PlugPortMessageBroker — On-Chain Pub/Sub

Overview

PlugPortMessageBroker.sol provides Redis-compatible Pub/Sub messaging backed by Monad smart contract events. Messages published to channels are emitted as on-chain events, making them immutable, verifiable, and decentralized.

Architecture

┌─────────────┐    PUBLISH     ┌──────────────────────┐    emit event     ┌────────────────────┐
│  redis-cli  │  ──────────▶   │   PlugPort Server    │  ──────────────▶  │   Monad Blockchain │
│  (ioredis)  │                │  (MessageBrokerAdap) │                   │  (MessageBroker)   │
└─────────────┘                └──────────────────────┘                   └────────────────────┘
                                        ▲                                         │
┌─────────────┐    SUBSCRIBE            │       eth_subscribe (WebSocket)          │
│  redis-cli  │  ──────────▶   ◀────────┼─────────────────────────────────────────┘
│  (ioredis)  │                         │
└─────────────┘                         │

How It Works

1. PUBLISH channel message

   • PlugPort server encodes the message and calls publishWithName() on the smart contract
   • The gas station wallet pays the gas fee (~45,000 gas)
   • The contract emits a MessagePublished event with the channel hash, sequence number, and message bytes

2. SUBSCRIBE channel

   • PlugPort server starts an eth_subscribe("logs") WebSocket listener filtered by the channel hash
   • When a MessagePublished event is received, the message is decoded and pushed to the subscriber

3. History Replay

   • The contract stores the last 100 messages per channel in a ring buffer
   • New subscribers can call getHistory(channelHash, count) to catch up

Contract API

Write Functions (cost gas)

Function	Gas (~)	Description
publish(channelHash, message)	45,000	Publish a message, emit event
publishWithName(channelHash, channelName, message)	50,000	Publish + register channel name (first use)
updateSubscriberCount(channelHash, count)	25,000	Update subscriber count (informational)
transferGasStation(newGasStation)	25,000	Transfer gas station rights

View Functions (free)

Function	Description
getHistory(channelHash, count)	Get last N messages for a channel
getSequence(channelHash)	Get current sequence number
getHistoryLength(channelHash)	Get number of messages in history
channelSubscriberCount(hash)	Get subscriber count
totalMessages()	Total messages across all channels

Gas Station

IMPORTANT: The MessageBroker uses a DEDICATED gas station wallet, separate from the PlugPortStore gas station.

This separation ensures that:

• Pub/Sub gas costs don't affect database operations
• Each gas station can be independently funded and monitored
• Rate limiting can be applied per-gas-station

Setting Up

1. Generate a new keypair for the Pub/Sub gas station
2. Fund it with MON on the Monad testnet
3. Deploy PlugPortMessageBroker.sol with the gas station address as constructor arg
4. Set MESSAGEBROKER_CONTRACT_ADDRESS and MONAD_WS_URL in .env

Usage Examples

Redis CLI

# Terminal 1: Subscribe
redis-cli -p 6379 SUBSCRIBE news

# Terminal 2: Publish
redis-cli -p 6379 PUBLISH news "Breaking: PlugPort v2 released!"

Node.js (ioredis)

import Redis from 'ioredis';

const sub = new Redis(6379);
const pub = new Redis(6379);

// Subscribe
sub.subscribe('chat:general', (err, count) => {
  console.log(`Subscribed to ${count} channels`);
});

sub.on('message', (channel, message) => {
  console.log(`${channel}: ${message}`);
  // This message was published on-chain and is immutable!
});

// Publish (triggers on-chain transaction)
await pub.publish('chat:general', 'Hello from PlugPort!');

History Replay

import { ethers } from 'ethers';

const provider = new ethers.JsonRpcProvider('https://monad-testnet.drpc.org');
const contract = new ethers.Contract(ADDRESS, ABI, provider);

// Get last 10 messages for a channel
const channelHash = ethers.keccak256(ethers.toUtf8Bytes('chat:general'));
const messages = await contract.getHistory(channelHash, 10);

for (const msg of messages) {
  console.log(ethers.toUtf8String(msg));
}

Deployment

# Using Hardhat or Remix:
# 1. Deploy PlugPortMessageBroker.sol with gas station address
# 2. Fund the gas station with MON
# 3. Update .env:

MESSAGEBROKER_CONTRACT_ADDRESS=0x...deployed_address...
MONAD_WS_URL=wss://monad-testnet.drpc.org

Gas Cost Estimates (Monad Testnet)

Operation	Gas	Cost at 0.1 gwei
publish()	~45,000	~0.0000045 MON
publishWithName()	~50,000	~0.000005 MON
updateSubscriberCount()	~25,000	~0.0000025 MON

Gas on Monad is extremely cheap. At 0.1 gwei, publishing 1 million messages costs ~0.5 MON.