Blockchain

A big file of bitcoin transactions

Diagram showing the blockchain as a file stored by nodes on the bitcoin network.
Current Blockchain Size:
762.18 GB
903,761 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: 903,761 (0 blocks away) ⇈
Next 0 blocks ↑
Height Block Hash Time (UTC) Size Txs Avg Feerate AFR
903,761 903,761 00000000000000000000f68cde03ff7039750593b5544dc0509b5662babe4409 ago 0.31 MB 536 2
903,760 903,760 00000000000000000001523c44aa311faa17b3c1385da9a4cc101b2a36407ea2 ago 2.16 MB 3,739 0
903,759 903,759 0000000000000000000113c560a05117aee1387eaa0f1ad632ea36bf6e34fa95 ago 1.48 MB 2,525 2
903,758 903,758 000000000000000000013fb0e143053f04b6abfd74a7ad339b3c1aa5b3c4d117 ago 1.04 MB 1,211 2
903,757 903,757 0000000000000000000067747cea1780111df5351b84e07dbcd6e6073f7bca33 ago 0.11 MB 172 2
903,756 903,756 000000000000000000005f920170a8ab2a367aa56e70e7be62f4117cc12adee8 0.36 MB 623 2
903,755 903,755 0000000000000000000029470e4e7427495d87b2abed96d5aae0c15cf24375b7 0.13 MB 320 3
903,754 903,754 00000000000000000001b08631e56b75dd4f443ef0a5d7353c7b1e35370d6724 1.91 MB 3,662 1
903,753 903,753 0000000000000000000111bfbdb122f0c03f90df192422c45343643175f2a8d0 1.66 MB 3,589 1
903,752 903,752 0000000000000000000190d82ecfff203c1b9014c136e51f4921de68eb94ac72 1.52 MB 2,169 2
903,751 903,751 000000000000000000004bbf29f1a6290cbafeff9e092c203d0c42132368c775 0.83 MB 1,273 1
903,750 903,750 000000000000000000025569dc1712e13de389cd0e10f6abed651d0695895195 1.65 MB 1,888 2
903,749 903,749 000000000000000000025dcb8d08e616117b6a4b4943065671cf464cdc270ea0 1.62 MB 4,019 2
903,748 903,748 0000000000000000000189e02693f164965f668975dfc0c709eb4080a46e16bc 1.48 MB 2,628 3
903,747 903,747 0000000000000000000223aa80d0d113206e5a4093d867a259df30983243526c 1.35 MB 2,623 3
903,746 903,746 000000000000000000013ab084d2773590d0af56c7d2bf7947493da7e21397c3 0.69 MB 886 7
903,745 903,745 00000000000000000001bb2b11b1ead2b4bd61548af5f105b742d748cc1bac91 1.19 MB 1,450 1
903,744 903,744 0000000000000000000155333114b7a5b7571e9dc85331f0c09b671d8b7fedbc 1.29 MB 2,172 3
903,743 903,743 00000000000000000000ef90f5a9c77b1a6d325231f80e2e34e9fce4263aeb5c 1.05 MB 2,597 2
903,742 903,742 0000000000000000000166e96f81414fd39ae0a81163a4e5813d3b6933cd09ca 1.39 MB 1,793 1
903,741 903,741 000000000000000000015abf94101eaa589f2666213f544e21780fec1ecec75d 1.49 MB 1,983 3
Previous 10 blocks ↓
Total Size: 762.18 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