Blockchain

A big file of bitcoin transactions

Diagram showing the blockchain as a file stored by nodes on the bitcoin network.
Current Blockchain Size:
769.91 GB
908,183 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: 908,183 (0 blocks away) ⇈
Next 0 blocks ↑
Height Block Hash Time (UTC) Size Txs Avg Feerate AFR
908,183 908,183 00000000000000000001e8fbd8a141bafab241428f02afbe48d557a9a489a3ae ago 2.93 MB 1,991 2
908,182 908,182 0000000000000000000003fba560de1b11325a6cf528afd258f6ce29b0a1c80b ago 2.61 MB 696 1
908,181 908,181 00000000000000000000fc58a71971236e396a3fa3d83db1bcd2427189275b0d ago 2.20 MB 2,703 3
908,180 908,180 00000000000000000000f6b9ec80d28d53c16bf1f6350f8cdd49f6208b02576d ago 2.16 MB 2,298 3
908,179 908,179 00000000000000000000710aa9bcab0bd1bf1cac8dbb791d303548ef36159b36 ago 3.75 MB 416 1
908,178 908,178 0000000000000000000008d1e7c432f6610d4512226180380158b35ea2f8299f 2.41 MB 2,165 2
908,177 908,177 00000000000000000001ed172146a99f2464df8383fd063cb43708d68e331551 3.82 MB 264 1
908,176 908,176 00000000000000000000d74e99b99f3098795f6a641f3a00bfdee335806d9072 3.49 MB 403 1
908,175 908,175 00000000000000000000f2403011f0513b9d283981dcd3abcd47e2ff02f5867a 2.62 MB 1,937 2
908,174 908,174 000000000000000000019867aa69da522262ec3e5ad793954d7fb70efe54205f 2.38 MB 1,989 2
908,173 908,173 00000000000000000001a91673c68e03369e4db833136527fc5f572ce3e0421e 2.24 MB 1,685 1
908,172 908,172 0000000000000000000115a4a19925d7e32c8d0fd7b3e26243bcd24000e61732 1.70 MB 1,090 2
908,171 908,171 00000000000000000001b2fa66a26c7fd3b566e96de3bdb662be6eb262819619 2.19 MB 3,146 5
908,170 908,170 0000000000000000000138170d6043d85edbfa5ce22b02cddad18b073f2d8e41 2.03 MB 2,574 4
908,169 908,169 00000000000000000002061d8753b9a371363175a60ed1286e4e10b4acd69433 2.28 MB 2,721 4
908,168 908,168 0000000000000000000160fb32c5f853f1e772d050825173a4d43d4ee5ba691b 1.91 MB 3,567 6
908,167 908,167 000000000000000000014a4c76094d86cecf7cd687559f1fe690188e67a81b28 1.87 MB 1,499 3
908,166 908,166 000000000000000000010d2876316d092d6988d2010a6fa410ef2ea8ff85ea41 2.05 MB 3,157 5
908,165 908,165 00000000000000000001af4101b79fc371548af0f8bb35b77a4b6a611d5b78fa 1.96 MB 1,690 2
908,164 908,164 0000000000000000000053d6d7a56745e15d14313d870f1e51d9da8406002073 2.23 MB 2,905 4
908,163 908,163 0000000000000000000051c418c483c932cd67863dcf463942184e9c81cddd5d 2.12 MB 3,236 7
Previous 10 blocks ↓
Total Size: 769.91 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