Aptos Redefines Blockchain Efficiency with Shoal++ DAG BFT

Joule Finance
5 min readJun 24, 2024


The blockchain landscape has evolved significantly, and two key innovations driving this progress are Directed Acyclic Graphs (DAG) and Byzantine Fault Tolerance (BFT).

In this blog, we’ll explore how DAG-based consensus mechanisms work, how they integrate with BFT, and their implementation in cutting-edge blockchain projects like Shoal++ by Aptos.

Directed Acyclic Graph (DAG)

Structure and Mechanics: In a DAG, each node represents a transaction or a block of transactions. Edges between nodes show dependencies, indicating which transactions need to be verified before others. This non-linear structure allows for multiple branches and merging paths, unlike the single chain of blocks in traditional blockchains.

The diagram illustrates how each vertex represents a transaction, and directed edges show dependencies, enabling multiple branches and merging paths for transaction verification.

Concurrency and Parallelism: One of the key advantages of DAGs is their ability to process transactions in parallel. Nodes can add transactions independently, as long as they follow the dependency rules. This parallel processing addresses bottlenecks in transaction throughput, making the network highly scalable.

Ordering and Finality: The challenge with DAGs is to order transactions and achieve finality without a single leader or sequential block production. The consensus mechanism must ensure that all honest nodes eventually agree on the transaction order.

Byzantine Fault Tolerance (BFT)

BFT is a well-established concept in the blockchain domain. In essence, BFT algorithms ensure that consensus is achieved even when some nodes are malicious or fail. Traditional BFT protocols involve complex communication patterns, with each node sending messages to every other node. However, newer approaches aim to reduce this overhead. There’s always a trade-off between low latency and high throughput, and achieving fast finality while maintaining high transaction rates remains a critical design challenge.

DAG BFT in Layer 1 Blockchains

Combining DAG and BFT: By integrating DAG’s parallel transaction processing with BFT’s robustness, Layer 1 blockchains can achieve high throughput without compromising security. This involves designing consensus algorithms that can efficiently order and validate transactions within a DAG structure.

Examples of DAG BFT Implementations:

  • Fantom Blockchain (Lachesis Protocol): Utilizes a DAG-based consensus protocol where nodes build local DAGs and periodically exchange information with other nodes. The protocol achieves asynchronous Byzantine Fault Tolerance (aBFT), providing high scalability and fast finality.
  • Hedera Hashgraph: Uses a gossip-about-gossip protocol combined with virtual voting to achieve consensus. The DAG structure allows for high transaction throughput, and the consensus mechanism ensures Byzantine fault tolerance.

Key Considerations for DAG BFT

Validator Selection and Sybil Resistance: Ensuring a diverse and secure set of validators is crucial to prevent Sybil attacks and maintain decentralization.

Handling Forks and Conflicts: Efficiently resolving conflicts and ensuring consistent transaction ordering across the network is vital. This often involves complex algorithms to merge branches and achieve consensus.

Security and Scalability Balance: Achieving a balance between high throughput and maintaining robust security guarantees is a constant challenge. Innovations in DAG BFT aim to optimize this balance for practical, large-scale deployments.

Aptos Innovations in DAG BFT: Shoal++

Overview: Shoal++ is a groundbreaking DAG BFT protocol developed by Aptos. It combines DAG’s parallel transaction processing with the robustness of Byzantine Fault Tolerance (BFT) to achieve unparalleled efficiency and security in blockchain transactions.

Evaluating Shoal++: Recent evaluations of Shoal++ against state-of-the-art DAG BFT protocols such as Bullshark, Shoal, and Mysticeti/Cordial Miners demonstrate its superior performance. In failure-free scenarios, Shoal++ reduces latency by up to 60%, significantly improving transaction processing times.

Shoal++ vs. Other DAG BFT Protocols — Performance Comparison

Key Latency Improvements: Understanding the end-to-end transaction latency in DAG BFT protocols involves breaking it down into three phases: anchor commit latency, anchoring latency, and queuing latency. Shoal++ introduces innovative enhancements to improve each phase, significantly reducing overall latency.

  • Anchor Commit Latency: In Bullshark, committing an anchor node requires 2 DAG rounds, equating to 6 message delays. Shoal++ introduces “Fast Anchors,” allowing validators to commit anchors in only 4 message delays.
  • Anchoring Latency: In Bullshark, nodes in even rounds require an additional DAG round, resulting in extra message delays. Shoal++ reduces anchoring latency by introducing “More Anchors,” treating most nodes as anchors and enabling faster linking.
  • Queuing Latency: In Bullshark, it takes 3 message delays to certify and add a node to the DAG. Shoal++ minimizes queuing latency by operating multiple DAGs in parallel, with a small offset between them. This allows transactions that miss a round to be quickly included in the next, reducing the average queuing latency from 1.5 to 0.5 message delays.

Throughput Enhancement

Throughput, a critical metric for scalability, was also evaluated. Shoal++ leverages multiple DAGs in parallel (Shoal++ introduces More DAGs!) to minimize queuing latency and optimize network utilization. This approach not only reduces average queuing latency from 1.5 to 0.5 message delays but also boosts throughput by effectively leveraging networking resources (streaming effect).

Additional Innovations

Shoal++ incorporates several other innovations to enhance its performance:

  • Fast Anchors: Validators can commit anchors more quickly.
  • More DAGs: Multiple parallel DAGs, staggered with a small offset, reduce waiting times for transactions to be included in the network, improving queuing latency and achieving higher throughput.


DAG BFT is a sophisticated approach combining the scalability advantages of DAG structures with the resilience of BFT consensus. Shoal++ exemplifies these advancements, setting a new standard for efficiency and performance, giving Aptos a significant performance edge.

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