To execute the instructions given in this chapter, you can use a machine with any operating system (Mac, Windows, or Linux) with a browser installed on it. Code for this chapter can be found on GitHub at https://github.com/PacktPublishing/Solidity-Programming-Essentials-Second-Edition/tree/main/Chapter01.

A blockchain is essentially a decentralized, distributed database or ledger, as follows:

• Decentralization: In simple terms, this means that the application or service continues to be available and usable even if a server or a group of servers on a network crashes or is not available. The service or application is deployed on a network in a way that no server has absolute control over data and execution; rather, each server has a current copy of data and execution logic.
• Distributed: This means that any server or node on a network is connected to every other node on the network. Rather than having one-to-one or one-to-many connectivity between servers, servers have many-to-many connections with other servers.
• Database: This refers to the location for storing durable data that can be accessed at any point in time. A database allows the storage and retrieval of data as functionality and also provides management functionalities to manage data efficiently, such as exporting, importing, backup, and restoration.
• Ledger: This is an accounting term. Think of it as specialized storage and retrieval of data. Think of ledgers that are available to banks – for example, when a transaction is executed with a bank. Let's say that Tom deposits USD 100 in his account; the bank enters this information in a ledger as credit. At some point in the future, Tom withdraws USD 25. The bank does not modify the existing entry and stored data from 100 to 75. Instead, it adds another entry in the same ledger as a debit of USD 25. This is because a ledger is a specialized database that does not allow modification of existing data. It allows you to create and append a new transaction to modify the current balance in the ledger. The blockchain is a database that has the same characteristics as a ledger. It allows newer transactions to be stored in an append-only pattern without any scope to modify past transactions. It is important here to understand that existing data can be modified by using a new transaction, but past transactions cannot be modified. A balance of USD 100 can be modified at any time by executing a new debit or credit transaction, but previous transactions cannot be modified. Take a look at the following diagram for a better understanding:

Figure 1.1 – A blockchain is essentially a chain of blocks

Blockchain means a chain of blocks – that is, having multiple blocks chained together, with each block storing transactions in such a way that it is not possible to change these transactions. We will discuss this in later sections when we talk about the storage of transactions and how immutability is achieved in a blockchain.

Because they are decentralized and distributed, blockchain solutions are stable, robust, durable, and highly available. There is no single point of failure. No single node or server is the owner of the data and solution, and everyone participates as a stakeholder.

Not being able to change and modify past transactions makes blockchain solutions highly trustworthy, transparent, and incorruptible.

The main objective of blockchain is to accept transactions from accounts, update the current state, and maintain this state till another transaction updates it again. This entire process can be divided into two phases in blockchain. There is a decoupling in between when a transaction is accepted by Ethereum and when the transaction is executed and written to the ledger. This decoupling is quite important for decentralization and distributed architecture to work as expected.

Blockchain helps primarily in the following three different ways:

• Trust: Blockchain helps in creating applications that are decentralized and collectively owned by multiple people. Nobody within this group has the power to change or delete previous transactions. Even if someone tries to do so, it will not be accepted by other stakeholders.
• Autonomy: There is no single owner for blockchain-based applications. No one controls the blockchain, but everyone participates in its activities. This helps in creating solutions that cannot be manipulated or corrupted.
• Intermediaries: Blockchain-based applications can help remove intermediaries from existing processes. Generally, there is a central body, such as vehicle registration and license issuing, that acts as a registrar for registering vehicles as well as issuing driver licenses. Without blockchain-based systems, there is no central body, and if a license is issued or a vehicle is registered after a blockchain mining process, that will remain a fact for an epoch (an epoch is a period of time – say, 5 seconds) without the need of any central authority vouching for it.

Blockchain is heavily dependent on cryptography technologies, as we will discuss in the following section.

Cryptography is the science of converting plain, simple text into secret, hidden, and meaningful text, and vice versa. It also helps in transmitting and storing data that cannot be easily deciphered using owned keys.

There are two types of cryptography in computing:

• Symmetric cryptography: This refers to the process of using a single key for both encryption and decryption. It means the same key should be available for multiple people if they want to exchange messages using this form of cryptography.
• Asymmetric cryptography: This refers to the process of using two keys for encryption and decryption. Any key can be used to encrypt and decrypt. Messages encrypted with a public key can be decrypted using a private key, and messages encrypted by a private key can be decrypted using a public key. Let's understand this with the help of an example. Tom uses Alice's public key to encrypt messages and sends it to Alice. Alice can use her private key to decrypt the message and extract its content. Messages encrypted with Alice's public key can only be decrypted by Alice, as only she holds her private key and no one else. This is the general use case of asymmetric keys. There is another use that we will see in the Digital signatures section.

Hashing

Hashing is the process of transforming any input data into fixed-length random character data, and it is not possible to regenerate or identify the original data from the resultant hash. Hashes are also known as fingerprints of input data. It is next to impossible to derive input data based on its hash value. Hashing ensures that even a slight change in input data will completely change the output data, and no one can ascertain the change in the original data.

Another important property of hashing is that no matter the size of input string data, the length of its output is always fixed. For example, using the SHA-256 hashing algorithm and function with any length of input will always generate 256-bit output data. This can especially become useful when large amounts of data can be stored as 256-bit output data. Ethereum uses the hashing technique quite extensively. It hashes every transaction, hashes the hash of two transactions at a time, and ultimately generates a single root transaction hash for every transaction within a block.

Another important property of hashing is that it is not mathematically feasible to identify two different input strings that will output the same hash. Similarly, it is not possible to find the input computationally and mathematically from the hash itself.

Ethereum uses Keccak256 as its hashing algorithm. The following screenshot shows an example of hashing. The Ritesh Modi input generates a hash, as shown in the following screenshot:

Figure 1.2 – A hashing example using the SHA-256 algorithm

Even a small modification of input generates a completely different hash, as shown in the following screenshot:

Figure 1.3 – A completely different hash output based on a small change in the original input

Digital signatures

Earlier, we discussed cryptography using asymmetric keys. One of the important uses for asymmetric keys is in the creation and verification of a digital signature. Digital signatures are very similar to a signature done by an individual on a piece of paper. Similar to a paper signature, a digital signature helps in identifying an individual. It also helps in ensuring that messages are not tampered with while in transit. Let's understand digital signatures with the help of an example.

Alice wants to send a message to Tom. How can Tom identify and ensure that the message has come from Alice only and that the message has not been changed or tampered with in transit? Instead of sending a raw message/transaction, Alice creates a hash of the entire payload and encrypts the hash with her private key. She appends the resultant digital signature to the hash and transmits it to Tom. When the transaction reaches Tom, he extracts the digital signature and decrypts it using Alice's public key to find the original hash. He also extracts the original hash from the rest of the message and compares both the hashes. If the hashes match, it means that it actually originated from Alice and that it has not been tampered with.

Digital signatures are used to sign transaction data by the owner of the asset or cryptocurrency, such as Ether. With a basic understanding of cryptography, it's time to introduce Ethereum and blockchain at a high level.

Reviewing blockchain and Ethereum architecture

Blockchain is an architecture comprising multiple components, and what makes blockchain unique is the way these components function and interact with each other. Ethereum allows you to extend its functionality with the help of smart contracts. (Smart contracts will be addressed in detail throughout this book.)

Some of the important Ethereum components are the Ethereum Virtual Machine (EVM), miner, block, transaction, consensus algorithm, account, smart contract, mining, Ether, and gas. We are going to discuss each of these components in this chapter.

A blockchain network consists of multiple nodes belonging to miners and some nodes that do not mine but help in the execution of smart contracts and transactions. These are known as EVMs. Each node is connected to another node on the network. These nodes use a peer-to-peer protocol to talk to each other. They, by default, use port 30303 to talk among themselves.

Each miner maintains an instance of a ledger. A ledger contains all blocks in the chain. With multiple miners, it is quite possible that each miner's ledger instance might have different blocks to another. The miners synchronize their blocks on an ongoing basis to ensure that every miner's ledger instance is the same as the other. Details about ledgers, blocks, and transactions are discussed in detail in subsequent sections in this chapter.

The EVM executes smart contracts and helps bring about changes to the global state. Smart contracts help in extending Ethereum by writing custom business functionality into it. These smart contracts can be executed as part of a transaction, and it follows the process of mining as discussed earlier.

A person with an account on a network can send a message for the transfer of Ether from their account to another or can send a message to invoke a function within a contract. Ethereum does not distinguish them as far as transactions are considered. The transaction must be digitally signed with an account holder's private key. This is to ensure that the identity of the sender can be established while verifying the transaction and changing the balances of multiple accounts. Let's take a look at the components of Ethereum in the following diagram:

Figure 1.4 – The relationship between blockchain and mining

The previous diagram illustrates some of the important components in Ethereum. The externally owned accounts are responsible for initiating transactions on Ethereum. The transactions that are executed within the Ethereum nodes are finally written as blocks on the blockchain. These blocks have header sections that help in chaining the blocks.

Relationship between blocks

In blockchain and Ethereum, every block is related to another block. There is a parent-child relationship between two blocks. There can be only one child to a parent and a child can have a single parent. This helps in forming a chain in blockchain, as shown. Blocks will be explained in a later section in this chapter:

Figure 1.5 – The relationship between blocks using a block hash

In this diagram, we can see three blocks apart from the Genesis Block – Block 1, Block 2, and Block 3. Block 1 is the parent of Block 2, and Block 2 is the parent of Block 3. The relationship is established by storing the parent block's hash in a child's block header. Block 2 stores the hash of Block 1 in its header and Block 3 stores the hash of Block 2 in its header. So, the question arises – who is the parent of the first block? Ethereum has a concept of the genesis block, also known as the first block. This block is created automatically when the chain is first initiated. You can say that a chain is initiated with the first block, the genesis block, and the formation of this block is driven through the genesis.json file.

The next chapter will show you how to use the genesis.json file to create the first block while initializing the blockchain.

How transactions and blocks are related to each other

Now that we know that blocks are related to each other, you will be interested in knowing how transactions are related to blocks. Ethereum stores transactions within blocks. Each block has an upper gas limit, and each transaction needs a certain amount of gas to be consumed as part of its execution. The cumulative gas from all transactions that are not yet written in a ledger cannot surpass the block gas limit. This ensures that all transactions do not get stored within a single block. As soon as the gas limit is reached, other transactions are removed from the block and mining begins thereafter. The gas concept will be covered in a subsequent section in this chapter. This section should be revisited after reading about gas.

The transactions are hashed and stored in the block. The hashes of two transactions are taken and hashed further to generate another hash. This process eventually provides a single hash from all transactions stored within the block. This hash is known as the transaction Merkle root hash and is stored in a block's header.

A change in any transaction will result in a change in its hash and, eventually, a change in the root transaction hash. It will have a cumulative effect because the hash of the block will change, and the child block has to change its hash because it stores its parent hash. This helps in making transactions immutable. This is also shown in the following diagram:

Figure 1.6 – The Merkle root representing all transaction hashes

Blocks and transactions are core to blockchain, but the question remains about the process of adding them to the chain. A generated block should be agreed upon and acceptable to all the nodes within a network. The process of coming to an agreement on a block and subsequently either adding it to the chain or rejecting it is based on the process known as consensus.

Consensus is one of the most important concepts in the world of blockchain. As we know, a blockchain comprises multiple nodes that are running the same software and storing the same data. There should be some agreement between the nodes to accept a given state as a known agreeable state. With thousands of nodes running together, intertwined using a Peer-to-Peer (P2P) network, they should agree on what is stored historically on the blocks and what should be included as transactions on the new block. It is quite possible that a node goes rogue and adds transactions that are not valid. The consensus mechanism or agreements among nodes would ensure that those transactions are not accepted by other ones. A rogue actor has to minimally control a majority of the nodes to make everyone agree on the dubious transactions.

There are various ways to implement and achieve consensus among nodes. Bitcoin is implemented using the Proof-of-Work (PoW) consensus algorithm. Ethereum also implements the PoW consensus algorithm as part of Ethereum 1.0. The new upcoming Ethereum 2.0 is changing its consensus algorithm to the Proof-of-Stake (PoS) consensus algorithm.

One of the main drawbacks of the PoW consensus algorithm is that is it quite costly to generate and agree on blocks due to its mining process, which will be explained later. Mining involves heavy usage of electricity and that has negative consequences toward environmental hazards and cost. Both PoW and PoS will be explained in the next subsections.

Proof of work

A miner is responsible for writing transactions to the Ethereum chain. A miner's job is very similar to that of an accountant. An accountant is responsible for writing and maintaining a ledger; similarly, a miner is solely responsible for writing a transaction to an Ethereum ledger. A miner is interested in writing transactions to a ledger because of the reward associated with it. Miners get two types of reward – a reward for writing a block to the chain and cumulative gas fees from all transactions in the block. There are generally many miners available within a blockchain network, each trying and competing to write transactions. However, only one miner can write the block to the ledger, and the rest will not be able to write the current block.

The miner responsible for writing the block is determined by way of a puzzle. The challenge is given to every miner, and they try to solve the puzzle using their computing power. The miner who solves the puzzle first writes the block containing transactions to their own ledger and sends the block and nonce value to other miners for verification. Once verified and accepted, the new block is written to all ledgers belonging to miners. In this process, the winning miner also receives Ether as a reward. Every mining node maintains its own instance of the Ethereum ledger, and the ledger is ultimately the same across all miners. It is the miner's job to ensure that their ledger is updated with the latest blocks. The following are the three important functions performed by miners or mining nodes:

• Mine or create a new block with a transaction and write this to the Ethereum ledger.
• Advertise and send a newly mined block to other miners.
• Accept new blocks mined by other miners and keep their own ledger instance up to date.

Mining nodes refer to nodes that belong to miners. These nodes are part of the same network where the EVM is hosted. At some point in time, miners will create a new block, collect all transactions from the transaction pool, and add them to the newly created block. Finally, this block is added to the chain. There are additional concepts such as consensus and the solving of a target puzzle before writing the block, which will be explained in the following section.

Miners are always looking forward to mining new blocks and are also listening actively to receive new blocks from other miners. They are listening for new transactions stored in the transaction pool. Miners also broadcast the new transactions to other connected nodes after validation. A miner collects all transactions available within the transaction pool, subject to constraints such as block size and block gas limits, and creates a block with them. This activity is done by all miners.

The miner constructs a new block and adds all transactions to it. Before adding these transactions, it will check whether any of the transactions are not already written in a block that it might receive from other miners. If so, it will discard those transactions.

The miner will add its own coinbase transaction for getting rewards for mining the block.

The next task for a miner is to generate the block header and perform the following tasks:

1. The miner takes hashes of two transactions at a time to generate a new hash till they get a single hash from all transactions. The hash is referred to as a root transaction hash or Merkle root transaction hash. This hash is added to the block header.
2. The miner also identifies the hash of the previous block. The previous block will become a parent to the current block, and its hash will also be added to the block header.
3. The miner calculates the state and receipts of the transaction root hashes and adds them to the block header.
4. A nonce and timestamp are also added to the block header.
5. A block hash consisting of both a block header and body is generated.
6. The mining process starts where the miner keeps changing the nonce value and tries to find a hash that will satisfy as an answer to the given puzzle. Keep in mind that everything mentioned here is executed by every miner in the network.
7. Eventually, one of the miners will be able to solve the puzzle and advertise the result to other miners in the network. The other miners will verify the answer and, if found correct, further verify every transaction, accept the block, and append it to their ledger instance.

This entire process is also known as PoW wherein a miner provides proof that they have worked on computing the final answer that is satisfactory as a solution to the puzzle. The header block and its content are shown in the following diagram:

Figure 1.7 – PoW generating a Merkle root and using transactions

Let's now discuss the next mechanism.

Proof of stake

Ethereum 2.0 will launch a new consensus mechanism known as PoS. It is another algorithm to reach consensus within a distributed network architecture. While PoW is computationally heavy, PoS does not perform any compute-heavy activities. Under this mechanism, interested stakeholders can stake their Ether (a minimum of 32 ETH) with the network. Once the Ethers are staked, they are locked, and these stakeholders become validators. These validators have their nodes running, and their job is to attest to new blocks for their validity. Attestation here means that the validator is vouching for the correctness of the block and its constituent transactions.

After a minimum number of validators attest, the block is finalized and becomes a permanent part of the chain. The network will at random also choose a validator to create a new block for every epoch. The validators are not competing with each other anymore as in the case of the PoW consensus mechanism. This makes PoS more environmentally friendly by being energy efficient and also anyone with 32 Ethers can potentially become a validator. Now that we understand the different consensus mechanisms, it's time to visit the different types of nodes supported by Ethereum.

Nodes represent the computers that are connected using a P2P protocol to form an Ethereum network. There are the following three types of nodes in Ethereum:

• The EVM
• Mining nodes (Ethereum 1.0)
• Validators (Ethereum 2.0)

Please note that this distinction is made to clarify concepts of Ethereum. In most scenarios, there is no dedicated EVM. Instead, all nodes act as miners as well as EVM nodes.

EVM

Think of an EVM as the execution runtime for smart contracts. EVMs are primarily responsible for providing a runtime that can execute code written in smart contracts. It can access accounts, both contract and externally owned, and its own storage data. It does not have access to the overall ledger but does have limited information about the current transaction.

EVMs are the execution components in Ethereum. The purpose of an EVM is to execute code in a smart contract line by line. However, when a transaction is submitted, the transaction is not executed immediately. Instead, it is added to a transaction pool. These transactions are not yet written to the Ethereum ledger.

Mining nodes

The mining nodes are responsible for generating, validating, and adding blocks to the chain. Mining nodes can be a full node, a light node, or an archive node. A full node has all the blocks (headers and transactions) since the genesis block and complete global state and can participate in generating and validating the blocks. Light nodes have blocks with headers only and are dependent on connected full nodes for any information they need. They require more bandwidth because of their chatty nature with full nodes but need less computing and storage. They are also faster while synchronizing blocks from full nodes. Archival nodes are again full nodes, but their usage is more for querying a node for historical information and reporting.

Ethereum validators

Validator nodes are again nodes responsible for generating new blocks for the chain. They do not run any mining process, and they collect all available transactions from the transaction pool and create a block. They broadcast the blocks to other validators for attestation. As part of the attestation process, validators execute each transaction and modify their state while adding a block to their chain.

Accounts are the main building blocks for the Ethereum ecosystem. It is an interaction between accounts that Ethereum wants to store as transactions in its ledger. There are two types of accounts available in Ethereum – externally owned accounts and contract accounts. Each account, by default, has a property named balance that helps in querying the current balance of Ether.

Externally owned accounts

Externally owned accounts are accounts that are owned by people on Ethereum. Accounts are not referred to by name in Ethereum. When an externally owned account is created on Ethereum by an individual, a public/private key is generated. The private key is kept safe with the individual while the public key becomes the identity of this externally owned account. This public key is generally of 256 characters; however, Ethereum uses the first 160 characters to represent the identity of an account.

If Bob, for example, creates an account on an Ethereum network, whether private or public, he will have his private key available to himself while the first 160 characters of his public key will become his identity. Other accounts on the network can then send Ether or other cryptocurrencies based on Ether to this account.

An account on Ethereum looks like the one shown in the following screenshot:

Figure 1.8 – An externally owned account identifier

An externally owned account can hold Ether in its balance and does not have any code associated with it. It can execute transactions with other externally owned accounts, and it can also execute transactions by invoking functions within contracts.

Contract accounts

Contract accounts are very similar to externally owned accounts. They are identified using their public address. They do not have a private key. They can hold Ether similar to externally owned accounts; however, they contain code for smart contracts consisting of functions and state variables.

Externally owned accounts are responsible for initiating transactions and each transaction in Ethereum requires gas, which is the cost of the transaction to be provided by the sender of the transaction. Gas has a relationship to its currency known as Ether. The next section will discuss the concepts related to Ether, gas, and transactions.

Ether, gas, and transactions

Ether is the currency of Ethereum. Every activity on Ethereum that modifies its state charges Ether as a fee. And miners who are successful in generating and writing a block in a chain are also rewarded Ether. Ether can easily be converted to dollars or other currencies through crypto exchanges.

Ethereum has a metric system of denominations known as units of Ether. The smallest denomination or base unit of Ether is called wei. The following is a list of the named denominations and their value in wei, which is available at https://github.com/ethereum/web3.js/blob/0.15.0/lib/utils/utils.js#L40:

var unitMap = {
   'wei' : '1'
   'kwei': '1000',
   'ada': '1000',
   'femtoether': '1000',
   'mwei': '1000000',
   'babbage': '1000000',
   'picoether': '1000000',
   'gwei': '1000000000',
   'shannon': '1000000000',
   'nanoether': '1000000000',
   'nano': '1000000000',
   'szabo': '1000000000000',
   'microether': '1000000000000',
   'micro': '1000000000000',
   'finney': '1000000000000000',
   'milliether': '1000000000000000',
   'milli': '1000000000000000',
   'ether': '1000000000000000000',
   'kether': '1000000000000000000000',
   'grand': '1000000000000000000000',
   'einstein': '1000000000000000000000',
   'mether': '1000000000000000000000000',
   'gether': '1000000000000000000000000000',
   'tether': '1000000000000000000000000000000'
};

Gas

In the previous section, it was mentioned that fees are paid using Ether for any transaction execution that leads to a state change in Ethereum. Ether is traded on public exchanges, and its price fluctuates daily. If Ether is used for paying fees, then the cost of using the same service can be high on a certain day while being low on other days. People will wait for the price of Ether to fall to execute their transactions. This is not ideal for a platform such as Ethereum. Gas helps in alleviating this problem. This is the internal currency of Ethereum. The execution and resource utilization costs are predetermined in Ethereum in terms of gas units. For each unit of gas, a price can be ascribed, known as the gas price. For example, for each unit of gas, a price of 10 gwei can be assigned. When both the number of gas units and gas price are multiplied together, it results in the gas cost. The gas price can be adjusted to a lower price when the price of Ether increases and a higher price when the price of Ether decreases.

Transactions

A transaction is an agreement between a buyer and a seller, a supplier and a consumer, or a provider and a consumer that there will be an exchange of assets, products, or services for currency, cryptocurrency, or some other asset, either in the present or in the future. Ethereum helps in executing the transaction. The following are the three types of transactions that can be executed in Ethereum:

• The transfer of Ether from one account to another: The accounts can be externally owned accounts or contract accounts. The following are the possible cases:
  • An externally owned account sending Ether to another externally owned account in a transaction
  • An externally owned account sending Ether to a contract account in a transaction
  • A contract account sending Ether to another contract account in a transaction
  • A contract account sending Ether to an externally owned account in a transaction
• Deployment of a smart contract: An externally owned account can deploy a contract using a transaction in an EVM.
• Using or invoking a function within a contract: Executing a function in a contract that changes state is considered a transaction in Ethereum. If executing a function does not change a state, it does not require a transaction.

A transaction has some of the following important properties related to it:

• The from account property denotes the account that originates the transaction and represents an account that is ready to send some gas or Ether. Both gas and Ether concepts were discussed earlier in this chapter. The from account can be externally owned or a contract account.
• The to account property refers to an account that is receiving Ether or benefits in lieu of an exchange. For transactions related to the deployment of the contract, the to field is empty. It can be externally owned or a contract account.
• The value account property refers to the amount of Ether that is transferred from one account to another.
• The input account property refers to the compiled contract bytecode and is used during contract deployment in an EVM. It is also used for storing data related to smart contract function calls along with their parameters. A typical transaction in Ethereum where a contract function is invoked is shown here. In the following screenshot, note the input field containing the function call to contract, along with its parameters:

Figure 1.9 – The transaction properties in a block

• The blockHash account property refers to the hash of the block to which this transaction belongs.
• The blockNumber account property is the block to which this transaction belongs.
• The gas account property refers to the amount of gas supplied by the sender who is executing this transaction.
• The gasPrice account property refers to the price per gas the sender was willing to pay in wei (we learned about wei in the Ether section earlier in this chapter). Total gas is computed at gas units x gas price.
• The hash account property refers to the hash of the transaction.
• The nonce account property refers to the number of transactions made by the sender prior to the current transaction.
• The transactionIndex account property refers to the serial number of the current transactions in the block.
• The value account property refers to the amount of Ether transferred in wei.
• The v, r, and s account properties relate to digital signatures and the signing of the transaction.

A typical transaction in Ethereum, where an externally owned account sends some Ether to another externally owned account, is shown here. Note that the input field is not used here. Since two Ethers were sent in the transaction, the value field is showing the value accordingly in wei, as shown in the following screenshot:

Figure 1.10 – Block properties in a block

One method to send Ether from an externally owned account to another externally owned account is shown in the following code snippet using the web3 JavaScript framework, which will be covered later in this book:

web.eth.sendTransaction({from: web.eth.accounts[0], to: "0x9d2a327b320da739ed6b0da33
c3809946cc8cf6a", value: web.
toWei(2, 'ether')})

A typical transaction in Ethereum where a contract is deployed is shown in the following screenshot. In the following screenshot, note the input field containing the bytecode of the contract:

Figure 1.11 – A sample transaction in a block