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Amir of Ekiti
Amir of Ekiti

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## What Really Happens When You Call a Smart Contract?

#webdev #programming #learning #blockchain

It was 2 AM. I had just deployed my first Solidity smart contract—a simple “wave at me” dApp—and was proudly calling wave() from my frontend. But nothing happened.

No wave.

No confirmation.

Just a pending transaction and a big mystery: what is actually happening when I “call” a smart contract?

If you've ever clicked "Submit" in a Web3 app and wondered what goes on behind the scenes, this post is for you.

🧠 TL;DR First (Then We Dive Deep)

When you call a smart contract:

  1. You create a transaction from your wallet (like MetaMask).
  2. That transaction is signed by you, proving it’s legitimate.
  3. It gets sent to the blockchain network (Ethereum, Polygon, etc).
  4. Nodes pick it up, run the contract logic, and validate it.
  5. If all goes well, it’s added to a block, and the change becomes permanent.

But the real magic? It’s what happens between those steps.

Let’s walk through it with a story.

👩‍💻 The Story: Alice Sends a Transaction

Say Alice wants to buy a sword in a blockchain game by calling a smart contract function:

function buySword(uint swordId) public payable {
    require(msg.value >= swordPrice, "Not enough ETH");
    ownerOf[swordId] = msg.sender;
}
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She clicks “Buy” in the dApp UI. Here’s what happens:

🧰 Step 1: The Frontend Prepares the Payload

The app uses Ethers.js or Web3.js to encode the function and arguments into something the Ethereum Virtual Machine (EVM) understands:

contract.buySword(2, { value: ethers.utils.parseEther("0.05") });
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This turns into a raw transaction object like:

{
  "to": "0xContractAddress",
  "data": "0xa9059cbb000000000000000000...",
  "value": "0.05 ETH"
}
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🔐 Step 2: Signing the Transaction

Before it can go anywhere, Alice’s wallet (MetaMask, WalletConnect, etc.) pops up and asks her to sign the transaction.

  • This doesn’t encrypt it.
  • It simply proves: “Yes, I want to do this.”

The private key never leaves her wallet, but the signature is added to the transaction.

🚀 Step 3: Broadcasting to the Network

Once signed, the transaction is sent to the network—usually to a full node like Infura or Alchemy, or directly to an Ethereum node if you're self-hosting.

This step is like saying:

“Hey miners or validators, here’s a new transaction! Please include it in the next block!”

🔄 Step 4: The EVM Executes the Code

When the transaction is picked up by a node (usually by a validator or miner), the Ethereum Virtual Machine (EVM):

  1. Loads the smart contract's bytecode.
  2. Locates the function via its function selector (first 4 bytes of the data field).
  3. Executes the code line by line.
  4. Updates blockchain state if successful.

If anything fails (like a failed require), the whole transaction is reverted — no changes are made.

⛓️ Step 5: It’s Mined & Stored On-Chain

Once the transaction passes validation:

  • It's included in the next block.
  • That block is appended to the blockchain.
  • The transaction becomes immutable history.

Anyone can verify the transaction later on block explorers like Etherscan.

🧼 Behind the Scenes (Gas, Nonces, and State)

A few more things happening under the hood:

  • Gas: The fee paid to miners/validators for computation and storage.
  • Nonce: A unique number to prevent replay attacks and ensure transaction order.
  • State transitions: Events and changes logged to update the ledger.

Smart contracts are deterministic: if the same inputs are given, every node will compute the same result or revert.

🤖 Read vs Write: Calls That Don’t Cost Gas

If Alice just wants to read data (like checking sword ownership), no transaction is created:

const sword = await contract.getSword(2);
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  1. No gas
  2. No miner
  3. No blockchain write
  4. Just a simulated EVM call against the latest state

These are called "view" functions and are free.

🎯 Summary

When you “call” a smart contract, you're not just hitting a function. You’re:

  1. Encoding the function and its arguments
  2. Signing the request
  3. Broadcasting it to the network
  4. Letting thousands of nodes simulate, verify, and agree on the outcome
  5. Getting it permanently recorded on the chain

That’s a lot of trustless magic behind one click.

🛠️ Want to See It in Action?

Try this:

  • Write a simple contract with a public function
  • Deploy it to a testnet (like Sepolia)
  • Call it from your dApp with Ethers.js
  • Then watch the tx on Etherscan

You’ll see exactly what happens, from pending to confirmed.

💬 Over to You

What’s the smartest (or weirdest) smart contract you’ve interacted with?

Ever had a tx fail mysteriously?

Drop your thoughts in the comments 👇

👋 Let’s Connect

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