Monitoring deployed contracts in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - Monitoring deployed contracts

Deploy Contract

↓

Get Contract Address

↓

Set Up Monitoring Tool

↓

Listen to Contract Events

↓

Receive Event Data

↓

Trigger Alerts or Logs

↓

Analyze Contract Behavior

↓

Repeat Monitoring Cycle

This flow shows how after deploying a contract, you get its address, set up monitoring, listen to events, and react to contract activity continuously.

Execution Sample

Blockchain / Solidity

const contract = new ethers.Contract(address, abi, provider);
contract.on('Transfer', (from, to, amount) => {
  console.log(`Transfer from ${from} to ${to} of ${amount.toString()}`);
});

This code listens for 'Transfer' events on a deployed contract and logs details when such events happen.

Execution Table

Step	Action	Evaluation	Result
1	Deploy contract	Contract deployed on blockchain	Contract address obtained
2	Initialize contract object	Using address and ABI	Contract instance ready
3	Set event listener for 'Transfer'	Listening for events	Waiting for events
4	Event 'Transfer' emitted	Event data: from=0xA, to=0xB, amount=100	Event handler triggered
5	Log event details	Console output	Printed: Transfer from 0xA to 0xB of 100
6	Continue listening	No new events yet	Idle, waiting for next event

💡 Monitoring continues indefinitely until stopped manually

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 4	Final
contractAddress	undefined	0x123...abc	0x123...abc	0x123...abc	0x123...abc
contractInstance	undefined	undefined	Contract Object	Contract Object	Contract Object
eventData	undefined	undefined	undefined	{from: '0xA', to: '0xB', amount: 100}	undefined

Key Moments - 3 Insights

Why do we need the contract address after deployment?

The contract address identifies the deployed contract on the blockchain. Without it, we cannot create a contract instance to listen for events, as shown in Step 2 of the execution_table.

What happens if no events are emitted after setting the listener?

The monitoring tool stays idle, waiting for events. This is shown in Step 6 where the system waits without output until an event occurs.

How does the event listener know when to trigger the callback?

When the blockchain emits an event matching the listener's filter (e.g., 'Transfer'), the callback runs automatically, as in Step 4 and 5.

Visual Quiz - 3 Questions

Look at the execution_table, what is the contractAddress value after Step 1?

A 0x123...abc

B undefined

C Contract Object

D Event data

💡 Hint

Check the variable_tracker row for contractAddress at After Step 1

At which step does the event handler get triggered?

A Step 3

B Step 5

C Step 4

D Step 6

💡 Hint

Look at the execution_table row where event data is received and handler runs

If the contract emits no events, what will the monitoring tool do?

A Throw an error

B Stay idle waiting for events

C Stop monitoring

D Restart the contract

💡 Hint

Refer to Step 6 in the execution_table showing the system waiting for events

Concept Snapshot

Monitoring deployed contracts:
1. Deploy contract and get its address.
2. Create contract instance with address and ABI.
3. Set event listeners for contract events.
4. On event, callback runs to log or alert.
5. Monitoring runs continuously until stopped.

Full Transcript

Monitoring deployed contracts involves first deploying the contract and obtaining its blockchain address. Then, a contract instance is created using this address and the contract's ABI. Event listeners are set up on this instance to listen for specific events like 'Transfer'. When such an event occurs, the listener triggers a callback function that logs or processes the event data. The monitoring process continues indefinitely, waiting for new events to react to.

Practice

1. What is the main purpose of monitoring deployed smart contracts?

easy

A. To track contract activity and events after deployment

B. To write new smart contracts

C. To compile smart contracts before deployment

D. To delete contracts from the blockchain

Solution

1. Step 1: Understand contract deployment

   Once a smart contract is deployed, it runs on the blockchain and can emit events or change state.

2. Step 2: Purpose of monitoring

   Monitoring helps track these events and state changes to stay informed and debug issues.

3. Final Answer:

   To track contract activity and events after deployment -> Option A

4. Quick Check:

   Monitoring = track activity [OK]

Hint: Monitoring means watching contract events after deployment [OK]

Common Mistakes:

• Confusing monitoring with writing or compiling contracts
• Thinking monitoring deletes contracts
• Assuming monitoring happens before deployment

2. Which of the following is the correct syntax to listen for an event named Transfer using Web3.js?

easy

A. contract.on('Transfer', callback);

B. contract.getEvent('Transfer', callback);

C. contract.listen('Transfer', callback);

D. contract.events.Transfer({}, callback);

Solution

1. Step 1: Recall Web3.js event listening syntax

   Web3.js uses contract.events.EventName(options, callback) to listen for events.

2. Step 2: Match syntax to options

   contract.events.Transfer({}, callback); matches this syntax exactly for the Transfer event.

3. Final Answer:

   contract.events.Transfer({}, callback); -> Option D

4. Quick Check:

   Web3.js event listener = contract.events.EventName [OK]

Hint: Web3.js event listeners use contract.events.EventName() [OK]

Common Mistakes:

• Using .on() which is for ethers.js, not Web3.js
• Using .listen() which is invalid
• Using .getEvent() which does not exist

3. Given this code snippet using Web3.js to fetch past events:

const events = await contract.getPastEvents('Approval', { fromBlock: 100, toBlock: 'latest' });
console.log(events.length);

What does events.length represent?

medium

A. The number of transactions in block 100

B. The number of Approval events emitted between block 100 and the latest block

C. The total number of blocks from 100 to the latest

D. The number of contracts deployed after block 100

Solution

1. Step 1: Understand getPastEvents usage

   The method fetches all events named 'Approval' emitted by the contract between specified blocks.

2. Step 2: Meaning of events.length

   The length of the returned array is the count of those events found in that block range.

3. Final Answer:

   The number of Approval events emitted between block 100 and the latest block -> Option B

4. Quick Check:

   events.length = count of fetched events [OK]

Hint: getPastEvents returns array; length = number of matching events [OK]

Common Mistakes:

• Confusing events with blocks or transactions
• Thinking length counts blocks or contracts
• Assuming it counts all events, not filtered by name

4. You wrote this code to listen for events but it never triggers:

contract.events.Transfer(callback);

What is the likely error?

medium

A. Callback function is not defined

B. Using wrong event name 'Transfer'

C. Missing empty options object before callback

D. Contract is not deployed

Solution

1. Step 1: Check Web3.js event listener syntax

   The correct syntax requires an options object before the callback, even if empty.

2. Step 2: Identify missing options object

   The code lacks the empty object {} before the callback, so the event listener does not register properly.

3. Final Answer:

   Missing empty options object before callback -> Option C

4. Quick Check:

   Event listener syntax needs options object [OK]

Hint: Always include {} before callback in Web3.js event listeners [OK]

Common Mistakes:

• Assuming event name is wrong without checking
• Ignoring syntax requirements for event listeners
• Not defining callback function properly

5. You want to monitor a deployed contract's Deposit events in real time and also fetch all past Deposit events from block 5000 onwards. Which approach correctly combines both tasks using Web3.js?

hard

A. Use contract.getPastEvents('Deposit', { fromBlock: 5000 }) for past events and contract.events.Deposit() for real-time listening

B. Use contract.events.Deposit({ fromBlock: 5000 }) for real-time and past events together

C. Use contract.events.Deposit() only, it covers past and real-time events

D. Use contract.getPastEvents('Deposit') only, it covers real-time events too

Solution

1. Step 1: Understand fetching past events

   Use getPastEvents with fromBlock to fetch historical events from a specific block.

2. Step 2: Understand real-time event listening

   Use contract.events.Deposit() without block filters to listen for new events as they happen.

3. Step 3: Combine both methods

   To monitor both past and real-time events, call getPastEvents for history, then set up events.Deposit() for live updates.

4. Final Answer:

   Use contract.getPastEvents('Deposit', { fromBlock: 5000 }) for past events and contract.events.Deposit() for real-time listening -> Option A

5. Quick Check:

   Past events + real-time = getPastEvents + events [OK]

Hint: Fetch past with getPastEvents; listen live with events.EventName() [OK]

Common Mistakes:

• Trying to get past and live events with one method
• Using events with fromBlock to get past events only
• Assuming getPastEvents listens for new events