Ep. 7 [3.2 'Blockchain']: There must be only one Transaction-Book.

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Note:
Gain a fundamental understanding of Cryptocurrency in an easy way. Before continue reading the story, I recommend to take a couple of minutes and read through the prior article again in order to refresh your present knowledge. The more often you read through the story, the more confident you will get with the technology behind cryptocurrencies like Bitcoin. Take your time, to understand cryptocurrencies, as a Steemian it's worth it.
If something in the story is not clear to you, don’t hesitate to ask. There are several people who might have the same question. So shoot me a comment, if something is unclear. Me and other readers will appreciate it!

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In the last article we saw that Bob sent some Bitcoins (envelope) to Charlie. This Bitcoin-Transaction was transmitted as the visitors on board verified that all Bitcoin-Transaction (envelopes) on the boat are legitimate.

Afterwards Bob tried to send a copy of the same Bitcoins to David. This transaction was not transmitted because the visitors saw in their Transaction-books(Blockchain) that Bobs envelope was already been sent with a prior boat.

Now I want to talk about the record-book, which all visitors brought with them, to verify the pending Bitcoin-Transactions (envelopes).
So each visitor has a record-book in which all prior Bitcoin-Transactions are recorded.

But as we said, each visitor is independent and therefore each of them has his own Transaction-Book(Blockchain). That means everybody can theoretically write whatever he or she wants in the own Transaction-Book(Blockchain).

But there must be only one record-book on which all Bitcoin-participants agree.
Otherwise it would cause some problems in the Bitcoin-Network. I want to make this point more clear by an example.
In the last post Bob sent Bitcoins to Charlie. This transaction got verified by the visitors on board and therefore was executed.

Now lets imagine this situation a bit differently. Lets say 50% of the visitors verified that Bobs transaction to Charlie is correct, and the other 50% of the visitors declined this transaction.
That means half of the visitors(=Full-Nodes) wrote down Bobs Transaction in their record-book and the other half of the visitors did not.

That means, there are two versions of the record-book(Blockchain). In Version1 Bobs transaction got executed and so the Bitcoins arrived to Charlie. In the Version2 of the record-book Bobs transaction was not written down and therefore the Bitcoins didn’t arrive to Charlie.
Now lets think about the copy of Bobs envelope, which he sends with the next boat to David.
Well now, 50% of the visitors see in their books, that this envelope already got sent with a prior boat to someone, and therefore can not be send again. But as we said, the other 50% of the visitors don’t have this envelope in their books written, and therefore this transaction looks legitimate to them.

So the question here: Did did David receive the Bitcoins or not?

Well, in this example you can see why it is important that there must be only ONE version of the record-book (Blockchain) on which every participant in the Bitcoin-Network agrees on.
There must be an universal agreement among all independent participants which Bitcoins-transactions got executed, so that all together can agree who holds how much Bitcoins at any time.

Summary
In Payment-Systems like Bitcoin it is important that there is only one right version of the Transaction-Book(=Blockchain) on which all Bitcoin-Participants fully agree on. But all visitors (Full-Nodes) are independent and therefore they can write whatever they want into their Transaction-Books. So how does all participants in the Bitcoin-Network agree on one version of the Transaction-Book(=Blockchain)?

This questions will be answered in the next articles.