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Building a Cross-Chain Bridge Event Monitor: Track Lock-Mint Events Across EVM Networks with Python

#crypto #python #opensource #web3

Cross-chain bridges are the backbone of DeFi interoperability, but monitoring token movements across chains remains surprisingly difficult for developers. In this tutorial, I will walk you through building a Python-based monitor that tracks lock-and-mint events across multiple EVM networks in real time.

The Problem

When users bridge tokens from Ethereum to Polygon (or any L2), the bridge contract locks tokens on the source chain and mints wrapped equivalents on the destination. If you are building a dApp that needs to react to these events, you need a reliable way to listen for them across chains.

What We Will Build

We will create a cross-chain event listener that:

  • Connects to multiple RPC endpoints (Ethereum, Polygon, Arbitrum, Optimism)
  • Monitors known bridge contracts for TokensLocked and TokensMinted events
  • Correlates lock events with their corresponding mint events using transaction hashes and user addresses
  • Stores results in a local SQLite database for querying

Prerequisites 

pip install web3 aiohttp sqlite-utils
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Step 1: Define Bridge Contract Addresses 

BRIDGES = {
    "polygon_pos": {
        "ethereum": "0x8484Ef722627bf18ca5Ae6BcF031c23E6e922B30",
        "polygon": "0x8484Ef722627bf18ca5Ae6BcF031c23E6e922B30"
    },
    "arbitrum": {
        "ethereum": "0x72Ce9c846789fdB6fC1f34aC4AD25Dd9ef7031ef",
        "arbitrum": "0x72Ce9c846789fdB6fC1f34aC4AD25Dd9ef7031ef"
    }
}
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Step 2: Connect to Multiple RPCs 

from web3 import Web3

RPCS = {
    "ethereum": "https://eth.llamarpc.com",
    "polygon": "https://polygon.llamarpc.com",
    "arbitrum": "https://arbitrum.llamarpc.com"
}

connections = {name: Web3(Web3.HTTPProvider(url)) for name, url in RPCS.items()}
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Step 3: Listen for Events 

BRIDGE_ABI = '[
  {"anonymous":false,"inputs":[
    {"indexed":true,"name":"user","type":"address"},
    {"indexed":true,"name":"token","type":"address"},
    {"indexed":false,"name":"amount","type":"uint256"},
    {"indexed":false,"name":"destChainId","type":"uint256"}
  ],"name":"TokensLocked","type":"event"}
]'

def scan_events(w3, contract_addr, from_block, to_block):
    contract = w3.eth.contract(address=contract_addr, abi=BRIDGE_ABI)
    return contract.events.TokensLocked.get_logs(
        fromBlock=from_block,
        toBlock=to_block
    )

# Scan the last 1000 blocks on each chain
for chain_name, w3 in connections.items():
    latest = w3.eth.block_number
    events = scan_events(w3, BRIDGES["polygon_pos"][chain_name], latest - 1000, latest)
    for evt in events:
        print(f"[{chain_name}] Lock: {evt.args.user} locked {evt.args.amount} of {evt.args.token}")
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Step 4: Correlate Cross-Chain Events 

import hashlib

def generate_correlation_id(user, amount, chain_id):
    raw = f"{user}-{amount}-{chain_id}".encode()
    return hashlib.sha256(raw).hexdigest()[:16]
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This correlation ID lets you match a lock event on Ethereum with its corresponding mint event on Polygon, even when the transaction hashes differ.

Full Source Code

The complete project with async support, database persistence, and Docker setup is available at github.com/Byaigo.

Why This Matters

Cross-chain monitoring is essential for:

  • Bridge security auditing
  • Automated cross-chain arbitrage bots
  • Real-time bridge health dashboards
  • Off-chain reconciliation systems

Final Thoughts

With just a few hundred lines of Python, you can build a production-ready cross-chain event monitor. The Web3.py library handles all the complex RLP encoding, and Python's async capabilities let you monitor dozens of chains simultaneously without breaking a sweat.

If you found this useful, consider supporting my open-source work:

ETH: 0x18da907cb9d981bc798acb87ac27b03a2dc3cbb7

Happy bridging! 🚀

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