Blockchain

A big file of bitcoin transactions

Diagram showing the blockchain as a file stored by nodes on the bitcoin network.
Current Blockchain Size:
643.28 GB
840,090 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: 840,090 (0 blocks away) ⇈
Next 0 blocks ↑
Height Block Hash Time (UTC) Size Txs Avg Fee Rate AFR
840,090 840,090 000000000000000000003eb740be632a4246f58f15822e9341faf6b056f2e419 ago 1.46 MB 4,404 546
840,089 840,089 00000000000000000000806e62118e63004b50245b62769fa011931aed701478 ago 1.45 MB 4,635 709
840,088 840,088 000000000000000000019898009dc57d32aba6e7ae43ab9e5e7760d827092829 ago 1.44 MB 4,850 1,003
840,087 840,087 00000000000000000002f0b43716d5b197adc2e29c918167fb3ce673be9643f3 ago 1.44 MB 4,686 927
840,086 840,086 000000000000000000011cfb32124d784a8d3e4b0067c630343fcb163aa63aba ago 1.34 MB 4,549 1,051
840,085 840,085 0000000000000000000090105c48b84aa1ebc6511ffd28ea81022fe8b25de37b 1.38 MB 5,225 1,085
840,084 840,084 000000000000000000032acb84ff0b9620f8584ce3b6b566c14e363e27d44fa4 1.42 MB 5,208 689
840,083 840,083 00000000000000000002956ca8c9e1bc3f8b64e2eaa105b76c147644d5ed7f6a 1.53 MB 3,935 441
840,082 840,082 0000000000000000000231d7a48c5ac96e666494e1f8f7cb41c4df8218ef73fe 1.44 MB 3,888 567
840,081 840,081 000000000000000000018122945cc6a571237351070bf51f1c0f718177a30ca3 1.48 MB 4,140 597
840,080 840,080 000000000000000000032dd29a3d5920be7c0248375af1e0b837a9ce35f31bd8 1.45 MB 4,301 662
840,079 840,079 00000000000000000002756411a2062772db88dfdd999b4e7ace2ea1029708ba 1.44 MB 4,179 781
840,078 840,078 0000000000000000000082bc6aec972ad9a2708b8f922fdc0672728d03366ce5 1.40 MB 4,588 937
840,077 840,077 000000000000000000018bd4948d21337c98ea27cd5586393a31c644ce9ab1ce 1.40 MB 5,294 1,120
840,076 840,076 00000000000000000002e72a1bafe4e7667a6c49c0ae0b9196e943c85309bc02 1.43 MB 5,843 481
840,075 840,075 000000000000000000020e131858f9e5ee21c487d21e626b67854c1a4554cf61 1.31 MB 4,551 394
840,074 840,074 00000000000000000000fbb0215b1067199e57e99b5b1fafc59afa8d2652a9de 1.41 MB 5,096 540
840,073 840,073 0000000000000000000322b7d9541a82a64e77efbd2deff1d6790849aa841383 1.51 MB 4,665 382
840,072 840,072 00000000000000000001049708d66e565743074367d9c3dc6341254973d0c512 1.47 MB 4,892 378
840,071 840,071 000000000000000000024617b4d2683c6170844f7bbd76ef0d3c5a2c5dcbed78 1.49 MB 4,303 417
840,070 840,070 00000000000000000001b4b41c5d7e1904da04facc8cce1a30fe202f5a83944c 1.46 MB 5,153 625
Previous 10 blocks ↓
Total Size: 643.28 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 can 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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