Why is Git not considered a "block chain"?

Question

Git's internal data structure is a tree of data objects, wherein each objects only points to its predecessor. Each data block is hashed. Modifying (bit error or attack) an intermediate block will be noticed when the saved hash and the actual hash deviate.

How is this concept different from block chain?
Git is not listed as an example of block chains, but at least in summaries, both data structure descriptions look alike: data block, single direction reverse linking, hashes, ...).

So where is the difference, that Git isn't called a block chain?

Dissonance

Git is not listed as an example of block chains When I first tried to learn what a blockchain was, I was referred to git as the most prominent example (I don't have the exact link now, but it was from the top of the list returned by Google search for "blockchain") – Hachman

Both Git and blockchain are using merkle trees as their fundamental underlying data structure. But that alone does not make Git a blockchain, or the other way around. – If you do know Git (and its internals), you do know merkle trees though, which can be a very helpful revelation to understand how blockchains work. – Saturant

It's your opinion that "it is NOT considered..." bitcoin.stackexchange.com/a/43627/77469 – Hexagon

@Hexagon Merkle trees exist since 1979. Just because two technologies are using Merkle trees prominently as part of their concept, that does not make them the same. It is incorrect to reduce either Git or block chains to just merkle trees as neither of them are merkle trees. They only use them. That makes the linked post completely irrelevant since it is actually talking about merkle trees, and not block chains. – Saturant

@Saturant it's talking about blockchains and a 91 paper. If the linked post was talking about Merkle trees, it'd be talking about Merkle trees and the 79 paper. It seems that you are trying to navigate the debate to the direction where we'd need to accept that blockchains are wonderful and new. But git is a blockchain by all definitions. And if you want to say that trustless consensus is way more than that, then we agree, though that is not blockchain related. – Hexagon

@Hexagon post your answer separately and I will upvote it because I completely agree with you that blockchain is a data structure the Git implements. And all the other things like consensus, peer network, etc - are not a part of this data structure. – Plantagenet

Can you cite the statement: "git is not considered a block chain"? – Geralyngeraniaceous

@JannisIoannou ?? I don't need to cite it.... You have lots of answers and comments that you can read here. – Dissonance

If block chains agree to have one branch, and will all agree to forget additional branches (not mutate, just forget). Then this is the same as git history rewrite. – Drape

This 1 hour video describes the various things you would need to add to Git, to make it operate like a cryptocurrency blockchain. Git as Blockchain - Michael Perry (NDC Conference Sydney). – Mumbletypeg

@v.oddou: no it is not the OP's opinion. It is a FACT that many people consider the blockchain to have been created for Bitcoin: en.wikipedia.org/wiki/Blockchain . It is a fact that many people have that opinion. OP did not ask for opinions, OP asked for facts which explain why so many people do not classify Git as using blockchain technology. Closing a question because it peripherally mentions an opinion is really unhelpful. OP is not asking "what's the best programming language/compiler/OS/editor/etc.?" – Soule

@JannisIoannou See Argument from ignorance – Swanherd

Answer 1

The question reads: Why is Git not considered a “block chain”? So this is asserting that there is a wide-spread opinion that Git is not a blockchain (an assertion that is illustrated and corroborated by the answers preceding mine on this page) and asking for the reason of the prevalence of this opinion. This is a good question.

Taking the question literally, the answer could be that the blockchain term and concept gained popularity as part of the digital currency operation called “Bitcoin”, and hence came to be associated with how Bitcoin does things: which is by using a lot of computing power to calculate a specific hash including a nonce to meet certain arbitrary requirements, which is by allegedly having no central authority, which is by being “independent”, maybe even “democratic”, and the rest of the kool aid; and as these things are not seen in Git, well, Git cannot be a blockchain, right? And so the question would be answered literally.

Hidden behind this prima facie question is another question: What is a block chain? Now you could look up a definition somehwere and copy it over here, but I didn't do that as I have made up my mind years ago, when listening to a podcast about Bitcoin that strove to explain the new concept of a blockchain, that a blockchain works like Git and I don't intend to let my precious understanding be misled by random claims on the internet.

So what is a blockchain? What's in the word?

Nothing in the term “blockchain” presupposes the requirement to include a nonce in the content so as to come up with a hash of so and so many leading zeros. (This requirement is only there to be able to control the blockchain by computing power and so, ultimately, by money.)

Nothing in the term “blockchain” presupposes the existence of a network, let alone a decentralized one.

Nothing in the term “blockchain” presupposes any “independence” from “central authority”.

The term “block chain” only presupposes blocks (of data) chained together. Now what is a chain? Is it just a link? No, it is a strong link designed to hold things together by force.

A simple linked list doesn't qualify as a blockchain because the contents of the chunks of data in the list could be altered while the list would continue to link back and forth just fine. This is not how a chain works.

To make a link of blocks of data into a chain of blocks of data, the contents of the blocks need to be checksummed (digested) in one way or another and this checksum (digest) must be part of the link, making it a strong link protecting the content, preventing it from being altered. This is a blockchain.

And this is what Git does, and hence Git is a blockchain, or works as one, if you prefer.

To close the circle, let's ask again: Why is Git not considered a “block chain”? It could be because many people, perhaps even a large majority, do not focus on the essence of a concept but on blinking accidents.

Gneiss

I agree with you (or at least I do not disagree) that Git should be considered a blockchain. But for corroborating evidence (beyond the other answers here) that many others think differently, see wikipedia: en.wikipedia.org/wiki/Blockchain – Soule

Blockchain is immutable (Git allows rebase), and also relies on consensus via redundant network mining and security is increased by the size of the network. – Whit

You can fork a blockchain and rewrite blocks just as you can fork a git repo and rewrite history. Both are immutable in a sense and mutable in another. – Mutiny

Answer 2

The reason why Git and blockchains appear similar is because they are both using merkle trees as their underlying data structure. A merkle tree is a tree where each node is labeled with the cryptographic hash value of their contents, which includes the labels of its children.

Git’s directed acyclic graph is exactly that, a merkle tree where each node (tag, commit, tree, or blob object) is labeled with the hash of its content and the label of its “child”. Note that for commits, the “child” term conflicts a bit with Git’s understanding of parents: Parent commits are the children of commits, you just need to look at the graph as a tree that keeps growing by re-rooting it.

Blockchains are very similar to this, since they also keep growing that way, and they are also using its merkle tree property to ensure data integrity. But usually, blockchains are understood as way more than just merkle trees which is where they are separating from the “stupid content tracker” Git. For example, blockchains usually also means having a highly decentralized system on a block level (not all blocks need to be in the same place).

Understanding blockchains is kind of difficult (personally, I’m still far away from understanding everything about it), but I consider understanding Git internals as a good way to understand merkle trees which definitely helps understanding a fundamental part about blockchains.

Saturant

I'm sorry but nowhere blockchains bring anything more than git does. blockchains are exactly as stupid as git. If you don't believe so, you are overhyped. The peer network and the consensus systems are a separate thing. – Hexagon

private ledgers (blockchains) are conceptually identical to git – Genteel

Typically, in a git repository there is one root commit but there can be any number of branches. If you see the last commit in a branch as a root commit and the parents as children, you have a tree with many roots that grow... I think it's just a variation on the Merkle tree where parent references are in the contents instead of child references. There can be multiple parents and children so it isn't even a tree. – Ideology

Git is a blockchain. Objections such as requiring "a highly decentralized system" are implementation details. – Gneiss

So, what this answer is trying to say is that Git is not a block chain because blockchains usually also means having a highly decentralized system on a block level.? This answer doesn't seem to describe why, except having this one "example" which many people considered false. What am I missing? – Guff

Git is a highly decentralized system, anyway. Not sure what "on the block level" means. Full Bitcoin nodes store a full copy of the Bitcoin blockchain and git users store a full copy of the Git repo. And new blocks in both cases can come from anywhere. – Passus

"blockchains usually also means having a highly decentralized system, and not all blocks need to be in the same place" - so does Git? You can do shallow or partial clones. – Quartz

An informative answer, but the argument you end on is weak in my opinion. You can use Git with several remotes, and do partial clones. Git can thus behave very decentralised if you want it to. I feel the point that Git is not a blockchain because crypto currencies are usually very decentralised and Git is usually mostly centralised is shaky at best. If Git is not a blockchain, there should be a property all blockchains share and Git does not, and this is not such a property. – Otisotitis

Answer 3

Blockchain is not just any chain of any blocks.

Blockchain is when there is a way of determining the main chain when two or more are diverted, and when no central authority is needed for that determination.

Superabound

Using this definition, "permissioned blockchain" makes no sense since they do in fact have a central authority. So your definition is contradicting actual usage of the word. See e.g. semi-centralised (federated) blockchains like Liquid. – Observation

@JanusTroelsen "permissioned blockchain" (or "private blockchain") is an oxymoron and indeed makes absolutely zero sense, so called centralised blockchains don't have a single difference from a functionality of regular servers or regular P2P networks. These terms (private/permissioned/centralised blockchain) are used only outside of the professional community. – Superabound

But R3 is a professional company and they use it: r3.com/blog/… – Observation

R3 had been ridiculed in the community exactly for abusing the word 'blockchain' to market their software which could easily exist even if the initial paper was not published back in 2008. Reference to the authority of R3 is not valid I'm afraid. – Superabound

The central authority in a blockchain such as Bitcoin which requires unnecessary and irrational computing is simply the party assembling the most computing power. – Gneiss

@ocodo what do you mean by "a way of determining the main chain when two or more are diverted, and when no central authority is needed for that determination" in git? – Superabound

Answer 4

Cyber Currencies like Bitcoin, use a distributed consensuses cryptographic chain of blocks (merkle tree). Common usage has shortened this to 'blockchain'

While git uses a chain of blocks (merkle tree), it lacks the distributed consensuses cryptographic components that common usage of the term 'BlockChain' imply.

Signpost

Without specifying what "distributed consensus" exactly requires, this distinction is irrelevant. If the PoW threshold is low, anybody can overwrite your blockchain. – Observation

The irrational and unnecessary cipher requirements in Bitcoin are only there so that block chaining requires computing power and thus can be dominated by purchasing computing power, and thus by financial power. – Gneiss

@JanusTroelsen I think what the author means is that there is an algorithm by which there is no central authority do decide on the main chain. In git there technically is no main chain, anyone can declare their own as such. Since there is no scarcity of some object (asset) then for git it doesn't matter. – Mcminn

Answer 5

Unlike cryptocurrency blockchains; git doesn't have a p2p trustless consensus mechanism.

Mammal

Why do you consider a trustless consensus system as part of a block chain? There are many ways to create trust in a block chain, for git it is just that you know that everything in your local copy cannot be removed by the next pull and you specifiy that you want the changes in the remote copy. You only need trustless consensus when it would otherwise be unclear what's right. In git multiple branches can be "right" and get eventuell merged together. – Crumpler

@Crumpler GitHub is typically used as the central source of truth but what's stopping an admin from force pushing and overriding history? If there was no GitHub and you pulled from your peers then how do you handle merge conflicts? How do you determine whose right? – Mammal

Nothing stops you from force pushing. But like a blockchain guarantees me, I can detect it because my chain cannot verify these commits as being based on it. That's the point with a blockchain, not the decentral consent. And in git I explicitely do not want to have a consent protocol for what I merge (development is not a democracy), but I actually read the new commits when merging them into my chain. So my copy is right, because it consists of stuff I already have and thus can verify (i.e. by seeing merge conflicts) and stuff I review and then accept into it. – Crumpler

@Crumpler you're correct in that regard, however I stated in the answer "cryptocurrency blockchains", not blockchains in general, but I now that I think about it my answer doesn't really seem to fit the question being asked because I was thinking about the system as a whole rather than the underlying data structures – Mammal

You're completely right about the difference of the block chains used in git and cryptocurrencies. It is just not an answer to the question why (or if) git is not considered a block chain, when using the term rigorouly. Even the currently accepted answer is similar to your answer. I still prefer the answer which got the bounty. – Crumpler

Answer 6

To sum it up (for me):

While Git offers you complete full freedom of choice, Blockchains are a highly political system, where you are forced to trust in others:

• Git is a Merkle Tree without a predefined consensus algorithm.

• Blockchains are Merkle Trees with a predefined consensus algorithm.

Hence if you are all alone, there is no difference between Git and a Blockchain. As you trust Git and yourself, you already have that predefined consensus.

But things start to become different, when you are in a Network.

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Notes:

• For Blockchains there is absolutely no requirement for the hash to be difficult to calculate or to define something like "Mining" or have some specific software which ensures you take part of a certain Network.
  This all might be a requirement for something like Bitcoin (which usually is referred to as Cryptocurrency, which I cannot fully agree to), but neither is BitCoin defining what a Blockchain is, nor does a Blockchain need to be something like BitCoin.

• The consensus algorithm does not necessarily be something which is based on some cryptographic protocol. For example it would be enough to publish your TIP in a local newspaper each day to (ab)use Git as some Blockchain.

Git readily offers multiple possible consensus algorithms you can chose from:

• Publishing the SHA in a Newspaper or similar (something which is distributed and hard to fake)

• If you are in the rare situation that you are already part of some GnuPG Web Of Trust, you readily can use Signed Commits (or Signed Tags) to agree to the consensus.

• The "Signed off:" variant does not offer cryptographically secure consensus, but in combination with something like Gerrit and Fast-Forward-Only pushes it is some pretty well defined consensus algorithm.

Hence to make Git a Blockchain, all you need is to add some air.

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Some different view:

Git is no Blockchain on itself. In contrast, it is far less than a Blockchain (lacking the predefined consensus algorithm) and much more than a Blockchain (allows a plethora of consensus algorithm to chose from, is meant as an SCM etc.).

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Some other observations:

• Git branches are the same as Blockchain splits. While Blockchain splits happen rarely, most Git repositories have less branches (master+HEAD) than BitCoin had splits.

• Git always has an explicite consensus done by you, that is, the TIP you push to. However this only applies to you and nobody else.
  Pushing the Git repository to some shared Git Service can also be seen as a consensus. There is no requirement for such a consensus to be based on Democratic principles.

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Very personal thoughts:

While Blockchain is some overhyped buzzword, something you can happily live without, Git is an inevitable fundamental tool for getting your work done, one of the basic must-haves you cannot live without, something as important as air and water. This is probably, why people like me do not refer to Git as a Blockchain ..

YMMV

Aleksandr

Fantastic summary. – Runoff

Answer 7

There is no reason to not consider Git as a blockchain. Git is focused in a very particular (and important) set of assets: source code. The consensus in this case is manual, and we can consider that a transaction (commit) is accepted when it is merged into the release branch. Actually, considering the number of transactions (commits), Git is by far the most successful blockchain.

Extracted from: https://arxiv.org/pdf/1803.00892.pdf "... ...We define“blockchain” and “blockchain network”, and then discuss two very different, well known classes of blockchain networks: cryptocurrencies and Git repositories..."

See also next paper that explain why Google use a single monorepo as single source of truth (basically, as a blockchain). https://research.google/pubs/pub45424/

Reformation

Answer 8

As poke said:

Git and Blockchains appear similar because they are both using Merkle Trees to store ordered timestamped transactions. A merkle tree is a tree data structure where each node is labeled with the cryptographic hash value of their contents, which includes the labels of its children.

The first difference is the Hash function: Blockchain has a very expensive hash function so that each block has to be mined, wheras a Git "block" can be created with a simple commit message.

The purpose of Bitcoin is to add trust to the order of transactions. The focus is on the longest chain, since that is most expensive to compute and thus most likely to be the truth.

Bitcoin accomplishes this by requiring that the hash meets certain parameters (begins with a specific number of 0s), by incrementing a value ("nonce") in the message until a satisfactory hash is found. This takes effort to find, but only 1 calculation to verify for a nonce; and if multiple nonces produce a satisfactory hash, then one will be lower and taken as the truth. Other authentication schemes make the hash trustworthy by centralizing the issuing of the hash to an authority, perhaps voted by network agreement, or some other method.

Blockchain data is limited to transactions, which must must conform to validation. Transaction must be valid to be included in the next block. A Bitcoin transaction corresponds to something important in the real world that justifies using an expensive block to record this transfer, like exchange of money value. We don't actually care about the final ledger, it's a metaphor for something in the real world.

By contrast, Git blocks are arbitrary, as a commit can contain any amount of data. The value lies in the changes of data being organized into the git tree because we care about the final product, it's validated by the existence of the git repository.

The purpose of Git is to allow cheap "ledgers" to track multiple product alternatives. The "ledger" in Git is what we care about, it's our final product; the transactions data just record how the product was built. We want to make it very cheap to make multiple versions of final products, just enough overhead to require the creator to record how they built this product. No explicit validation is done on the data, you maintain the end-product if it looks good, and that existence makes it useful to have the chain of this product's creation. If the end-product is bad or the order of commits is invalid, this "ledger" gets deleted during garbage collection.

The second difference is that Blockchain transactions must come from a prior valid source. In Git, we don't care what data you use to extend the tree. In Blockchain, the transactions must come from a prior valid source. In that sense, Git tracks the extension of our environment, whereas Blockchain tracks the exchange of value within a closed environment.

Exhortation

Answer 9

The Goals are different for blockchain and git although both use merkle trees as data structure.

A blockchain is typically managed by a peer-to-peer network adhering to a protocol for inter-node communication and validating new blocks. Once recorded, the data in any given block cannot be altered retroactively without alteration of all subsequent blocks, which requires consensus of the network majority.

As According to Bitcoin whitepaper :

A purely peer-to-peer version of electronic cash would allow online payments to be sent directly from one party to another without going through a financial institution. Digital signatures provide part of the solution, but the main benefits are lost if a trusted third party is still required to prevent double-spending. We propose a solution to the double-spending problem using a peer-to-peer network. The network timestamps transactions by hashing them into an ongoing chain of hash-based proof-of-work, forming a record that cannot be changed without redoing the proof-of-work. The longest chain not only serves as proof of the sequence of events witnessed, but proof that it came from the largest pool of CPU power. As long as a majority of CPU power is controlled by nodes that are not cooperating to attack the network, they'll generate the longest chain and outpace attackers. The network itself requires minimal structure. Messages are broadcast on a best effort basis, and nodes can leave and rejoin the network at will, accepting the longest proof-of-work chain as proof of what happened while they were gone

While Git is a distributed version-control system for tracking changes in source code during software development.It is designed for coordinating work among programmers, but it can be used to track changes in any set of files. Its goals include speed, data integrity, and support for distributed, non-linear workflows.

As according to Linus Torvalds:

In many ways you can just see git as a filesystem – it's content-addressable, and it has a notion of versioning, but I really designed it coming at the problem from the viewpoint of a filesystem person (hey, kernels is what I do), and I actually have absolutely zero interest in creating a traditional SCM system.

Rachealrachel

Answer 10

A great way to understand any given technology is to ask, "what problem does it solve"? Git's use case is quite simple in that it's intended use is for version control / source code control.

What is Git?

"Git is a free and open source distributed version control system designed to handle everything from small to very large projects with speed and efficiency." See https://www.git-scm.com/

So it is clear that the intended use case problem to solve is "distributed version control". That is all, nothing more, nothing less. Many proofs of this are readily available.

"Version control — also known as source control or revision control — is an important software development practice for tracking and managing changes made to code and other files. It is closely related to source code management." gitlab source

What is Blockchain?

"Blockchain is a peer-to-peer decentralized distributed ledger technology that makes the records of any digital asset transparent and unchangeable and works without involving any third-party intermediary. It is an emerging and revolutionary technology that is attracting a lot of public attention due to its capability to reduce risks and fraud in a scalable manner." blockchain-council.org

Without repeating the technical details of blockchain already outlined in previous answers(i.e. mining, distributed networking), simply put, blockchain is a solution to an entirely different problem than those solved by Git.

Whit