What is Blockchain?
There are countless approaches to explain what Blockchain is. This is just one of them :)
As its name says, Blockchain is a chain of blocks, interconnected through encryption.
These blocks store transactions. Depending on the Blockchain, these transactions may have different objectives.
When we talk about Bitcoin Blockchain, transactions record cryptocurrency transfers from one address to another.
But when we talk about Ethereum or RSK, in addition to cryptocurrency and token transactions, we can send transactions to publish a smart contract or interact with another already published.
It is very common to say that Blockchain is a ledger, accounting, known as ledger given its great use for transfers.
Blockchain works on a computer network, and public Blockchains on the biggest network of all: the Internet. So another important aspect to be considered a Blockchain is that it is distributed. That is, there must be several identical copies, scattered on computers from different places.
There are also rules for validating a transaction and allowing it to be included in a Blockchain block. These rules are called the consensus protocol.
The fact that the blocks are connected through encryption makes the Blockchain considered immutable, given that, to change a transaction from a past block, you need to change all the later blocks, and, according to the consensus protocol, a large number of computers on the network must accept this change.
Computers that validate transactions are known as miners. In the popular consensus protocol, called Pow - Proof of Work, or proof of work, you have to find a number by trial and error, known as nonce, for a block to be valid on the network and this quest is known as mining.
It is important to say that there are financial rewards for validating blocks. There is a competition to earn the reward in Bitcoins for creating a valid block, that is, a block that has only valid transactions and a correct nonce so that the block is valid according to the network rules.
This is the guarantee that efforts will be made to do everything correctly, and not to waste time and energy, which would be a huge loss.
Mempool
When we send a transaction to the Blockchain, it doesn’t immediately go to a block, it stays in a queue, or "waiting room", called mempool.
For Bitcoin, for example, if a transaction is not mined / validated in 2 weeks, it is deleted from the mempool.
For Ethereum, a transaction can stay in the mempool forever. How Ethereum controls the order of an account's transactions by a transaction counter, called nonce, so you are unable to validate a higher nonce transaction if there is a previous nonce transaction pending.
Timestamp
The timestamp is an information stored in each block, to determine the exact moment when the block was created and validated by the Blockchain network.
In this way, every transaction included in a block has proof of the moment of its inclusion.
This can be useful to determine a sequence of transfers from a cryptocurrency, it is a mechanism to avoid double spending.
It can also be used to record that a particular hash representing a document or other file has been written to a block in that timestamp, proving the moment of its existence.
Anders Blockchain Demo
Anders Brownworth created an excellent visual and interactive Blockchain demo:
Conclusion
An introduction to Blockchain was presented.
In the next class you learn the fundamentals of Blockchain.