Learn Blockchains by Building One in Python — 3. Decentralization

This story follows the Python Blockchain series. You can find the other stories here:

There’s also a GitHub repo associated with this series, find it here: Python Blockchain Series.

In the last story, we’ve turned our blockchain into an API. But for now, our blockchain lacks a prominent feature: it’s not decentralized because everything happens on our computer. To fix this, we’ll have to implement something called the Consensus Algorithm. It allows the blockchain to be the same on several computers.

Nodes Registration Endpoints

To implement a Consensus Algorithm, we have first to define new endpoints allowing a node to know more about the other nodes connected to the blockchain.

Let’s start with a node registration endpoint. We need to modify our Blockchain class to allow our blockchain to keep a list of connected nodes, and to add a method to register new nodes.

We initialize our nodes as a set so that we avoid duplicates. In the register_node method, we just parse the URL and extract its content.

Now, we can add our endpoint to our app. We just need to retrieve nodes' addresses sent through a POST request and register them using the previous method. The response is just a message and the list of the nodes registered.

The Consensus Algorithm

In the blockchain, a conflict occurs when several nodes don’t have the same version of the blockchain. To fix this, we’ll add a rule to our blockchain saying that the valid chain is the longest.

We’ll also add a method to check for each block if it is correctly chained to the next block and if the proof (from the Proof of Work) is valid.

Now, we have to use this in our method to find the longest chain.

Here is what happens for each node:

1. We get the blockchain as it is seen by the node.
2. We check if its length is the maximum length and if the chain is valid
3. If it is, we update the blockchain.

Now, we can add a new endpoint that will return the longest chain when requested. It will just call our resolve_conflicts method and return a message and the blockchain.

Playing with the Blockchain

Now, you have a fully functional blockchain. You can try to mine blocks, make transactions, etc… You can’t really try the Consensus feature because it requires the program to be hosted on a server, but you have at least an idea about how it works.

You can also extend the blockchain and develop new unique features. You can also change the algorithms if you want, add rules, etc... Currently, data stored on our blockchain are transactions, but why not store other things, such as invoices if you want?

Final Note

Now, you know how blockchains work and you have some tools to build yours. The possibilities are endless, be creative!

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