Building Smart Contracts on the Ethereum Blockchain

Last week’s Comcast Lab Week gave me another opportunity to dig deeper into Blockchains. In my previous writeup on CodeCoin, I had used Ethereum to create a bounty system for Github issues. However under the hood, we had cloud servers managing various wallets belonging to the different issues. Smart Contracts offer a better way to handle this.

What Are Smart Contracts?

Smart Contracts are pieces of code that execute on the Blockchain. Think of them as classes in an Object Oriented Programming model. Once deployed, you can invoke methods on the Contract from any wallet on the Blockchain. The method, when called, gets not only the parameters that you explicitly sent as part of the method call but also the caller’s wallet address and any value (Ether or its smaller fraction Wei) that the contract was sent. The contract can then keep part or all of the value sent in return for executing the code.

The world’s simplest Smart Contract.

Note that the above contract is a “hello world” contract. Ours was a bit more complicated.

Tools and Setup

Similar to the last time, we used the TestRPC program, now rebranded as Ganache, or specifically it’s CLI  version, the Ganache-CLI) to develop and test the application. The app itself was pretty simple: It allowed users to rent an asset in our store by sending a particular amount of ether to our smart contract.

ganache setup
Configuring Ganache

In addition to Ganache, we also used the Truffle framework to build the application as well as MetaMask to run the transaction.

The Smart Contract itself was written in Solidity. Even though there are may editor plugins you can add to your favorite editor, I found using the the cloud hosted Remix IDE the best to get started. It’s already configured with linters and debug tools that make developing your contract much simpler.

The Truffle framework can be thought of as an equivalent to Ruby on Rails, just for Ethereum projects. When you start a new project, it creates a project structure with folders for contracts, migrations, tests and a truffle.js configuration file that lets you deploy your code to various dev/test/prod environments (think of truffle.js as a config.yml from Rails)

Once you place your contract in the contracts/ folder, running truffle compile compiles the Solidity code to the bytecode that can be deployed to the Ethereum Blockchain. Running truffle migrate deploys the contract to the chain. Truffle also provides a handy REPL console that you can use to interact with your contract from the command line. Very convenient to find your contract’s deployed address for example by calling

Truffle Console
Find the address of the SimpleStorage Contract

Running the Application

Our client application ran a small React app that would send a little bit of Ether to a deployed smart contract. If the transaction was successful, we’d send the successful Transaction ID to a middleware server that would validate it and authorize the user to a piece of content

The client code communicates with the Blockchain via the Web3 library injected into the browser by MetaMask. The code to use the contract looks something like this:

That’s pretty much it.

Random Learnings

  • We had issues with MetaMask and Ganache seeing each other’s wallets. This might be by design but to get anything done, I had to write another small server script that funded Metamask accounts with Ether from Ganache accounts
  • All transactions are done in Wei (10^-18 Ether). During development we’d send small values like 10 Wei across and were perplexed that Metamask or other wallets didn’t change their displays till we realized that the number was too small for MetaMask to show in its UI
  • We assumed a successful transaction on the contract assumed successful payment. We did not wait on the transaction to be mined to declare payment success. We should be waiting on that using the Web3’s filter API but we just ran out of time on the project.
  • To connect with a contract using Web3, you need to point it to its address and the JSON interface of the contract. Truffle’s console has a `toJSON()` method but that is not the JSON you are looking for. The right JSON file is located in the `/build/contracts/` directory. Once you have that JSON file you can create the Contract object in Web3 by using
    var myContract = new web3.eth.Contract(SimpleStorageABI, address)
  • We found an interesting project called OpenZeppelin that seems to be a public repo/tool for often used Contracts. I need to try that next.

Final Thoughts

This was a fun 5 day project that demystified a lot of things around Smart Contracts that I wasn’t sure about. And as always, it was great to work with a bunch of smart engineers I usually don’t get the opportunity to work with otherwise 🙂

Author: Arpit Mathur

Arpit Mathur is a Principal Software Engineer at Comcast Labs where he is currently working on Virtual and Augmented Reality as well as investigating Blockchains and the decentralized internet.

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