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✅ Master Event Logs & Real-Time dApps

Emit events, use indexed parameters, and listen for blockchain updates

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🎓 Module Complete: Key Takeaways

🎉 Congratulations!

You've mastered blockchain event logs! You now understand how to emit, filter, and listen to events—essential skills for building reactive dApps.

📚 What You've Learned

📋

Event Fundamentals

•Logs stored in transaction receipts, not contract storage
•50x cheaper than storage (375 gas vs 20,000)
•Accessible off-chain only, not by other contracts

🔍

Event Structure

•Topic 0: Event signature (keccak256 hash)
•Topics 1-3: Up to 3 indexed parameters
•Data field: Unlimited non-indexed parameters

🏷️

Indexed Parameters

•Enable efficient filtering via Bloom filters
•Max 3 indexed params per event
•Use for addresses, IDs, and filter criteria

👂

Event Listening

•WebSocket listeners for real-time updates
•Polling with getLogs for batch processing
•The Graph for complex historical queries

⚡

Best Practices

•Index searchable params (addresses, IDs)
•Always remove listeners on cleanup
•Handle WebSocket reconnections gracefully

🎯

Use Cases

•Real-time dApp UI updates
•Transaction notifications
•Analytics and monitoring dashboards

🎯 Interactive: Event Logs Knowledge Quiz

Test your understanding with 5 questions about blockchain events and logging.

🛠️ Quick Reference Guide

Event Declaration

event Transfer(
  address indexed from,
  address indexed to,
  uint256 amount
);

Event Emission

emit Transfer(
  msg.sender,
  recipient,
  amount
);

Event Listening (ethers.js)

contract.on("Transfer",
  (from, to, amount) => {
    console.log(from, to);
  }
);

Event Filtering

const filter = contract
  .filters.Transfer(
    null,
    myAddress
  );

💡 Real-World Applications

💼

DeFi Protocols

Track swaps, liquidity adds/removes, yield farming rewards in real-time.

🎨

NFT Marketplaces

Monitor mints, transfers, sales, and listings with instant UI updates.

🗳️

DAO Governance

Log votes, proposals, executions for transparency and auditability.

👛

Wallet Applications

Notify users of incoming transactions, approvals, and balance changes.

🚀 Next Steps

1️⃣

Build a dApp with events: Create a simple token contract and build a React frontend that listens for Transfer events in real-time.

2️⃣

Explore The Graph: Deploy a subgraph to index your contract events and query historical data with GraphQL.

3️⃣

Study production contracts: Examine how Uniswap, Aave, and other protocols use events for monitoring and analytics.

4️⃣

Implement error handling: Build robust event listeners with reconnection logic and proper error handling.

📋

Event Logs Mastered!

You now have the knowledge to emit efficient events, filter them strategically, and build reactive dApps that respond to blockchain activity in real-time.

"Events are the nervous system of the blockchain, carrying signals from smart contracts to the outside world."