How to Implement DApps in Just 5 Minutes
As Skillsoft instructors Kishan Iyer and Ubaid Rehman Qureshi recently explained, blockchain is everywhere. One of the main reasons for its popularity is that once you write data to a blockchain, it is impossible to tamper with it. This capability makes blockchain the perfect fit for securely logging supply chain transactions, recording votes in an election, and for potentially so much more.
Since blockchain is primarily used to record transactions, you must first establish these transactions. To do this, you first need to build smart contracts. Smart contracts use a programming language to determine the operations. Users of blockchain-enabled applications do not interact with smart contracts directly. Rather, they use a web application that invokes these contracts. In the Ethereum network, these are called decentralized applications or DApps.
Why DApps are so critical
In a previous blog post, we heard from Roberto Infante, author of Building Ethereum DApps. He provided an introduction to DApps, explaining how they run on each server of a dedicated peer-to-peer network, where a different organization owns every server rather than running in a central server (or cluster of servers) owned or leased by the organization that developed it. Consequently, there is no central control or ownership. This capability means that a user doesn’t need to trust the owner of the application since each transaction is validated independently by each node of the network
In this post, Robert provides a lesson on taking the next step and using DApps to start coding the smart contract of a custom cryptocurrency.
Building SimpleCoin, a basic cryptocurrency
Most DApps are designed on functionality based on the exchange of cryptocurrency or tokens through rules encoded in one or more smart contracts. You’ll start to get a feel for DApps programming by building SimpleCoin, a basic cryptocurrency which presents useful preliminary concepts about smart contracts and the Ethereum platform.
Open a web browser and go to: https://ethereum.github.io/browser-solidity/
You should see a screen like in figure 1.
The left-hand side of the IDE is a code editor. The right-hand hand side contains various panels to run the code and interact with it.
In your first encounter with Solidity you’ll implement the simplest possible smart contract. If you think of a smart contract as the equivalent of a class in an object-oriented language, you’ll write a single class with only one-member field, one constructor and one method. Then you’ll run it and interact with it.
Enter the code of the following listing in the Remix editor, on the left side of the screen.
❶ Pragma directive indicating the supported version of the Solidity compiler. The code supports a compiler later than 0.4.0 but earlier than 0.5.0.
❷ Defines a contract, which is like a class in other languages.
❸ Defines a state variable as a “mapping” between an address and an integer. A state variable is the equivalent of a member variable. A mapping is the equivalent to a hash table or hash map.
❹ Starts defining the contract constructor.
❺ Assigns 10000 SimpleCoin tokens to the coin account with address 0x14723a09acff6d2a60dcdf7aa4aff308fddc160c at contract creation.
❻ Defines a function which moves a number of SimpleCoin tokens from the coin account of the function caller to a specified coin account.
❼ Decreases the coin balance of the message sender by the specified number of tokens. The special implicit property msg.sender represents the address of the sender of the transaction.
❽ Increases the coin balance by the specified number of tokens.
Let’s examine this code in detail. A contract, in Solidity, is a type similar to a class in any other language: it has state variables (such as coinBalance), a constructor (such as function SimpleCoin), functions (such as transfer), and events.
The coinBalance state variable is defined as a mapping. A mapping is a hash map, equivalent to a hashMap in Java, Dictionary in C# or dict in Python. In this example, the type of the key is an address, whereas the value is a uint256, an unsigned 256-bit integer. An address holds a 20-byte value and it can identify a specific smart contract account or a specific user account. An account, as we’ll see later in detail, is the sender or the receiver of a transaction. coinBalancetherefore represents a collection of coin accounts, each holding several SimpleCoin tokens.
The transfer function is meant to move a number of SimpleCoin tokens from the coin account of the function caller to a specified coin account. In smart contract terminology, a function caller is the transaction sender. msg is a special implicitly defined variable that represents the incoming message. It has various properties, among which msg.sender represents the address of the sender of the transaction, who is the caller of transfer. The body of the transfer function is simple to understand. It’s subtracting the specified amount from the cash account associated with the function caller and adding the amount specified in the _amount parameter to the account associated with the address specified in the _to parameter. To keep this initial code simple, this implementation isn’t performing yet any boundary checks on the number of SimpleCoin tokens owned by the transaction sender, who, for example, shouldn’t be allowed to send more tokens than the owned ones.
At this point you should understand that our SimpleCoin “contract” is, in practice, a class with a constructor (SimpleCoin function), some state (coinBalance variable), and a method (transferfunction). Table 1 gives a quick summary of the Solidity keywords you’ve come across.
Table 1. A summary of Solidity keywords met in the first code sample
Let’s run the contract
Move now to the right-hand side of the screen to instantiate the SimpleCoin contract.
If you’ve typed your code correctly, and there are no compilation errors, you should see the following buttons in the Contract panel: Publish, At Address and Create, as shown in figure 2. Ignore for the moment Publish and At Address and focus your attention on Create. By clicking this button, you’ll instantiate the SimpleCoin contract.
The contract is stored against an address identifying its location on the Ethereum blockchain. You can see it by clicking on the “Copy Address” button.
Now that the SimpleCoin contract has been instantiated you can perform simple operations against it: you’ll check SimpleCoin token balances and move tokens across accounts.
After clicking on Create, two new buttons appear: coinBalance and transfer, as shown in figure 3.
The Remix IDE shows two types of buttons:
- Blue buttons. They perform read operations against the contract, such as to check the value of state variables or call read-only functions.
- Red buttons. They perform “write” operations against the contract, such as instantiating the contract through the constructor (Create) or, call functions which modify any state variables.
In our case, coinBalance is blue because it allows you to read the coin balance associated with an address. transfer is red because by clicking it you alter the state of the contract, specifically by changing values contained in the coinBalance mapping state variable.
Let’s check that the coinBalance associated with the address specified in the constructor has indeed the full initial supply of SimpleCoin you set at construction. Wrap the address with double quotes: “0x14723a09acff6d2a60dcdf7aa4aff308fddc160c”, enter it in the textbox and click coinBalance. Some output appears, and at its bottom you should see the expected number of SimpleCoin tokens you specified in the constructor: 10000.
0x14723a09acff6d2a60dcdf7aa4aff308fddc160c is the address of one of the five test accounts present on the Remix IDE. You can see them in the Transaction Origin dropdown list box on the top right of the screen. Although they aren’t fully visible on the screen, their full addresses are reported in table 2. (I have retrieved them by copying them one by one by clicking on the Copy Address icon next to the Account dropdown):
Table 2. Remix test accounts whose full address is hidden behind the HTML
You can double check that the amount of SimpleCoin tokens associated with any address different from 0x14723a09acff6d2a60dcdf7aa4aff308fddc160c is zero. For instance, enter the following address, wrapped with double quotes as you did earlier, in the coinBalancetextbox: “0x583031d1113ad414f02576bd6afabfb302140225”. After clicking the button, you’ll see, as expected, an amount equal to 0.
Recapping, when you instantiated the contract, an amount of 10000 SimpleCoin tokens got assigned, as initial money supply, to the address starting with 0x14723a09. No other address owns any token yet, as summarized in table 3.
Table 3. Balance of each Remix test account after contract instantiation
Now you can call the transfer function to move some tokens from the account with the address starting with 0x14723a09 to a different test account. Because the transfer function moves tokens from the account of its caller, the function must be called from the contract creator’s address starting with 0x14723a09. Pick this address from the Transaction Origin dropdown, then enter in the textbox of the transfer method the destination address (let’s pick, for example, the address starting with 0x4b0897b0) and a number of tokens to be transferred (for instance, 150 tokens). The values of these parameters should be separated by a comma:
"0x4b0897b0513fdc7c541b6d9d7e929c4e5364d2db", 150 Now click the transfer button. Apart from the usual information on transaction and execution costs, there’s no actual return value from the function, as expected.
Check the number of tokens present in the contract creator’s address by clicking on the coinBalance button after having entered again the contract creator’s address (“0x14723a09acff6d2a60dcdf7aa4aff308fddc160c”) in the related textbox: the value is now 9850, as expected.
If you perform the same check on the destination address (“0x4b0897b0513fdc7c541b6d9d7e929c4e5364d2db”) you get: 150. All other addresses still have zero tokens, as summarized in Table 4:
Table 4. Balance of each Remix test account after a transfer operation
As an exercise, you can try to transfer coins from the address starting with 0x4b0897b05 to a different address and recheck to see if the amounts are correct. While doing this, please don’t perform any “crazy” transactions, such as trying to move more coins than a certain address holds. As you know, in order to keep the code simple for the moment, you haven’t coded yet any boundary conditions.
Although the code you’ve written for this far is simple, your main objective, at this stage, is only starting to familiarize yourself with smart contracts, with the Solidity language, and with Remix. Hopefully you’ve achieved the objective. You now understand how contract instantiation works and how to interact with a contract from different accounts.
Want to learn more?
The App Developer to Blockchain Engineer Skillsoft Aspire learning journey takes a learner on a path that starts with courses covering areas that app developers typically are involved with on a day to day basis. Topics such as Smart Contracts and Ethereum Wallets then help them progress to the next role. By the end of the journey, the learner will have worked up to building decentralized applications and building blockchains on the cloud.
To ensure anyone taking the four-step journey fully understands what is necessary for their new role, we have assessments and hands-on practice labs (done virtually on real equipment/applications) so each learner can demonstrate their knowledge and applicability of the material covered. At the culmination of the journey, users must pass a rigorous final exam and complete a capstone project to earn their credential.
This “How to” is from Roberto Infante’s Building Ethereum DApps, published by Manning Publications. Skillsoft is partnering with Manning Publications, a leading publisher of technology and developer content, to deliver more than 250 current and backlist Manning titles to Skillsoft customers. Manning’s bestsellers include titles across a wide range of languages, technologies, and new tech, including Kubernetes in Action, Deep Learning with Python, Java 8 in Action, Grokking Algorithms, Spring Microservices in Action, Serverless Architectures on AWS, Functional Programming in C#, and Deep Learning with R. Manning’s highly respected content on foundational technology topics and emerging areas greatly expands Skillsoft Tech & Developer book offerings and enhances our Skillsoft Aspire learning journeys. Manning will add new titles to Skillsoft Books on a monthly basis.
Roberto Infante is a software development consultant who specializes in finance. He currently works on financial risk management systems and on blockchain technology.