Where is the blockchain stored?

Following my previous article where I talked about what a blockchain is, taking the example of the Bitcoin blockchain, a natural question comes to mind: where is that blockchain (the big spreadsheet storing all the transactions ever made) stored?

Well as you have probably heard, a blockchain is decentralized. Thus there cannot be a single place where the blockchain is stored (that would make it centralized).

Blockchains are stored in a lot of people’s computers. These people are called nodes. They can be simple users or miners (more on that later)

Ok, so each node has a copy of all transactions ever made. So what happens when a new transaction is written to the blockchain (added to the spreadsheet)? Well, all computers storing the blockchain have to update it!

Here is an example:

1. Alice sends 1 bitcoin to Bob. She creates a transaction and sends it to every computer that she knows are running the Blockchain (they are called nodes). Alice’s bitcoin wallet has a pre-filled list of other nodes so she does not have to worry about actually knowing other users. This process is transparent to her.
2. Each node that receives the transaction now knows that Alice is sending 1 bitcoin to Bob. They all send the transaction to all the other nodes they know which will in turn do the same until the whole network knows about the transaction.
3. Some of these nodes are miners and are tasked to verify the transaction. But that is for another time.

The time it takes for the transaction to propagate to the whole network is around 15 seconds, which is pretty fast. This can be seen here (in “transaction 90th percentile”, which is the time it takes for a transaction to reach 90% of the nodes). This time can vary day-to-day.

Want to know more?

I like to think I suffer from attention disorder so not only I can’t read long articles, but I can’t write them either. I am currently writing very short article about cryptocurrencies. If you are interested in reading more here is the list of the articles I have written.

• What is a blockchain?
• Have another question? Ask in the comments and I will try to answer with a very short article!