Blockchain

A big file of bitcoin transactions

Diagram showing the blockchain as a file stored by nodes on the bitcoin network.
Current Blockchain Size:
762.43 GB
903,901 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 reorganizations 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: 903,901 (0 blocks away) ⇈
Next 0 blocks ↑
Height Block Hash Time (UTC) Size Txs Avg Feerate AFR
903,901 903,901 00000000000000000000c0ae9b8c4d4a8aef2a248ca3bd85a79a335b5d209e1d ago 1.42 MB 1,918 2
903,900 903,900 0000000000000000000193e07ef688cf7a9b86b7581e4604baa4fc35368b7201 ago 1.44 MB 3,080 3
903,899 903,899 00000000000000000001b566e92ab7495f94ca8711760a7fe10ac73a8e01c393 ago 0.25 MB 675 2
903,898 903,898 0000000000000000000197420019ecc11e4f74b92297330aa43863fe473b014a ago 1.37 MB 2,126 1
903,897 903,897 000000000000000000012e7adc359c06868132c408cf32fb90b4fe9edb2f8a40 ago 1.57 MB 1,742 1
903,896 903,896 000000000000000000016e2baec0a4d5dd19a43f72b21b2e6b71afd7ed8eef7f 1.76 MB 2,249 2
903,895 903,895 000000000000000000024b08003ead7e6fc17d83103d212b0b227bd07db0b413 1.39 MB 1,621 2
903,894 903,894 000000000000000000008f2d0a79c42a9c9b5b654554892a979800583b958969 1.40 MB 2,347 3
903,893 903,893 00000000000000000001ed4a2c59e4774c57c414c51e68b1e0e9d4978fdb1ac0 1.51 MB 4,643 5
903,892 903,892 0000000000000000000023254fad4246f5c6ade045ea28fd1153ffe241cc4e27 1.48 MB 2,583 3
903,891 903,891 000000000000000000000903723c7b15e708cf2fc0b9464316eb25391e8898d7 1.40 MB 2,117 3
903,890 903,890 00000000000000000001f712d91aa4a52207e491de79e4242f35520506993f15 1.29 MB 1,014 2
903,889 903,889 00000000000000000000aaf2a9dd0f78a6603c84259aa77909211138deeb20f0 1.58 MB 3,953 5
903,888 903,888 00000000000000000000fe9f74520f9f18c2423fc6f2b00e3632f2cea1cc9cb8 1.60 MB 3,622 3
903,887 903,887 00000000000000000000a15bdcf4f50a141e2e7795b326d47f0a6cc242e703da 1.64 MB 3,707 4
903,886 903,886 000000000000000000000d7828b4fe369bc35339dbc87a9691442cb789672076 1.63 MB 2,712 7
903,885 903,885 00000000000000000001d80840d73d277b0b2cce3669681f39a06cd7ffebfecd 1.54 MB 2,634 1
903,884 903,884 0000000000000000000180f6406e83a526875698f70af2d517026064524e8fd6 1.39 MB 2,214 2
903,883 903,883 000000000000000000004c918a929cb2710b5376dcdd89e719c521dc5255115c 1.57 MB 3,949 4
903,882 903,882 0000000000000000000179ec2b0078d64f49045bce3cc6b1fd5a83d1417ba053 1.52 MB 2,501 3
903,881 903,881 000000000000000000017e71733448d2c8e2f4c08105d97f7c1868acb4acc7ba 1.38 MB 2,636 3
Previous 10 blocks ↓
Total Size: 762.43 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 into 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.

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Block Hash

Create a block hash from a block header.

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Used internally inside raw block headers

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Used externally when searching for blocks on block explorers

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Target Adjustment

Calculate the next target value based on the current target and the time between blocks.

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Time (seconds)
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The target adjustment period is 2016 blocks. A block is mined on average every 600 seconds (10 minutes), so the expected time is 2016 * 600 = 1209600 seconds.

Ratio

The actual time divided by the expected time. We multiply the current target by this ratio to get the new target.

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Note: This target value has been truncated slightly for storage in the bits field of the block header, and that's the target value that's actually used when mining.

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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 Reorganizations

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 be built 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 reorganization to move out blocks from their old active chain in favor of blocks that make up a new longer chain.

Diagram showing a temporary fork in the blockchain being resolved via a chain reorganization.
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. This 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 top of 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 reorganization (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 into 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 reorganization.

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.

Resources