Blockchain, Ethereum and Smart Contracts

Usually, I try to make all the content I create original, based on readings, investigations, or just based on situations or questions I have heard or have been asked while working. Today, this article is a little bit different. It is not a copy from somewhere but, it has pieces that have been taken from other articles. You can find all the references at the end of this article. This article was just built as a part of some hack days I enjoyed recently where I decided to dig a little bit deeper on Blockchain, Ethereum, and Smart Contracts. This article is my field notes just in case someone else finds them useful.


What is Blockchain?

A blockchain is essentially an immutable digital ledger of transactions that is duplicated and distributed (shared) across the entire network of computer systems on the blockchain. Each block in the chain contains several transactions, and every time a new transaction occurs on the blockchain, a record of that transaction is added to every participant’s ledger. Blockchain facilitates the process of recording transactions and tracking assets in a business network. An asset can be tangible (i.e., a house, car, cash, land) or intangible (i.e., intellectual property, patents, copyrights, branding). Virtually anything of value can be tracked and traded on a blockchain network, reducing risk and cutting costs for all involved.

A decentralised database managed by multiple participants is known as Distributed Ledger Technology (DLT).

Blockchain is a type of DLT in which transactions are recorded with an immutable cryptographic signature called a hash.

The properties of Distributed Ledge Technology are:

  • Programmable: A blockchain is programmable i.e., smart contracts.
  • Secure: All records are individually encrypted.
  • Anonymous: The identity of participants is either anonymous or pseudo-anonymous.
  • Unanimous: All members of the network agree to the validity of each of the records.
  • Time-stamped: A transaction timestamp is recorded on a block.
  • Immutable: Any validated records are irreversible and cannot be changed.
  • Distributed: All network participants have a copy of the ledger for complete transparency.

How is Blockchain used?

Blockchain technology is used for many different purposes, from providing financial services to administering voting systems.


The most common use of blockchain today is as the backbone of cryptocurrencies, like Bitcoin or Ethereum. When people buy, exchange or spend cryptocurrency, the transactions are recorded on a blockchain. The more people use cryptocurrency, the more widespread blockchain could become.


Beyond cryptocurrency, blockchain is being used to process transactions in fiat currency, like dollars and euros. This could be faster than sending money through a bank or other financial institution as the transactions can be verified more quickly and processed outside of normal business hours.

Asset Transfers

Blockchain can also be used to record and transfer the ownership of different assets. This is currently very popular with digital assets like NFTs, a representation of ownership of digital art and videos.

However, blockchain could also be used to process the ownership of real-life assets, like the deed to real estate and vehicles. The two sides of a party would first use the blockchain to verify that one owns the property and the other has the money to buy; then they could complete and record the sale on the blockchain. Using this process, they could transfer the property deed without manually submitting paperwork to update the local county’s government records; it would be instantaneously updated in the blockchain.

Smart Contracts

Another blockchain innovation is self-executing contracts commonly called “smart contracts”. These digital contracts are enacted automatically once conditions are met. For instance, a payment for a good might be released instantly once the buyer and seller have met all specified parameters for a deal. Another example of this is to automate legal contracts such as “A properly coded smart legal contract on a distributed ledger can minimize, or preferably eliminate, the need for outside third parties to verify performance.”

Supply Chain Monitoring

Supply chains involve massive amounts of information, especially as goods go from one part of the world to the other. With traditional data storage methods, it can be hard to trace the source of problems, like which vendor poor-quality goods came from. Storing this information on the blockchain would make it easier to go back and monitor the supply chain, such as with IBM’s Food Trust, which uses blockchain technology to track food from its harvest to its consumption.


Experts are looking into ways to apply blockchain to prevent fraud in voting. In theory, blockchain voting would allow people to submit votes that could not be tampered with as well as would remove the need to have people manually collect and verify paper ballots.


Health care providers can leverage blockchain to securely store their patients’ medical records. When a medical record is generated and signed, it can be written into the blockchain, which provides patients with the proof and confidence that the record cannot be changed. These personal health records could be encoded and stored on the blockchain with a private key, so that they are only accessible by certain individuals, thereby ensuring privacy.

Advantages of Blockchain

Higher Accuracy of Transactions

Because a blockchain transaction must be verified by multiple nodes, this can reduce error. If one node has a mistake in the database, the others would see it is different and catch the error.

In contrast, in a traditional database, if someone makes a mistake, it may be more likely to go through. In addition, every asset is individually identified and tracked on the blockchain ledger, so there is no chance of double spending it (like a person overdrawing their bank account, thereby spending money twice).

No Need for Intermediaries

Using blockchain, two parties in a transaction can confirm and complete something without working through a third party. This saves time as well as the cost of paying for an intermediary like a bank. This can bring greater efficiency to all digital commerce, increase financial empowerment to the unbanked or underbanked populations of the world and power a new generation of internet applications as a result.

Extra Security

Theoretically, a decentralized network, like blockchain, makes it nearly impossible for someone to make fraudulent transactions. To enter forged transactions, they would need to hack every node and change every ledger. While this is not necessarily impossible, many cryptocurrency blockchain systems use proof-of-stake or proof-of-work transaction verification methods that make it difficult, as well as not in participants’ best interests, to add fraudulent transactions.

More Efficient Transfers

Since blockchains operate 24/7, people can make more efficient financial and asset transfers, especially internationally. They do not need to wait days for a bank or a government agency to manually confirm everything.

Disadvantages of Blockchain

Limit on Transactions per Second

Given that blockchain depends on a larger network to approve transactions, there is a limit to how quickly it can move. For example, Bitcoin can only process 4.6 transactions per second versus 1,700 per second with Visa. In addition, increasing numbers of transactions can create network speed issues. Until this improves, scalability is a challenge.

High Energy Costs

Having all the nodes working to verify transactions takes significantly more electricity than a single database or spreadsheet. Not only does this make blockchain-based transactions more expensive, but it also creates a large carbon burden for the environment.

Risk of Asset Loss

Some digital assets are secured using a cryptographic key, like cryptocurrency in a blockchain wallet. You need to carefully guard this key, there is no centralised entity that can be called to recover the access key.

Potential for Illegal Activity

Blockchain’s decentralization adds more privacy and confidentiality, which unfortunately makes it appealing to criminals. It is harder to track illicit transactions on blockchain than through bank transactions that are tied to a name.

Common misconception: Blockchain vs Bitcoin

The goal of blockchain is to allow digital information to be recorded and distributed, but not edited. Blockchain technology was first outlined in 1991 but it was not until almost two decades later, with the launch of Bitcoin in January 2009, that blockchain had its first real-world application.

The key thing to understand here is that Bitcoin merely uses blockchain as a means to transparently record a ledger of payments, but blockchain can, in theory, be used to immutably record any number of data points. As discussed above, this could be in the form of transactions, votes in an election, product inventories, state identifications, deeds to homes, and much more.

How does a transaction get into the blockchain?

For a new transaction to be added to the blockchain a few steps need to happen:


The original blockchain was designed to operate without a central authority (i.e. with no bank or regulator controlling who transacts), but transactions still have to be authenticated.

This is done using cryptographic keys, a string of data (like a password) that identifies a user and gives access to their “account” or “wallet” of value on the system.

Each user has their own private key and a public key that everyone can see. Using them both creates a secure digital identity to authenticate the user via digital signatures and to “unlock” the transaction they want to perform.


Once the transaction is agreed between the users, it needs to be approved, or authorised, before it is added to a block in the chain.

For a public blockchain, the decision to add a transaction to the chain is made by consensus. This means that the majority of “nodes” (or computers in the network) must agree that the transaction is valid. The people who own the computers in the network are incentivised to verify transactions through rewards. This process is known as “proof of work”.

Proof of Work

Proof of Work requires the people who own the computers in the network to solve a complex mathematical problem to be able to add a block to the chain. Solving the problem is known as mining, and “miners” are usually rewarded for their work in cryptocurrency.

But mining is not easy. The mathematical problem can only be solved by trial and error and the odds of solving the problem are about 1 in 5.9 trillion. It requires substantial computing power which uses considerable amounts of energy. This means the rewards for undertaking the mining must outweigh the cost of the computers and the electricity cost of running them, as one computer alone would take years to find a solution to the mathematical problem.

The Problem with Proof of Work

To create economies of scale, miners often pool their resources together through companies that aggregate a large group of miners. These miners then share the rewards and fees offered by the blockchain network.

As a blockchain grows, more computers join to try and solve the problem, the problem gets harder and the network gets larger, theoretically distributing the chain further and making it even more difficult to sabotage or hack. In practice though, mining power has become concentrated in the hands of a few mining pools. These large organisations have the vast computing and electrical power now needed to maintain and grow a blockchain network based around Proof of Work validation.

Proof of Stake

Later blockchain networks have adopted “Proof of Stake” validation consensus protocols, where participants must have a stake in the blockchain – usually by owning some of the cryptocurrency – to be in with a chance of selecting, verifying, and validating transactions. This saves substantial computing power resources because no mining is required.

In addition, blockchain technologies have evolved to include “Smart Contracts” which automatically execute transactions when certain conditions have been met.

Risk in public blockchains

51% Attacks

Where blockchains have consensus rules based on a simple majority, there is a risk that malign actors will act together to influence the outcomes of the system. In the case of a cryptocurrency, this would mean a group of miners controlling more than 50% of the mining computing power can influence what transactions are validated and added (or omitted) from the chain. On a blockchain that uses the Proof of Work (PoW) consensus protocol system, a 51% attack can also take the form of a “rival” chain – including fraudulent transactions – being created by malicious parties.

Through their superior mining capacity, these fraudsters can build an alternative chain that ends up being longer than the “true” chain, and therefore – because part of the Bitcoin Nakamoto consensus protocol is “the longest chain wins” – all participants must follow the fraudulent chain going forward.

In a large blockchain like Bitcoin this is increasingly difficult, but where a blockchain has “split” and the pool of miners is smaller, as, in the case of Bitcoin Gold, a 51% attack is possible.

A 51% double spend attack was successfully executed on the Bitcoin Gold and Ethereum Classic blockchains in 2018, where fraudsters misappropriated millions of dollars of value.

Proof of Work vs Proof of Stake

A 51% attack on a new blockchain called Ethereum Classic in January 2019 prompted a change in strategic direction from Proof-of-Work (PoW) mining to Proof-of-Stake (PoS) voting for the Ethereum blockchain.

However, Proof of Stake is more vulnerable to schisms or splits known as “forks”, where large stakeholders make different decisions about the transactions that should comprise blocks and end up creating yet another new currency. Ethereum briefly tried this validation method but, due to forking issues, reverted back to Proof of Work. It is expected to introduce a revised Proof of Stake validation system in 2020.

Double Spending

There is a risk that a participant with, for example, one bitcoin can spend it twice and fraudulently receive goods to the value of two bitcoins before one of the providers of goods or services realises that the money has already been spent. But this is, in fact, an issue with any system of electronic money, and is one of the principal reasons behind clearing and settlement systems in traditional currency systems.

How Does Blockchain Work?

Blockchain consists of three important concepts: blocks, nodes, and miners.


Every chain consists of multiple blocks and each block has three basic elements:

  • The data in the block.
  • A 32-bit whole number is called a nonce. The nonce is randomly generated when a block is created, which then generates a block header hash.
  • The hash is a 256-bit number wedded to the nonce. It must start with a huge number of zeroes (i.e., be extremely small).

When the first block of a chain is created, a nonce generates the cryptographic hash. The data in the block is considered signed and forever tied to the nonce and hash unless it is mined.


Miners create new blocks on the chain through a process called mining.

In a blockchain every block has its own unique nonce and hash, but also references the hash of the previous block in the chain, so mining a block isn’t easy, especially on large chains.

Miners use special software to solve the incredibly complex math problem of finding a nonce that generates an accepted hash. Because the nonce is only 32 bits and the hash is 256, there are roughly four billion possible nonce-hash combinations that must be mined before the right one is found. When that happens miners are said to have found the “golden nonce” and their block is added to the chain.

Making a change to any block earlier in the chain requires re-mining not just the block with the change, but all of the blocks that come after. This is why it’s extremely difficult to manipulate blockchain technology. Think of it is as “safety in maths” since finding golden nonces requires an enormous amount of time and computing power.

When a block is successfully mined, the change is accepted by all of the nodes on the network and the miner is rewarded financially.


One of the most important concepts in blockchain technology is decentralization. No one computer or organization can own the chain. Instead, it is a distributed ledger via the nodes connected to the chain. Nodes can be any kind of electronic device that maintains copies of the blockchain and keeps the network functioning.

Every node has its own copy of the blockchain and the network must algorithmically approve any newly mined block for the chain to be updated, trusted, and verified. Since blockchains are transparent, every action in the ledger can be easily checked and viewed. Each participant is given a unique alphanumeric identification number that shows their transactions.

Combining public information with a system of checks and balances helps the blockchain maintain the integrity and creates trust among users. Essentially, blockchains can be thought of as the scalability of trust via technology.


What is Ehtereum?

Ethereum is often referred to as the second most popular cryptocurrency, after Bitcoin. But unlike Bitcoin and most other virtual currencies, Ethereum is intended to be much more than simply a medium of exchange or a store of value. Instead, Ethereum calls itself a decentralized computing network built on blockchain technology.

It is distributed in the sense that everyone participating in the Ethereum network holds an identical copy of this ledger, letting them see all past transactions. It is decentralised in that the network is not operated or managed by any centralised entity – instead, it is managed by all of the distributed ledger holders.

In Ethereum’s case, participants, when mining, are rewarded with cryptocurrency tokens called Ether (ETH).

Ether can be used to buy and sell goods and services, like Bitcoin. It’s also seen rapid gains in price over recent years, making it a de-facto speculative investment. But what’s unique about Ethereum is that users can build applications that “run” on the blockchain like software “runs” on a computer. These applications can store and transfer personal data or handle complex financial transactions. This is one of the big differences with Bitcoin, the Ethereum network can perform computations as part of the mining process, this basic computational capability turns a store of value and medium of exchange into a decentralized global computing engine and openly verifiable data store.

While inferring from the previous paragraphs, it is worth it to emphasise here the meanings of the terms Ethereum and Ether are different: Ether as a digital currency for financial transactions, while Ethereum is the blockchain network on which Ether is held and exchanged, and the network offers a variety of other functions outside of ETH.

As said before, the Ethereum network can also be used to store data and run decentralized applications. Rather than hosting software on a server owned and operated by Google or Amazon, where the one company controls the data, people can host applications on the Ethereum blockchain. This gives users control over their data and they have open use of the app as there’s no central authority managing everything.

Perhaps one of the most intriguing use cases involving Ether and Ethereum are self-executing contracts or so-called smart contracts. Like any other contract, two parties make an agreement about the delivery of goods or services in the future. Unlike conventional contracts, lawyers are not necessary: The parties code the contract on the Ethereum blockchain, and once the conditions of the contract are met, it self-executes and delivers Ether to the appropriate party.

Ethereum Benefits

  • Large, existing network: Ethereum is a tried-and-true network that has been tested through years of operation and billions of value trading hands. It has a large and committed global community and the largest ecosystem in blockchain and cryptocurrency.
  • Wide range of functions: Besides being used as a digital currency, Ethereum can also be used to process other types of financial transactions, execute smart contracts and store data for third-party applications.
  • Constant innovation: A large community of Ethereum developers is constantly looking for new ways to improve the network and develop new applications. Because of Ethereum’s popularity, it tends to be the preferred blockchain network for new and exciting (and sometimes risky) decentralised applications.
  • Avoids intermediaries: Ethereum’s decentralised network promises to let users leave behind third-party intermediaries, like lawyers who write and interpret contracts, banks that are intermediaries in financial transactions or third-party web hosting services.

Ethereum Disadvantages

  • Rising transaction costs: Ethereum’s growing popularity has led to higher transaction costs. Ethereum transaction fees, also known as “gas,” hit a record $23 per transaction in February 2021, which is great if you’re earning money as a miner but less so if you’re trying to use the network. This is because unlike Bitcoin, where the network itself rewards transaction verifiers, Ethereum requires those participating in the transaction to cover the fee.
  • Potential for crypto inflation: While Ethereum has an annual limit of releasing 18 million Ether per year, there’s no lifetime limit on the potential number of coins. This could mean that as an investment, Ethereum might function more like dollars and may not appreciate as much as Bitcoin, which has a strict lifetime limit on the number of coins.
  • The steep learning curve for developers: Ethereum can be difficult for developers to pick up as they migrate from centralised processing to decentralised networks.
  • Unknown future: Ethereum continues to evolve and improve, and the release of Ethereum 2.0 (1st of December 2020) holds out the promise of new functions and greater efficiency. This major update to the network, however, is creating uncertainty for apps and deals currently in use. Is someone familiar with migrations?

Ethereum as a Platform for Applications

Single Source of Truth

The Ethereum blockchain establishes a single source of truth for the ecosystem by maintaining a public transaction database. This shared database captures all transactions that occur between users and applications. Unique virtual addresses identify actors, and each transaction captures participating addresses. These addresses do not reveal any personal information, allowing users to remain anonymous. Transactions are batched together into blocks and validated by thousands of computers or “nodes” before joining the public ledger.

Once posted, no one can remove or alter transactions. Because records are sealed, bad actors cannot revert a transaction after the fact or tamper with the public record. Balances are known and transactions are settled in real-time, so the entire ecosystem is on the same page. This append-only approach assures users and applications that the current state of the blockchain is final and trustworthy.

Platform for Applications

In addition to providing a shared source of truth, the Ethereum blockchain provides a platform for applications. The ability to store and power these applications sets the Ethereum blockchain apart from the Bitcoin blockchain.

The Ethereum blockchain provides critical application infrastructure similar to web services. Thousands of nodes that maintain the single source of truth also supply resources like storage, processing power, and bandwidth. Because people run these nodes all over the globe, Ethereum is referred to as the “world computer” because the collective resources function as a single machine.

Ethereum is different from centralized web services in that transaction data and applications are distributed across thousands of nodes, rather than a few data centres controlled by a corporation. This feature, known as decentralization, leads to a highly redundant and resilient ecosystem that cannot be controlled or censored by a single entity.

The code for these applications lives on the blockchain just like the transactions created by users and other applications. As a result, applications deployed on Ethereum are open and auditable. These apps are also designed to interoperate with other apps in the ecosystem (a stark departure from the traditional “black box” approach for software). Once applications are deployed to Ethereum, they operate autonomously, meaning that they will execute the programs they are designed to run without manual intervention. They are controlled by code, not by individuals or companies. For this reason, applications are referred to as “smart contracts”.

A common analogy for smart contracts is vending machines. Vending machines are programmed to automatically deliver specific items based on specific inputs. Users punch in a code and receive the corresponding item. Likewise, smart contracts receive inputs from users, execute their programmed code, and produce an output.

Decentralized Finance (DeFi)

Because the most fundamental data recorded on the Ethereum blockchain is accounts, balances, and transactions, it makes sense to build financial applications on top of Ethereum. Users and other applications can freely interact with these financial applications because they are public and permissionless by default.

Simple financial services like lending and borrowing can be programmed and deployed on the blockchain as applications, allowing users (and other applications) to earn interest on digital assets or take out loans. Order book exchanges can autonomously pair buyers and sellers at no charge. Automated market makers can minimise spreads by creating liquidity pools that automatically rebalance according to predefined logic. Derivatives can be deployed on the blockchain so that contract terms are known and the underlying assets are priced in real-time.

Smart contracts

A smart contract is a self-executing contract with the terms of the agreement between buyer and seller being directly written into lines of code. The code and the agreements contained therein exist across a distributed, decentralised blockchain network. The code controls the execution, and transactions are trackable and irreversible. In other words, a smart contract is simply a piece of code that is running on Ethereum and can control valuable things like ETH or other digital assets.

Smart contracts permit trusted transactions and agreements to be carried out among disparate, anonymous parties without the need for a central authority, legal system, or external enforcement mechanism.

The fact that smart contracts are computer programs deployed on a blockchain network brings to the table some inherent characteristics:

  • Immutable: Once deployed, the code of a smart contract cannot change. Unlike traditional software, the only way to modify a smart contract is to deploy a new instance.
  • Deterministic: The outcome of the execution of a smart contract is the same for everyone who runs it, given the context of the transaction that initiated its execution and the state of the Ethereum blockchain at the moment of execution.
  • Ethereum Virtual Machine (EVM) context: Smart contracts operate with a very limited execution context. They can access their own state, the context of the transaction that called them, and some information about the most recent blocks.
  • Decentralized world computer: The EVM runs as a local instance on every Ethereum node, but because all instances of the EVM operate on the same initial state and produce the same final state, the system as a whole operates as a single “world computer” (as mentioned before).

Languages to write smart contracts

  • LLL: A functional (declarative) programming language, with Lisp-like syntax. It was the first high-level language for Ethereum smart contracts but is rarely used today.
  • Serpent: A procedural (imperative) programming language with a syntax similar to Python. Can also be used to write functional (declarative) code, though it is not entirely free of side effects.
  • Solidity: A procedural (imperative) programming language with a syntax similar to JavaScript, C++, or Java. The most popular and frequently used language for Ethereum smart contracts. (We will be using this one)
  • Vyper: A more recently developed language, similar to Serpent and again with Python-like syntax. Intended to get closer to a pure-functional Python-like language than Serpent, but not to replace Serpent.
  • Bamboo: A newly developed language, influenced by Erlang, with explicit state transitions and without iterative flows (loops). Intended to reduce side effects and increase audibility. Very new and yet to be widely adopted.

Non-fungible tokens (NFT)

NFTs are tokens that we can use to represent ownership of unique items. They let us tokenise things like art, collectables, even real estate. They can only have one official owner at a time and they’re secured by the Ethereum blockchain – no one can modify the record of ownership or copy/paste a new NFT into existence.

NFT stands for non-fungible token. Non-fungible is an economic term that you could use to describe things like your furniture, a song file, or your computer. These things are not interchangeable with other items because they have unique properties.

Fungible items, on the other hand, can be exchanged because their value defines them rather than their unique properties. For example, ETH or dollars are fungible because 1 ETH / 1 USD is exchangeable for another 1 ETH / 1 USD.


An NFT internetThe internet today
NFTs are digitally unique, no two NFTs are the same.A copy of a file, like a .mp3 or .jpg, is the same as the original.
Every NFT must have an owner and this is of public record and easy for anyone to verify.Ownership records of digital items are stored on servers controlled by institutions – you must take their word for it.
NFTs are compatible with anything built using Ethereum. An NFT ticket for an event can be traded on every Ethereum marketplace, for an entirely different NFT. You could trade a piece of art for a ticket!Companies with digital items must build their own infrastructure. For example, an app that issues digital tickets for events would have to build its own ticket exchange.
Content creators can sell their work anywhere and can access a global market.Creators rely on the infrastructure and distribution of the platforms they use. These are often subject to terms of use and geographical restrictions.
Creators can retain ownership rights over their own work, and claim resale royalties directly.Platforms, such as music streaming services, retain the majority of profits from sales.
Items can be used in surprising ways. For example, you can use digital artwork as collateral in a decentralised loan.


Solidity is a contract-oriented, high-level language for implementing smart contracts. It was influenced by C++, Python, and JavaScript and is designed to target the Ethereum Virtual Machine (EVM).

Solidity is statically typed, supports inheritance, libraries, and complex user-defined types among other features.

Let’s see an example. Let’s build a simple token contract:

pragma solidity ^0.4.0;

contract SimpleToken {
    int64 constant TOTAL_UNITS = 100000;
    int64 outstanding_tokens;
    address owner;
    mapping(address => int64) holdings;

    function SimpleToken() public { // Constructor
        outstanding_tokens = TOTAL_UNITS;
        owner = msg.sender; // msg.sender represents the address that initiated this contract call

    // Declaring some events
    event TokenAllocation(address holder, int64 number, int64 remaining);
    event TokenMovement(address from, address to, address value);
    event InvalidTokenUsage(string reason);

    function getOwner() public constant returns (address) {
        return owner;

    // Allocate tokens
    function allocate(address newHolder, int64 value) public {
        if (msg.sender != owner) {
            InvalidTokenUsage('Only owner can allocate tokens');

        if (value < 0) {
            InvalidTokenUsage('Cannot allocate negative value');

        if (value <= outstanding_tokens) {
            holdings[newHolder] += value;
            outstanding_tokens -= value;
            TokenAllocation(newHolder, value, outstanding_tokens);
        } else {
            InvalidTokenUsage('Value to allocate longer that outstanding tokens');

    // Move tokens
    function move(address destination, int64 value) public {
        address source = msg.sender;

        if (value < 0) {
            InvalidTokenUsage('Must move value greater than zero');

        if (holdings[source] >= value) {
            holdings[destination] += value;
            holdings[source] -= value;
            TokenMovement(source, destination, value);
        } else {
            InvalidTokenUsage('Value to move longer than holdings');

    // Getters & fallback
    function myBalance() constant public returns (int64) {
        return holdings[msg.sender];

    function holderBalance(address holder) constant public returns (int64) {
        if (msg.sender != owner) {

        return holdings[holder];

    function outstandingBalance() constant public returns (int64) {
        if (msg.sender != owner) {

        return outstanding_tokens;

    function() public {

For reasons related to the environment where I am working, it is a bit restricted, the contract does not follow the latest style described on the documentation of the most recent version but, as an example, it should work. You can find the latest version here.

Building stuff

Setting up the environment

Dependencies required:

  • nodejs
  • Truffle framework: Framework to create Ethereum smart contracts
  • Ganache: Quickly fire up a personal Ethereum blockchain which you can use to run tests, execute commands, and inspect state while controlling how the chain operates.
  • Metamask: Configure out fake ether addresses to interact with the apps we are building. It is a Chrome Extension.

Installing dependencies

# Installing node
$ sudo apt install nodejs
$ node -v
# Install Truffle
$ sudo npm install -g truffle
# Installing Ganache
$ wget
$ chmod a+x ganache-2.5.4-linux-x86_64.AppImage
$ ./ganache-2.5.4-linux-x86_64.AppImage
# Installing Solidity compiler
$ sudo add-apt-repository ppa:ethereum/ethereum
$ sudo apt update
$ sudo apt install solc

Project 1: Memory Game with Blockchain

Memory Game, also known as the Concentration card game or Matching Game, is a simple card game where you need to match pairs by turning over 2 cards at a time. Once this match has been done, we can keep the card forever adding it to the blockchain.

Elements involved:

  • Smart contract
  • NFT

Source code can be found here.

Project 2: Decentralised Twitter

Just a very basic decentralized Twitter.

Elements involved:

  • Drizzle: A collection of front-end libraries that make writing Dapp user interfaces easier and more predictable.
  • Smart contract

Source code can be found here.


Ether, Gas, Gas Cost, Fees

  • Ether – the cryptocurrency underpinning Ethereum.
  • Gas – the unit used to measure the execution of your transaction.
  • Gas Cost – the price of one “gas unit” that you are prepared to pay.
  • Set the higher gas costs to get faster confirmation.
  • Fee – the (gas * gasCost) cost you pay to run your transaction.


  • Ethereum nodes
    • Geth: It is an Ethereum-client, which means that we can run our own private blockchain with it. Command-line
    • parity: Parity Ethereum is a software stack that lets you run blockchains based on the Ethereum Virtual Machine (EVM) with several different consensus engines.
    • Ganache: It allows you to create your own private blockchain mainly for testing purposes It has UI
  • Cloud environments
    • Infura’s development suite provides instant, scalable API access to the Ethereum and IPFS networks.
    • Microsoft Azure
  • IDEs
    • Normal IDEs. IntelliJ have a plugin for Solidity
    • Javascript editors are good for building the tests and any app
  • Dev environment
    • Web3j: Web3j is a library for working with Smart Contracts and integrating with Ethereum blockchains. This allows you to work with Ethereum blockchains, without the additional overhead of having to write your own integration code for the platform.
    • Embark: The all-in-one developer platform for building and deploying decentralized applications
    • Truffle: Framework to create Ethereum smart contracts
    • Brownie: Brownie is a Python-based development and testing framework for smart contracts targeting the EVM.
  • Tools
    • Etherchain: makes the Ethereum blockchain accessible to non-technical end-users.
    • remix: Remix IDE is an open-source web and desktop application. It fosters a fast development cycle and has a rich set of plugins with intuitive GUIs. Remix is used for the entire journey of contract development as well as being a playground for learning and teaching Ethereum.
    • etherscan: Etherscan is a Block Explorer and Analytics Platform for Ethereum, a decentralized smart contracts platform
    • EthGasStation: ETH Gas Station aims to increase the transparency of gas prices, transaction confirmation times, and miner policies on the Ethereum network.
    • Metamask: MetaMask is a software cryptocurrency wallet used to interact with the Ethereum blockchain. It allows users to access their Ethereum wallets through a browser extension or mobile app.

Blockchain for Development

EmulatorsGanache, Embark
Lightweight nodesEthereumjs-vm, Pyethereum
Local Regular BlockchainsGeth, Parity
Hosted Nodes or ChainsInfura, Azure
Public Testing BlockchainsRinkeby, Ropsten
Public BlockchainMainnet


What Is Blockchain?

Blockchain explained

Blockchain technology defined

Mastering Ethereum (book)

Ethereum Whitepaper

What is Ethereum and How does it work?

Understanding Ethereum

Intro to Ethereum programming (video)

Blockchain, Ethereum and Smart Contracts