The Fossil version control system shares a lot of similarities with blockchain based technologies, but it also differs from the more common sorts of blockchains. This document will discuss the term’s applicability, so you can decide whether applying the term to Fossil makes sense to you.

The Dictionary Argument

Wikipedia defines "blockchain" in part as

"…a growing list of records, called blocks, which are linked using cryptography. Each block contains a cryptographic hash of the previous block, a timestamp, and transaction data…"

By that partial definition, Fossil is indeed a blockchain. The blocks are "manifests" artifacts. Each manifest has a SHA1 or SHA3 hash of its parent or parents, a timestamp, and other transactional data. The repository grows by adding new manifests onto the list.

Nevertheless, there are many reasons to regard Fossil as not a blockchain.

Cryptocurrency

Because blockchain technology was first popularized as Bitcoin, many people associate the term with cryptocurrency. Since Fossil has nothing to do with cryptocurrency, someone using the term “blockchain” to refer to Fossil is likely to fail to communicate their ideas clearly.

Cryptocurrency also has unfortunate implications in certain circles, its anonymity and lack of regulation leading it to become associated with bad actors. Even if we ignore all of the other criticisms in this document, our unwillingness to be so associated may be enough of a reason for us to avoid using it.

Marketing Capture

The fact that blockchain technology has become a hot marketing buzzword should affect your choice of whether to use the term “blockchain” to refer to Fossil. Your choice may well vary based on the audience:

• Executive Board: At the quarterly all-hands meeting, the big boss — who just read about blockchains in PHB Weekly — asks if your development organization “has a blockchain.” With Fossil and a suitably narrow definition of the term “blockchain” in mind, you could answer “Yes,” except that you know they’re then going to go to the shareholders and happily report, “Our development organization has been using blockchain technology for years!” You may decide that this makes you responsible for a public deception, putting the organization at risk of an SEC investigation for making false statements.

  Yet if you answer “No,” knowing you’ll be punished for not being on top of the latest whiz-bang as the technologically gormless PHB sees it, are you advancing the organization’s actual interests? If the organization has no actual need for a proper blockchain tech base, isn’t it better to just say “Yes” and point at Fossil so you can get back to useful work?

• Middle Management: Your project leader asks the same question, so you point them at this document, which tells them the truth: kinda yes, but mostly no.

• Developer Lunch: A peer asks if you’re doing anything with blockchains. Knowing the contents of this document, you decide you can’t justify using that term to refer to Fossil at a deep technical level, so you admit that you are not.

Distributed Ledgers

Cryptocurrencies are a type of distributed ledger technology. Is Fossil a distributed ledger?

A key tenet of DLT is that records be unmodifiable after they’re committed to the ledger, which matches quite well with Fossil’s design and everyday use cases.

Yet, Fossil also has purge and shunning. Doesn’t that mean Fossil cannot be a distributed ledger?

What if you removed those features from Fossil, creating an append-only variant? Is it a DLT then? Arguably still not, because today’s Fossil is an AP-mode system in the CAP theorem sense, which means there can be no guaranteed consensus on the content of the ledger at any given time. If you had an AP-mode accounts receivable system, it could have different bottom-line totals at different sites, because you’ve cast away “C” to get AP-mode operation.

What are the prospects for CA-mode or CP-mode Fossil? We don’t want CA-mode Fossil, but CP-mode could be useful. Until the latter exists, this author believes Fossil is not a distributed ledger in a technologically defensible sense. If you restrict your definition’s scope to cover only the most common uses of “blockchain,” which are all DLTs, that means Fossil is not a blockchain.

Distributed Partial Consensus

If we can’t get DLT, can we at least get some kind of distributed consensus at the level of individual Fossil’s commits?

Many blockchain based technologies have this property: given some element of the blockchain, you can make certain proofs that it either is a legitimate part of the whole blockchain, or it is not.

Unfortunately, this author doesn’t see a way to do that with Fossil. Given only one “block” in Fossil’s putative “blockchain” — a commit, in Fossil terminology — all you can prove is whether it is internally consistent, not corrupt. That then points you at the parent(s) of that commit, which you can repeat the exercise on, back to the root of the DAG. This is what the enabled-by-default repo-cksum setting does.

If cryptocurrencies worked this way, you wouldn’t be able to prove that a given cryptocoin was legitimate without repeating the proof-of-work calculations for the entire cryptocurrency scheme!

What would it even mean to prove that a given Fossil commit “belongs” to the repository you’ve extracted it from? For a software project, isn’t that tantamount to automatic code review, where the server would be able to reliably accept or reject a commit based solely on its content? That sounds nice, but this author believes we’ll need to invent AGI first.

A better method to provide distributed consensus for Fossil would be to rely on the natural intelligence of its users: that is, distributed commit signing, so that a commit is accepted into the blockchain only once some number of users countersign it. This amounts to a code review feature, which Fossil doesn’t currently have.

Solving that problem basically requires solving the PKI problem first, since you can’t verify the proofs of these signatures if you can’t first prove that the provided signatures belong to people you trust. This is a notoriously hard problem in its own right.

A future version of Fossil could instead provide consensus in the CAP sense. For instance, you could say that if a quorum of servers all have a given commit, it “belongs.” Fossil’s strong hashing tech would mean that querying whether a given commit is part of the “blockchain” would be as simple as going down the list of servers and sending it an HTTP GET /info query for the artifact ID, returning “Yes” once you get enough HTTP 200 status codes back. All of this is hypothetical, because Fossil doesn’t do this today.

Even with all of the above solved, you’d still have another problem: Fossil currently has no way to do partial cloning of a repository. The only way to remotely extract individual “blocks” — commits — from a remote repository is to make /artifact, /info, or /raw queries to its HTTP interface. For Fossil to be a true blockchain, we’d want a way to send around as little as one commit which could be individually verified as being “part of the blockchain” using only intra-block consistency checks.

Conclusion

This author believes it is technologically indefensible to call Fossil a “blockchain” in any sense likely to be understood by a majority of those you’re communicating with.

Within a certain narrow scope, you can defend this usage, but if you do that, you’ve failed any goal that requires clear communication: it doesn’t work to use a term in a nonstandard way just because you can defend it. The people you’re communicating your ideas to must have the same concept of the terms you use.

What term should you use instead? A blockchain is a type of Merkle tree, also called a hash tree, and Fossil is certainly that.

Fossil and “blockchain” are technological peers. They are related technologies, but neither is a subset or instance of the other in any useful sense.