Hashgraph-like Consensus

A lightweight Rust library for making binary decisions in peer-to-peer or gossipsub networks. Perfect for group governance, voting systems, or any scenario where you need distributed agreement.

Features

• Fast - Reaches consensus in O(log n) rounds
• Byzantine fault tolerant - Correct even if up to 1/3 of peers are malicious
• Pluggable storage - In-memory by default; implement ConsensusStorage for persistence
• Network-agnostic - Works with both Gossipsub (fixed 2-round) and P2P (dynamic rounds) topologies
• Event-driven - Subscribe to consensus outcomes via a broadcast event bus
• Cryptographic integrity - Votes are signed with secp256k1 and chained in a hashgraph structure

Based on the Hashgraph-like Consensus Protocol RFC.

Installation

Add to your Cargo.toml:

[dependencies]
hashgraph-like-consensus = { git = "https://github.com/vacp2p/hashgraph-like-consensus" }

Quick Start

use hashgraph_like_consensus::{
    scope::ScopeID,
    service::DefaultConsensusService,
    types::CreateProposalRequest,
};
use alloy::signers::local::PrivateKeySigner;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let service = DefaultConsensusService::default();
    let scope = ScopeID::from("example-scope");
    let signer = PrivateKeySigner::random();

    // Create a proposal
    let proposal = service
        .create_proposal(
            &scope,
            CreateProposalRequest::new(
                "Upgrade contract".into(),   // name
                b"Switch to v2".to_vec(),     // payload (bytes)
                signer.address().as_slice().to_vec(), // owner
                3,                           // expected voters
                60,                          // expiration (seconds from now)
                true,                        // liveness: silent peers count as YES at timeout
            )?,
        )
        .await?;

    // Cast a vote
    let vote = service
        .cast_vote(&scope, proposal.proposal_id, true, signer)
        .await?;
    println!("Recorded vote {}", vote.vote_id);

    Ok(())
}

Core Concepts

Scopes and Proposals

A scope groups related proposals together and carries default configuration (network type, threshold, timeout). Proposals inherit scope defaults unless overridden individually.

Scope (group / channel)
  +-- ScopeConfig (defaults for all proposals)
  +-- Proposals
       +-- Proposal 1 -> Session (inherits scope config)
       +-- Proposal 2 -> Session (inherits scope config)
       +-- Proposal 3 -> Session (overrides scope config)

Network Types

Architecture

ConsensusService is the single entry point. All consensus business logic lives there. It is generic over two pluggable backends:

• ConsensusStorage — where sessions and votes are persisted (in-memory, database, etc.)
• ConsensusEventBus — how consensus events are delivered (broadcast channel, message queue, etc.)

Use service.storage() for reads, queries, and cleanup. Use service.event_bus() for event subscription.

What the Library Does vs. What You Do

This library handles consensus calculation — vote validation, hashgraph chain verification, threshold math, and liveness rules. It does not handle orchestration. Your application is responsible for:

API Reference

Creating a Service

use hashgraph_like_consensus::service::DefaultConsensusService;

// Default: in-memory storage, 10 max sessions per scope
let service = DefaultConsensusService::default();

// Custom session limit
let service = DefaultConsensusService::new_with_max_sessions(20);

// Fully custom: plug in your own storage and event bus
let service = ConsensusService::new_with_components(my_storage, my_event_bus, 10);

Configuring a Scope

use hashgraph_like_consensus::{
    scope::ScopeID,
    scope_config::NetworkType,
    service::DefaultConsensusService,
};
use std::time::Duration;

let service = DefaultConsensusService::default();
let scope = ScopeID::from("team_votes");

// Initialize with the builder
service
    .scope(&scope)
    .await?
    .with_network_type(NetworkType::P2P)
    .with_threshold(0.75)
    .with_timeout(Duration::from_secs(120))
    .with_liveness_criteria(false)
    .initialize()
    .await?;

// Update later (single field)
service
    .scope(&scope)
    .await?
    .with_threshold(0.8)
    .update()
    .await?;

Built-in presets are also available:

// High confidence (threshold = 0.9)
service.scope(&scope).await?.strict_consensus().initialize().await?;

// Low latency (threshold = 0.6, timeout = 30 s)
service.scope(&scope).await?.fast_consensus().initialize().await?;

Working with Proposals

// Create a proposal
let proposal = service
    .create_proposal(&scope, CreateProposalRequest::new(
        "Upgrade contract".into(),
        b"Switch to v2".to_vec(),
        owner_address,
        3,     // expected voters
        60,    // expiration (seconds from now)
        true,  // liveness: silent peers count as YES at timeout
    )?)
    .await?;

// Process a proposal received from the network
service.process_incoming_proposal(&scope, proposal).await?;

Casting and Processing Votes

// Cast your vote (yes = true, no = false)
let vote = service.cast_vote(&scope, proposal_id, true, signer).await?;

// Cast a vote and get the updated proposal (useful for gossiping)
let proposal = service
    .cast_vote_and_get_proposal(&scope, proposal_id, true, signer)
    .await?;

// Process a vote received from the network
service.process_incoming_vote(&scope, vote).await?;

Reading State (via Storage)

All reads go through service.storage():

use hashgraph_like_consensus::storage::ConsensusStorage;

// Get the consensus result for a proposal (Ok(true) = YES, Ok(false) = NO)
let result: bool = service.storage().get_consensus_result(&scope, proposal_id).await?;

// Get a proposal by ID
let proposal = service.storage().get_proposal(&scope, proposal_id).await?;

// List active proposals (empty Vec if none)
let active: Vec<Proposal> = service.storage().get_active_proposals(&scope).await?;

// List finalized proposals (proposal_id -> result)
let reached: HashMap<u32, bool> = service.storage().get_reached_proposals(&scope).await?;

// Delete all state for a scope (e.g. when a user leaves a group)
service.storage().delete_scope(&scope).await?;

Handling Timeouts

The library does not schedule timeouts automatically. Your application must set up a timer for each proposal and call handle_consensus_timeout when it fires. Without this, proposals with offline voters will stay Active forever and the silent-peer liveness logic will never run.

When handle_consensus_timeout is called, silent peers (those who never voted) are counted toward quorum so that the liveness_criteria_yes flag can take effect:

• liveness_criteria_yes = true — silent peers are counted as YES votes. A proposal passes unless there are enough explicit NO votes to block it.
• liveness_criteria_yes = false — silent peers are counted as NO votes. A proposal fails unless there are enough explicit YES votes to carry it.

The only case where timeout produces no result is a tie (equal YES and NO weight after counting silent peers), which marks the session as failed.

// Schedule a timeout (typically via tokio::time::sleep)
tokio::time::sleep(config.consensus_timeout()).await;

match service.handle_consensus_timeout(&scope, proposal_id).await {
    Ok(true)  => println!("Consensus: YES"),
    Ok(false) => println!("Consensus: NO"),
    Err(ConsensusError::InsufficientVotesAtTimeout) => {
        println!("Tied — no consensus");
    }
    Err(e) => eprintln!("Error: {e}"),
}

During normal voting (before timeout), the quorum gate still requires ceil(2n/3) actual votes — silent peers are not counted until timeout.

Subscribing to Events

use hashgraph_like_consensus::events::ConsensusEventBus;
use hashgraph_like_consensus::types::ConsensusEvent;

let mut rx = service.event_bus().subscribe();

tokio::spawn(async move {
    while let Ok((scope, event)) = rx.recv().await {
        match event {
            ConsensusEvent::ConsensusReached { proposal_id, result, timestamp } => {
                println!("Proposal {} -> {}", proposal_id, if result { "YES" } else { "NO" });
            }
            ConsensusEvent::ConsensusFailed { proposal_id, timestamp } => {
                println!("Proposal {} failed to reach consensus", proposal_id);
            }
        }
    }
});

Statistics

let stats = service.get_scope_stats(&scope).await;
println!(
    "Active: {}, Reached: {}, Failed: {}",
    stats.active_sessions, stats.consensus_reached, stats.failed_sessions
);

Advanced Usage

Custom Storage

Implement the ConsensusStorage trait to persist proposals to a database. You only need to implement the primitive methods — query helpers like get_consensus_result, get_active_proposals, and get_reached_proposals are provided as default implementations for free.

use hashgraph_like_consensus::storage::ConsensusStorage;

// Required primitives (you implement these):
//   save_session, get_session, remove_session,
//   list_scope_sessions, replace_scope_sessions,
//   update_session, update_scope_sessions,
//   stream_scope_sessions, list_scopes,
//   get_scope_config, set_scope_config, update_scope_config,
//   delete_scope
//
// Free query helpers (default implementations):
//   get_consensus_result, get_proposal, get_proposal_config,
//   get_active_proposals, get_reached_proposals

Custom Event Bus

Implement ConsensusEventBus for alternative event delivery:

use hashgraph_like_consensus::events::ConsensusEventBus;

pub trait ConsensusEventBus<Scope> {
    type Receiver;
    fn subscribe(&self) -> Self::Receiver;
    fn publish(&self, scope: Scope, event: ConsensusEvent);
}

Utility Functions

The utils module provides low-level helpers for advanced use cases:

Building

# Build
cargo build

# Run tests
cargo test

# Generate docs
cargo doc --open

Note: Requires a working protoc (Protocol Buffers compiler) since the library generates code from .proto files at build time.

License

MIT