This document discusses improvements made to a basic blockchain implementation including: - Restructuring the Block class to include a transactions field instead of a data field. - Defining a Transaction class with sender, receiver, and amount fields. - Adding pending transactions storage to the Blockchain class constructor. - Rewriting the addBlock() method and adding a createTransaction() method. - Adding a minePendingTransactions() method to mine new blocks with pending transactions and send rewards. - Including an addressBalance() method to check account balances.

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Blockchain Training
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Building a Blockchain
Lesson 05

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Blockchain
At this point we have built a basic Blockchain in JavaScript and implemented proof-
of-work to protect it from spammers and attackers.
We still have a couple of things to fix:
● Our blockchain can only store 1 transaction in a block
● There are no rewards for miners.

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Restructuring Block Class
The first step is to restructure the
Block Class and the
calculateHash() function. We need
to include a transactions field
instead of the data one and the
index field can be removed since
it doesn't make sense with our
implementation.

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Transaction Class
On each block we should be able to store multiple transactions. For this we will
have to define a Transaction class. In this case it will be very simple, containing a
sender (fromAddress) a receiver (toAddress) and an amount. You can add more
fields to a transaction, but these are the minimum.

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Adjust the Blockchain class
The first thing that we need is a place to store pending transactions.
As you know, blockchains create blocks on a steady interval thanks to the proof-of-
work algorithm. However, it should be possible to submit new transactions in
between the creation of two blocks.
To do that, the Blockchain’s constructor needs some changes to include a place to
store pending transactions.

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Adjust the Blockchain class
This is how the Blockchain class constructor will look. Notice that there is a
property to define the reward for the miner:

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Review addBlock() method
This method needs to be rewritten completely, and a new method
createTransaction will be use instead:

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Block Mining
A new method
minePendingTransactions()
will help us mine a new
block with all the pending
transactions while sending
the reward to the miner.

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Block Mining
Note that this implementation takes all the pending transactions and adds them to a
block.
The mininghRewardAddress argument can be passed along with the wallet
address. Once a block is successfully mined, a new transaction with the reward will
be created.

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Address Balance
We need to
include a new
method to
check the
balances of
the addresses
on the
blockchain

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Run the application
By including this code we
can test our application
and creating 2 new
transaction in pending
state. We will have to start
the miner to get them
confirmed.

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Run the application
While adding the following code we will be starting the miner and showing the
whole chain:

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Run the application
We can check the balance for the test address with the following code:
However if you look closely at the code, you'll see that the system creates a new
block and then adds your mining rewards as a new pending transaction. That
transaction will be included in the next block. Until the miner is started again the
reward will not be received.

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Run the application
Let's start the miner again and run the application to receive the reward and check
the current chain status.

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Results
Here you have the whole terminal
output:

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Lesson Review
While this implementation is far from complete. It is capable of storing multiple
transactions in a block and of giving rewards to miners. A basic example of how to
implement blockchain and moreover how to build a basic cryptocurrency.
The complete project code can be found in the following Git repository:
https://github.com/juliomacr/blockchain-demo