Scaling the Celo Blockchain with Narwhal


This tutorial will discuss the concept of scaling the Celo blockchain using Narwhal, focusing on consensus properties and the integration of a high traffic mempool. We will explore the basics of consensus, the challenges of leader-based consensus, and the potential of leaderless consensus. Additionally, we will delve into the implementation of Narwhal, a consensus algorithm, and its benefits for scaling the Celo blockchain.

Understanding Consensus

Consensus is crucial for the state machine replication paradigm, where programs need to run reliably and consistently across multiple replicas. In the consensus setting, there is a need for communication among nodes to achieve consensus. The worst-case scenario requires N square communication, where N represents the number of authorities. However, leader-based consensus offers a more efficient approach by leveraging a leader to collect and broadcast messages. This approach significantly reduces communication overhead.

Challenges of Leader-Based Consensus

While leader-based consensus seems promising, there are challenges to overcome. One major issue is network utilization, as sending messages to each node individually results in poor bandwidth utilization. The leader becomes saturated with network traffic, while other validators are underutilized. Furthermore, transactions are often submitted twice, once in the mempool and again when included in a block proposal. This redundancy negatively impacts efficiency.

Exploring Leaderless Consensus

Leaderless consensus offers a solution to the challenges faced by leader-based consensus. It eliminates the bottleneck of a single leader and enables all machines to work together organically. In a leaderless consensus system, small sequences of transactions called blocks are organized in asynchronous rounds. Each node can make a proposal, which is broadcasted to others for voting. Transitive acknowledgments of prior valid blocks allow the construction of a directed acyclic graph (DAG), which serves as a basis for building a mempool and consensus protocol.

Introducing Narwhal

Narwhal is a leaderless consensus algorithm that aims to optimize data availability and network utilization. It leverages the deterministic nature of DAGs to build a consensus protocol with zero message overhead. By using collections of transactions instead of individual transactions, Narwhal reduces the size of block proposals and enhances consensus performance. The mempool in Narwhal consists of tiny, hashable blocks called collections, which can be referred to in blog proposals without transmitting the transactions themselves.

Achieving Efficient Blog Proposals

To achieve efficient blog proposals, Narwhal reduces many transactions to collection hashes and further reduces multiple collection hashes to a single collection hash. This simplifies the mempool, making it easier to handle. However, a challenge arises when prioritizing transactions in the mempool. While a graph walk like depth-first search (DFS) provides a fast way to process transactions, it limits the mempool to a FIFO (First-In-First-Out) approach. To address this, Narwhal employs algorithmic techniques based on the union-find data structure to optimize the handling of collections and achieve a more flexible transaction prioritization strategy.


Narwhal presents a scalable solution for the Celo blockchain by introducing leaderless consensus and optimizing network utilization. By leveraging DAGs and efficient mempool management, Narwhal reduces redundancy and improves the overall performance of the blockchain. Understanding the principles and implementation of Narwhal can help in scaling the Celo blockchain effectively.

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