal & Preprocessing: The block executor calls EE.ProcessProposal (abci.RequestProcessProposal).
2. Speculative Execution: The EE receives an unfinalized block and executes it optimistically.
3. Consensus Decision: The CE continues its process to finalize the block.

4. Commit or Discard:
   • If the block is finalized, the precomputed execution results are used immediately.
   • If the block is rejected, the speculative execution is discarded.

This approach ensures that the majority of proposed blocks—often over 99%—can leverage precomputed execution results, significantly reducing processing time.

4 - Why Parallel Optimistic Execution Increases Throughput

Comparing Execution Timelines: In the traditional, sequential execution model, consensus must be reached before transaction execution can begin. The total time is therefore the sum of the consensus time and the execution time.

With parallel optimistic execution, consensus and execution occur concurrently. The total time is then determined by whichever process takes longer, effectively overlapping the two phases.

Key Takeaways

• Eliminates Sequential Bottlenecks: Execution begins as soon as a block is proposed, rather than waiting for consensus.
• Reduces Block Finalization Latency: If consensus confirms the block, execution results are immediately available.
• Minimal Downsides: If a different block wins consensus, the speculative work is discarded without delaying finalization.

5 - Why Doesn’t Every Blockchain Implement OE?

Technical Dependencies

Parallel optimistic execution is enabled by ABCI++ (introduced in CometBFT), which provides:

• PrepareProposal – Allows block proposers to modify blocks before submission.
• ProcessProposal – Enables nodes to see and execute unfinalized blocks before consensus completion.

Implementation Challenges

• Infrastructure Requirements: Chains must support ABCI++ or equivalent mechanisms.
• Speculative Execution Complexity: Systems must efficiently manage speculative results to prevent excessive wasted computation.

6 - Future Directions and Open Quetions

Parallel optimistic execution offers a significant path to increased blockchain throughput by overlapping consensus and execution. While ABCI++ facilitates its implementation, efficient management of speculative state and conflict resolution remain key engineering challenges. Further research is needed to optimize dependency tracking, minimize wasted computation, and develop robust mechanisms for handling complex state interactions within a parallel execution environment. Exploring the security implications of speculative execution and developing appropriate validation strategies are also crucial for wider adoption. As blockchain technology matures, addressing these open questions will be essential for realizing the full potential of parallel optimistic execution and achieving truly scalable decentralized systems.