Blockchain

A big file of bitcoin transactions

Diagram showing the blockchain as a file stored by nodes on the bitcoin network.
Current Blockchain Size:
669.21 GB
854,122 blocks
Note: This is the size of the blockchain for my local node.
The size of your blockchain will differ depending on how many chain reorganisations your node has experienced and how many stale blocks you have stored on disk.

The blockchain is a file of transactions. It's the most important file that a bitcoin node maintains.

It is called the "blockchain" because new transactions are added to the file in blocks, and these blocks are built on top of one another to create a chain of blocks. Hence, blockchain.

But ultimately, the blockchain is permanent storage for bitcoin transactions.

Live Bitcoin Blockchain:

Tip: 854,122 (0 blocks away) ⇈
Next 0 blocks ↑
Height Block Hash Time (UTC) Size Txs Avg Fee Rate AFR
854,122 854,122 000000000000000000008fafb602a88201e670da301710a2d49db54d2e474887 ago 1.56 MB 3,693 4
854,121 854,121 0000000000000000000166cce92283f1800ef677c099f0bf4528827ae2eb9ee3 ago 1.55 MB 3,951 6
854,120 854,120 00000000000000000000cccd6239fa1fd6882e62d73762c32fa1e744e2bf97c6 ago 1.48 MB 3,549 6
854,119 854,119 000000000000000000020c3e1cfd97c729c675cd8c937ce3cfb033a7cf42276d ago 1.83 MB 2,317 5
854,118 854,118 000000000000000000022508ed1942f53ff70ca091419a9b1d0136095a7f176d ago 2.07 MB 1,315 4
854,117 854,117 000000000000000000004d7c5dd13a600409c8d65fccd31745ad1b5dd2526979 1.60 MB 3,404 7
854,116 854,116 000000000000000000008ffde1950ee78e8040ea597e6ee035e9c5cefe6a39cb 1.96 MB 1,587 5
854,115 854,115 00000000000000000000a8a9e7b76b081e061fddc7f65728fee5173a6c9fef26 2.03 MB 1,282 4
854,114 854,114 000000000000000000021d0549a314959508147167304c029b56bb99a5f2bb55 2.00 MB 1,206 4
854,113 854,113 00000000000000000001990b5bea92d50913faaad60a51efdc9f3f711fb602be 1.59 MB 3,268 6
854,112 854,112 000000000000000000012ea59bc3279d3e025bb04c00c58f2aa0c5c717c7a74d 1.47 MB 2,929 11
854,111 854,111 00000000000000000000efac316bbf11d516c1e3bfacac25daf25a85a72d94ec 2.15 MB 436 4
854,110 854,110 00000000000000000000c51051873ed4e3f597e8d78c7355b7ac10fc3b71f770 2.02 MB 816 4
854,109 854,109 000000000000000000032304183620018cea43493af4f343595c89eb9dbaed7d 2.02 MB 1,273 4
854,108 854,108 00000000000000000002b6c44c521612555229c2ff515edb18dfface2cd1002a 1.79 MB 2,558 6
854,107 854,107 00000000000000000001cb86ae6b0073019fcb8996bfc0d0931829c1b9fa897b 2.10 MB 480 4
854,106 854,106 0000000000000000000318962abd785addeda00b486878f8a5b8ef0e456ba524 1.56 MB 2,897 6
854,105 854,105 0000000000000000000160198735a9c3bc1c900d91dcd9745322b93bdd0fb276 1.73 MB 2,299 5
854,104 854,104 00000000000000000000824bfc185120cdcfc32016bda96f4b5292b7ce39108f 1.60 MB 3,334 7
854,103 854,103 0000000000000000000082e159f05bc019806b03f5c5d0d9a936be02abe6cbab 1.64 MB 3,581 8
854,102 854,102 000000000000000000015993cf7ddebcae5e06de4970469020771c3fe29b79d3 1.40 MB 2,769 13
Previous 10 blocks ↓
Total Size: 669.21 GB

Download

How do you get a copy of the blockchain?

Diagram showing the blockchain being downloaded from other nodes on the network.

The easiest way to get a copy of the blockchain is to run a Bitcoin node.

When you run the Bitcoin program (e.g. Bitcoin Core) your node will automatically download blocks from other nodes on the network until you have an up-to-date copy of the blockchain on your computer.

When nodes connect to each other, they tell each other the height of their chain (how many blocks they have) during the initial handshake. If another node has more blocks than you, your node will request these blocks from the other nodes until you have a full copy of the blockchain.

As a result, nodes are constantly communicating with each other to replicate the blockchain across every computer on the network.

There is no single or definitive version of "the blockchain". Every node keeps their own local copy of the blockchain, and it can vary from computer to computer at any given time.

It can take a while to download the full blockchain when you run Bitcoin for the first time. This is referred to as the Initial Block Download (IBD).

Mining

How are new blocks added to the blockchain?

Diagram showing the a block being mined on to the blockchain by a node on the network.

New blocks of transactions must be mined on to the blockchain.

In short, the process of mining involves collecting transactions from the memory pool in to a candidate block, and then using processing power to produce a block hash that is below a specific target value. This means that any node on the network can mine a new block, but you need to use energy to be able to do so.

Block Hash
Target Recaluclator

When a node (or "miner") successfully mines a new block, they will share it with the other nodes on the network. When other nodes receive this new block, they will add it to their blockchain, and miners will start trying to mine a new block on top of it.

Diagram showing a newly-mined block being propagated to other on the network.

As a result, miners are constantly working to extend the blockchain with new blocks of transactions.

Here's a video on how mining works in Bitcoin.

Chain Reorganisations

Can two blocks be mined at the same time?

As the blockchain is being built, it's perfectly normal for two blocks to be mined at the same time.

Diagram showing a temporary fork in the blockchain due to two blocks being mined at the same time.
If two blocks are mined at the same time it will cause a temporary "fork" in the chain.

In this situation, nodes will consider the first block they receive as part of their blockchain, but also keep the second block they receive just in case. However, the second block to arrive (and the transactions inside it) will not be considered as part of their active blockchain.

Consequently, nodes on the network will be in temporary disagreement about which of these two blocks belongs at the top of the chain.

This disagreement is resolved when the next block is mined. The next block will build on top of one of these blocks, creating a new longest chain of blocks, and as a rule nodes will always adopt the longest known chain of blocks as their active blockchain.

As a result, nodes with the shorter chain will perform a chain reorganisation to move out blocks from their old active chain in favour of blocks that make up a new longer chain.

Diagram showing a temporary fork in the blockchain being resolved via a chain reorganisation.
A fork is resolved when a new block is mined, as this will create a new longest chain.

So although there may be disagreements across the network about which block(s) belong at the top of the blockchain at any given time, the mining of new blocks and the adoption of the longest chain means that nodes will always eventually be in sync.

A temporary fork like this is rare. It happens about once a month (roughly), and usually only affects the top block on the blockchain.

Longest Chain

Can blocks in the blockchain be replaced?

Due to the way the blockchain is built, it's possible for blocks at the top of the chain to be replaced.

Nodes always adopt the longest chain as the "true" version of the blockchain. Therefore, you could always try and build a new longer chain of blocks to replace an existing one, and every node on the network will adopt it.

As a result, this allows you to "undo" or reverse a bitcoin transaction from the blockchain.

Diagram showing nodes on the network adopting the longest chain of blocks as their blockchain.
If you build a new longest chain of blocks, other nodes will adopt it as their blockchain.

However, the problem is that all miners are incentivized to always be building on to the longest known chain. This means that the combined processing power of miners on the network will be focused on building one single chain, which will be built faster than any chain you could build on your own.

Diagram showing nodes on the network adopting the longest chain of blocks as their blockchain.
Miners naturally work to extend the current longest chain.

In other words, the combined processing power of the network working to build the blockchain helps to protect blocks (and transactions) that have already been mined on to the blockchain.

So the only way you could perform an intentional chain reorganisation (to "undo" a transaction in an existing block) would be to have more processing power than every other miner combined so that you could out-mine the network and build a longer chain for everyone to adopt. This is referred to as a "51% Attack".

Nobody has performed a successful 51% attack on the Bitcoin blockchain.

Location

Where is the blockchain stored?

If you're running a Bitcoin Core node, the blockchain files can be found in the following location on your computer:

The blockchain is split in to multiple files named blk00000.dat, blk00001.dat, blk00002.dat, and so on. This is because it's easier to work with multiple small files than it is to work with one giant file. See blk.dat for details.

Summary

Diagram showing a blockchain being built by nodes across a network of computers.
Click on the image to see a nice and slow visualization of a blockchain being built over time, including a chain reorganisation.

The blockchain is permanent storage for bitcoin transactions. New transactions are added to the file in blocks, and these blocks are built on top of each other to create a chain.

New blocks are added to the blockchain through mining, which involves the use of computer processing power. This means it takes energy to mine a block, but any node can work to try and add the next block on to the chain.

When a new block is mined, it will be relayed across the network, which nodes will verify and add on to their chain. This makes the blockchain a constantly growing ledger of transactions, distributed across multiple computers on a network.

Nodes always adopt the longest chain of blocks as the active version of the blockchain, which resolves disagreements about which blocks belong at the top of the chain. This also protects blocks that are already in the blockchain, as it would require large amounts of energy to build a chain that replaces blocks lower down in the chain.

The mechanism of mining and adopting the longest chain allows multiple computers over a network to agree on the same set of blocks and transactions, whilst also making it difficult for anyone to make historic changes to the blocks (and therefore transactions) in the blockchain.

As a result the blockchain is a secure, distributed, and regularly updated file of transactions.

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