If there's one thing that's true about the word "blockchain", it's that these days people have strong opinions about it. Open your social media feed and you'll see people either heaping praises on blockchains, calling them the saviors of humanity, or condemning them as destroying and burning down the planet and making the rich richer and the poor poorer and generally all the other kinds of fights that people like to have about capitalism (also a quasi-vague word occupying some hotly contested mental real estate).
There are good reasons to hold opinions about various aspects of what are called "blockchains", and I too have some pretty strong opinions I'll be getting into in a followup article. The followup article will be about "cryptocurrencies", which many people also seem to think of as synonymous with "blockchains", but this isn't particularly true either, but we'll deal with that one then.
In the meanwhile, some of the fighting on the internet is kind of confusing, but even more importantly, kind of confused. Some of it might be what I call "sportsballing": for whatever reason, for or against blockchains has become part of your local sportsball team, and we've all got to be team players or we're gonna let the local team down already, right? And the thing about sportsballing is that it's kind of arbitrary and it kind of isn't, because you might pick a sportsball team because you did all your research or you might have picked it because that just happens to be the team in your area or the team your friends like, but god almighty once you've picked your sportsball team let's actually not talk against it because that might be giving in to the other side. But sportsballing kind of isn't arbitrary either because it tends to be initially connected to real communities of real human beings and there's usually a deeper cultural web than appears at surface level, so when you're poking at it, it appears surface-level shallow but there are some real intricacies beneath the surface. (But anyway, go sportsball team.)
But I digress. There are important issues to discuss, yet people aren't really discussing them, partly because people mean different things. "Blockchain" is a strange term that encompasses a wide idea space, and what people consider or assume essential to it vary just as widely, and thus when two people are arguing they might not even be arguing about the same thing. So let's get to unpacking.
"Blockchain" as handwaving towards decentralized networks in general
Years ago I was at a conference about decentralized networked technology, and I was having a conversation with someone I had just met. This person was telling me how excited they were about blockchains... finally we have decentralized network designs, and so this seems really useful for society!
I paused for a moment and said yes, blockchains can be useful for some things, though they tend to have significant costs or at least tradeoffs. It's good that we also have other decentralized network technology; for example, the ActivityPub standard I was involved in had no blockchains but did rely on the much older "classic actor model."
"Oh," the other person said, "I didn't know there were other kinds of decentralized network designs. I thought that 'blockchain' just meant 'decentralized network technology'."
It was as if a light had turned on and illuminated the room for me. Oh! This explained so many conversations I had been having over the years. Of course... for many people, blockchains like Bitcoin were the first ever exposure they had (aside from email, which maybe they never gave much thought to as being decentralized) of something that involved a decentralized protocol. So for many people, "blockchain" and "decentralized technology" are synonyms, if not in technical design, but in terms of understanding of a space.
Mark S. Miller, who was standing next to me, smiled and gave a very interesting followup: "There is only one case in which you need a blockchain, and that is in a decentralized system which needs to converge on a single order of events, such as a public ledger dealing with the double spending problem."
Two revelations at once. It was a good conversation... it was a good start. But I think there's more.
Blockchains are the "cloud" of merkle trees
As time has gone on, the discourse over blockchains has gotten more dramatic. This is partly because what a "blockchain" is hasn't been well defined.
All terminology exists on an ever-present battle between fuzziness and crispness, with some terms being much clearer than others. The term "boolean" has a fairly crisp definition in computer science, but if I ask you to show me your "stove", the device you show me today may be incomprehensible to someone's definition a few centuries ago, particularly in that today it might not involve fire. Trying to define as in terms of its functionality can also cause confusion: if I asked you to show me a stove, and you showed me a computer processor or a car engine, I might be fairly confused, even though technically people enjoy showing off that they can cook eggs on both of these devices when they get hot enough. (See also: Identity is a Katamari, language is a Katamari explosion.)
Still, some terms are fuzzier than others, and as far as terms go, "blockchain" is quite fuzzy. Hence my joke: "Blockchains are the 'cloud' of merkle trees."
This ~joke tends to get a lot of laughs out of a particular kind of audience, and confused looks from others, so let me explain. The one thing everyone seems to agree on is that it's a "chain of blocks", but all that really seems to mean is that it's a merkle tree... really, just an immutable datastructure where one node points at the parent node which points at the parent node all the way up. The joke then is not that this merkle tree runs on a cloud, but that "cloud computing" means approximately nothing: it's marketing speak for some vague handwavey set of "other peoples' computers are doing computation somewhere, possibly on your behalf sometimes." Therefore, "cloud of merkle trees" refers to the vagueness of the situation. (As everyone knows, jokes are funnier when fully explained, so I'll turn on my "STUDIO LAUGHTER" sign here.)
So, a blockchain is a chain of blocks, ie a merkle tree, and I mean, technically speaking, that means that Git is a blockchain (especially if the commits are signed), but when you see someone arguing on the internet about whether or not blockchains are "good" or "bad", they probably weren't thinking about git, which aside from having a high barrier of entry in its interface and some concerns about the hashing algorithm used, isn't really something likely to drag you into an internet flamewar.
"Blockchain" is to "Bitcoin" what "Roguelike" is to "Rogue"
These days it's common to see people either heaping praises on blockchains or criticizing them, and those people tend to be shouting past one another. I'll save unpacking that for another post. In the meanwhile though, it's worth noting that people might not be talking about the same things.
What isn't in doubt is whether or not Bitcoin is a blockchain... trying to understand and then explore the problem space around Bitcoin is what created the term "blockchain". It's a bit like the video game genre of roguelikes, which started with the game Rogue, particularly explored and expanded upon in NetHack, and then suddenly exploding into the indie game scene as a "genre" of its own. Except the genre has become fuzzier and fuzzier as people have explored the surrounding space. What is essential? Is a grid based layout essential? Is a non-euclidean grid acceptable? Do you have to provide an ascii or ansi art interface so people can play in their terminals? Dare we allow unicode characters? What if we throw out terminals altogether and just play on a grid of 2d pixelart? What about 3d art? What about permadeath? What about the fantasy theme? What about random level generation? What are the key features of a roguelike?
Well now we're at the point where I pick up a game like Blazing Beaks and it calls itself a "roguelite", which I guess is embracing the point that terminology has gotten extremely fuzzy... this game feels more like Robotron than Rogue.
So... if "blockchain" is to Bitcoin what "roguelike" is to Rogue, then what's essential to a blockchain? Does the blockchain have to be applied to a financial instrument, or can it be used to store updateable information about eg identity? Is global consensus required? Or what about a "trusted quorum" of nodes, such as in Hyperledger? Is "mining" some kind of asset a key part of the system? Is proof of work acceptable, or is proof of stake okay? What about proof of space, proof of space-time, proof of pudding?
On top of all this, some of the terms around blockchains have been absorbed as if into them. For instance, I think to many people, "smart contract" means something like "code which runs on a blockchain" thanks to Ethereum's major adoption of the term, but the E programming language described "smart contracts" as the "likely killer app of distributed capabilities" all the way back in 1999, and was borrowing the term from Nick Szabo, but really the same folks working on E had described many of those same ideas in the Agoric Papers back in 1988. Bitcoin wasn't even a thing at all until at least 2008, so depending on how you look at it, "smart contracts" precede "blockchains" by one or two decades. So "blockchain" has somehow even rolled up terms outside of its space as if within it. (By the way, I don't think anyone has given a good and crisp definition for "smart contract" either despite some of these people trying to give me one, so let me give you one that I think is better and embraces its fuzziness: "Smart contracts allow you to do the kinds of things you might do with legal contracts, but relying on networked computation instead of a traditional state-based legal system." It's too bad more people also don't know about the huge role that Mark Miller's "split contracts" idea plays into this space because that's what makes the idea finally makes sense... but that's a conversation for another time.) (EDIT: Well, after I wrote this, Kate Sills lent me her definition, which I think is the best one: "Smart contracts are credible commitments using technology, and outside a state-provided legal system." I like it!)
So anyway, the point of this whole section is to say that kind of like roguelike, people are thinking of different things as essential to blockchains. Everyone roughly agrees on the jumping-off point of ideas but since not everyone agrees from there, it's good to check in when we're having the conversation. Wait, you do/don't like this game because it's a roguelike? Maybe we should check in on what features you mean. Likewise for blockchains. Because if you're blaming blockchains for burning down the planet, more than likely you're not condemning signed git repositories (or at least, if you're condemning them, you're probably doing so about it from an aspect that isn't the fundamental datastructure... probably).
This is an "easier said than done" kind of thing though, because of course, I'm kind of getting into some "in the weeds" level of details here... but it's the "in the weeds" where all the substance of the disagreements really are. The person you are talking with might not actually even know or consider the same aspects to be essential that you consider essential though, so taking some time to ask which things we mean can help us lead to a more productive conversation sooner.
"Blockchain" as an identity signal
First, a digression. One thing that's kind of curious about the term "virtue signal" is that in general it tends to be used as a kind of virtue signal. It's kind of like the word hipster in the previous decade, which weirdly seemed to be obsessively and pejoratively used by people who resembled hipsters than anyone else. Hence I used to make a joke called "hipster recursion", which is that since hipsters seem more obsessesed with pejorative labeling of hipsterism than anyone else, there's no way to call someone a "hipster" without yourself taking on hipster-like traits, and so inevitably even this conversation is N-levels deep into hipster recursion for some numerical value of N.
"Virtue signaling" appears similar, but even more ironically so (which is a pretty amazing feat given how much of hipsterdom seems to surround a kind of inauthentic irony). When I hear someone say "virtue signaling" with a kind of sneer, part of that seems to be acknowledging that other people are sending signals merely to impress others that they are some kind of the same group but it seems as if it's being raised as in a you-know-and-I-know-that-by-me-acknowledging-this-I'm-above-virtue-signaling kind of way. Except that by any possible definition of virtue signaling, the above appears to be a kind of virtue signaling, so now we're into virtue signaling recursion.
Well, one way to claw our way out of the rabbithole of all this is to drop the pejorative aspect of it and just acknowledge that signaling is something that everyone does. Hence me saying "identity signaling" here. You can't really escape identity signaling, or even sportsballing, but you can acknowledge that it's a thing that we all do, and there's a reason for it: people only have so much time to find out information about each other, so they're searching for clues that they might align and that, if they introduce you to their peer group, that you might align with them as well, without access to a god-like view of the universe where they know exactly what you think and exactly what kinds of things you've done and exactly what way you'll behave in the future or whether or not you share the same values. (After all, what else is virtue ethics but an ethical framework that takes this in its most condensed form as its foundation?) But it's true that at its worst, this seems to result in shallow, quick, judgmental behavior, usually based on stereotypes of the other side... which can be unfortunate or unfair to whomever is being talked about. But also on the flip side, people also do identity signal to each other because they want to create a sense of community and bonding. That's what a lot of culture is. It's worth acknowledging then that this occurs, recognizing its use and limitations, without pretending that we are above it.
So wow, that's quite a major digression, so now let's get back to "identity signaling". There is definitely a lot of identity signaling that tends to happen around the word "blockchain", for or against. Around the critiques of the worst of this, I tend to agree: I find much of the machismo hyper-white-male-privilege that surrounds some of the "blockchain" space uncomfortable or cringey.
But I also have some close friends who are not male and/or are people of color and those ones tend to actually suffer the worst of it from these communities internally, but also seem to find things of value in them, but particularly seem to feel squeezed externally when the field is reduced to these kinds of (anti?-)patterns. There's something sad about that, where I see on the one hand friends complaining about blockchain from the outside on behalf of people who on the inside seem to be both struggling internally but then kind of crushed by being lumped into the same identified problems externally. This is hardly a unique problem but it's worth highlighting for a moment I think.
But anyway, I've taken a bunch of time on this, more than I care to, maybe because (irony again?) I feel that too much of public conversation is also hyperfocusing on this aspect... whether there's a subculture around blockchain, whether or not that subculture is good or bad, etc. There's a lot worthwhile in unpacking this discourse-wise, but some of the criticisms of blockchains as a technology (to the extent it even is coherently one) seem to get lumped up into all of this. It's good to provide thoughtful cultural critique, particularly one which encourages healthy social change. And we can't escape identity signaling. But as someone who's trying to figure out what properties of networked systems we do and don't want, I feel like I'm trying to navigate the machine and for whatever reason, my foot keeps getting caught in the gears here. Well, maybe that itself is pointing to some architectural mistakes, but socially architectural ones. But it's useful to also be able to draw boundaries around it so that we know where this part of the conversation begins and ends.
"Blockchain" as "decentralized centralization" (or "decentralized convergence")
One of the weird things about people having the idea of "blockchains" as being synonymous with "decentralization" is that it's kind of both very true and very untrue, depending on what abstraction layer you're looking at.
For a moment, I'm going to frame this in harsh terms: blockchains are decentralized centralization.
What? How dare I! You'll notice that this section is in harsh contrast to the "blockchain as handwaving towards decentralized networks in general" section... well, I am acknowledging the decentralized aspect of it, but the weird thing about a blockchain is that it's a decentralized set of nodes converging on (creating a centrality of!) a single abstract machine.
Contrast with classic actor model systems like CapTP in Spritely Goblins, or as less good examples (because they aren't quite as behavior-oriented as they are correspondence-oriented, usually) ActivityPub or SMTP (ie, email). All of these systems involve decentralized computation and collaboration stemming from sending messages to actors (aka "distributed objects"). Of CapTP this is especially clear and extreme: computations happen in parallel across many collaborating machines (and even better, many collaborating objects on many collaborating machines), and the behavior of other machines and their objects is often even opaque to you. (CapTP survives this in a beautiful way, being able to do well on anonymous, peer to peer, "mutually suspicious" networks. But maybe read my rambling thoughts about CapTP elsewhere.)
While to some degree there are some very clever tricks in the world of cryptography where you may be able to get back some of the opacity, this tends to be very expensive, adding an expensive component to the already inescapable additional expenses of a blockchain. A multi-party blockchain with some kind of consensus will always, by definition be slower than a single machine operating alone.
If you are irritated by this framing: good. It's probably good to be irritated by it at least once, if you can recognize the portion of truth in it. But maybe that needs some unpacking to get there. It might be better to say "blockchains are decentralized convergence", but I have some other phrasing that might be helpful.
"Blockchain" as "a single machine that many people run"
There's value in having a single abstract machine that many people run. The most famous source of value is in the "double spending problem". How do we make sure that when someone has money, they don't spend that money twice?
Traditional accounting solves this with a linear, sequential ledger, and it turns out that the right solution boils down to the same thing in computers. Emphasis on sequential: in order to make sure money balances out right, we really do have to be able to order things.
Here's the thing though: the double spending problem was in a sense solved in terms of single-computers a long time ago in the object capability security community. Capability-based Financial Instruments was written about a decade before blockchains even existed and showed off how to make a "mint" (kind of like a fiat-currency bank) that can be implemented in about 25 lines of code in the right architecture (I've ported it to Goblins, for instance) and yet has both distributed accounts and is robust against corruption on errors.
However, this seems to be running on a "single-computer based machine", and again operates like a fiat currency. Anyone can create their own fiat currency like this, and they are cheap, cheap, cheap (and fast!) to make. But it does rely on sequentiality to some degree to operate correctly (avoiding a class of attacks called "re-entrancy attacks").
But this "single-computer based machine" might bother you for a couple reasons:
We might be afraid the server might crash and service will be interrupted, or worse yet, we will no longer be able to access our accounts.
Or, even if we could trade these on an open market, and maybe diversify our portfolio, maybe we don't want to have to trust a single operator or even some appointed team of operators... maybe we have a lot of money in one of these systems and we want to be sure that it won't suddenly vanish due to corruption.
Well, if our code operates deterministically, then what if from the same initial conditions (or saved snapshot of the system) we replay all input messages to the machine? Functional programmers know: we'll end up with the same result.
So okay, we might want to be sure this doesn't accidentally get corrupted, maybe for backup reasons. So maybe we submit the input messages to two computers, and then if one crashes, we just continue on with the second one until the other comes up, and then we can restore the first one from the progress the second machine made while the first one was down.
Oh hey, this is already technically a blockchain. Except our trust model is that we implicitly trust both machines.
Hm. Maybe we're now worried that we might have top-down government pressure to coerce some behavior on one of our nodes, or maybe we're worried that someone at a local datacenter is going to flip some bits to make themselves rich. So we actually want to spread this abstract machine out over three countries. So okay, we do that, and now we set a rule agreeing on what all the series of input messages are... if two of three nodes agree, that's good enough. Oh hey look, we've just invented the "small-quorum-style" blockchain/ledger!
Well, okay. This is probably good enough for a private financial asset, but what about if we want to make something more... global? Where nobody is in charge!
Well, we could do that too. Here's what we do.
First, we need to prevent a "swarming attack" (okay, this is generally called a "sybil attack" in the literature, but for a multitude of reasons I won't get into, I don't like that term). If a global set of peers are running this single abstract machine, we need to make sure there aren't invocations filling up the system with garbage, since we all basically have to keep that information around. Well... this is exactly where those proof-of-foo systems come in the first time; in fact Proof of Work's origin is in something called Hashcash which was designed to add "friction" to disincentivize spam for email-like systems. If we don't do something friction-oriented in this category, our ledger is going to be too easily filled with garbage too fast. We also need to agree on what the order of messages is, so we can use this mechanism in conjuction with a consensus algorithm.
When are new units of currency issued? Well, in our original mint example, the person who set up the mint was the one given the authority to make new money out of thin air (and they can hand out attenuated versions of that authority to others as they see fit). But what if instead of handing this capability out to individuals we handed it out to anyone who can meet an abstract requirement? For instance, in zcap-ld an invoker can be any kind of entity which is specified with linked data proofs, meaning those entities can be something other than a single key... for instance, what if we delegated to an abstract invoker that was specified as being "whoever can solve the state of the machine's current proof-of-work puzzle"? Oh my gosh! We just took our 25-line mint and extended it for mining-style blockchains. And the fundamental design still applies!
With these two adjustments, we've created a "public blockchain" akin to bitcoin. And we don't need to use proof-of-work for either technically... we could swap in different mechanisms of friction / qualification.
If the set of inputs are stored as a merkle tree, then all of the system types we just looked at are technically blockchains:
A second machine as failover in a trusted environment
Three semi-trusted machines with small-scale private consensus
A public blockchain without global trust, with swarming-attack resistance and an interesting abstract capability accessible to anyone who can meet the abstract requirement (in this case, to issue some new currency).
The difference for choosing any of the above is really a question of: "what is your trust/failover requirements?"
Blockchains as time travel plus convergent inputs
If this doesn't sound believable to you, that you could create something like a "public blockchain" on top of something like Goblins so easily, consider how we might extend time travel in Terminal Phase to add multiplayer. As a reminder, here's an image:
Now, a secret thing about Terminal Phase is that the gameplay is deterministic (the random starfield in the background is not, but the gameplay is) and runs on a fixed frame-rate. This means that given the same set of keyboard inputs, the game will always play the same, every time.
Okay, well let's say we wanted to hand some way for someone to replay our last game. Chess games can be fully replayed with a very condensed syntax, meaning that merely handing someone a short list of codes they can precisely replay the same game, every time, deterministically.
Well okay, as a first attempt at thinking this through, what if for some game of Terminal Phase I played we wrote down each keystroke I entered on my keyboard, on every tick of the game? Terminal Phase runs at 30 ticks per second. So okay, if you replay these, each one at 30 ticks per second, then yeah, you'd end up with the same gameplay every time.
It would be simple enough for me to encode these as a linked list (cons, cons, cons!) and hand them to you. You could descend all the way to the root of the list and start playing them back up (ie, play the list in reverse order) and you'd get the same result as I did. I could even stream new events to you by giving you new items to tack onto the front of the list, and you could "watch" a game I was playing live.
So now imagine that you and I want to play Terminal Phase together now, over the network. Let's imagine there are two ships, and for simplicity, we're playing cooperatively. (The same ideas can be extended to competitive, but for narrating how real-time games work it's easier to to start with a cooperative assumption.)
We could start out by wiring things up on the network so that I am allowed to press certain keys for player 1 and you are allowed to press certain keys for player 2. (Now it's worth noting that a better way to do this doesn't involve keys on the keyboard but capability references, and really that's how we'd do things if we were to bring this multiplayer idea live, but I'm trying to provide a metaphor that's easy to think about without introducing the complicated sounding kinds of terms like "c-lists" and "vat turns" that we ocap people seem to like.) So, as a first attempt, maybe if we were playing on a local area network or something, we could synchronize at every game tick: I share my input with you and you share yours, and then and only then do both of our systems actually input them into that game-tick's inputs. We'll have achieved a kind of "convergence" as to the current game state on every tick. (EDIT: I wrote "a kind of consensus" instead of "a kind of convergence" originally, and that was an error, because it misleads on what consensus algorithms tend to do.)
Except this wouldn't work very well if you and I were living far away from each other and playing over the internet... the lag time for doing this for every game tick might slow the system to a crawl... our computers wouldn't get each others' inputs as fast as the game was moving along, and would have to pause until we received each others' moves.
So okay, here's what we'll do. Remember the time-travel GUI above? As you can see, we're effectively restoring from an old snapshot. Oh! So okay. We could save a snapshot of the game every second, and then both get each other our inputs to each other as fast as we can, but knowing it'll lag. So, without having seen your inputs yet, I could move my ship up and to the right and fire (and send that I did that to you). My game would be in a "dirty state"... I haven't actually seen what you've done yet. Now suddenly I get the last set of moves you did over the network... in the last five frames, you move down and to the left and fire. Now we've got each others' inputs... what our systems can do is secretly time travel behind the scenes to the last snapshot, then fast forward, replaying both of our inputs on each tick up until the latest state where we've both seen each others' moves (but we wouldn't show the fast forward process, we'd just show the result with the fast forward having been applied). This can happen fast enough that I might see your ship jump forward a little, and maybe your bullet will kill the enemy instead of mine and the scores shift so that you actually got some points that for a moment I thought I had, but this can all happen in realtime and we don't need to slow down the game at all to do it.
Again, all the above can be done, but with actual wiring of capabilities instead of the keystroke metaphor... and actually, the same set of ideas can be done with any kind of system, not just a game.
And oh hey, technically, technically, technically if we both hashed each of our previous messages in the linked list and signed each one, then this would qualify as a merkle tree and then this would also qualify as a blockchain... but wait, this doesn't have anything to do with cryptocurrencies! So is it really a blockchain?
"Blockchain" as synonym for "cryptocurrency" but this is wrong and don't do this one
By now you've probably gotten the sense that I really was annoyed with the first section of "blockchain" as a synonym for "decentralization" (especially because blockchains are decentralized centralization/convergence) and that is completely true. But even more annoying to me is the synonym of "blockchain" with "cryptocurrency".
"Cryptocurrency" means "cryptographically based currency" and it is NOT synonymous with blockchains. Digicash precedes blockchains by a dramatic amount, but it is a cryptocurrency. The "simple mint" type system also precedes blockchains and while it can be run on a blockchain, it can also run on a solo computer/machine.
But as we saw, we could perceive multiplayer Terminal Phase as technically, technically a blockchain, even though it has nothing to do with currencies whatsoever.
So again a blockchain is just a single, abstract, sequential machine, run by multiple parties. That's it. It's more general than cryptocurrencies, and it's not exclusive to implementing them either. One is a kind of programming-plus-cryptography-use-case (cryptocurrencies), the other one is a kind of abstracted machine (blockchains).
So please. They are frequently combined, but don't treat them as the same thing.
Blockchains as single abstract machines on a wider network
One of my favorite talks is Mark Miller's Programming Secure Smart Contracts talk. Admittedly, I like it partly because it well illustrates some of the low-level problems I've been working on, and that might not be as useful to everyone else. But it has this lovely diagram in it:
This is better understood by watching the video, but the abstraction layers described here are basically as follows:
"Machines" are the lowest layer of abstraction on the network, but there a variety of kinds of machines. Public blockchains are one, quorum blockchains are another, solo computer machines yet another (and the simplest case, too). What's interesting then is that we can see public chains and quorums abstractly demonstrated as machines in and of themselves... even though they are run by many parties.
Vats are the next layer of abstraction, these are basically the "communicating event loops"... actors/objects live inside them, and more or less these things run sequentially.
Finally, at the top are "erights" and "smart contracts", which feed into each other... "erights" are "exclusive electronic rights", and "smart contracts" are generally patterns of cooperation involving achieving mutual goals despite suspicion, generally involving the trading of these erights things (but not necessarily).
Okay, well cool! This finally explains the worldview I see blockchains on. And we can see a few curious things:
The "public chain" and "quorum" kinds of machines still boil down to a single, sequential abstract machine.
Object connections exist between the machines... ocap security. No matter whether it's run by a single computer or multiple.
Public blockchains, quorum blockchains, solo-computer machines all talk to each other, and communicate between object references on each other.
Blockchains are not magical things. They are abstracted machines on the network. Some of them have special rules that let whoever can prove they qualify for them access some well-known capabilities, but really they're just abstracted machines.
And here's an observation: you aren't ever going to move all computation to a single blockchain. Agoric's CEO, Dean Tribble, explained beautifully why on a recent podcast:
One of the problems with Ethereum is it is as tightly coupled as possible. The entire world is a single sequence of actions that runs on a computer with about the power of a cell phone. Now, that's obviously hugely valuable to be able to do commerce in a high-integrity fashion, even if you can only share a cell phone's worth of compute power with the entire rest of the world. But that's clearly gonna hit a brick wall. And we've done lots of large-scale distributed systems whether payments or cyberspace or coordination, and the fundamental model that covers all of those is islands of sequential programming in a sea of asynchronous communication. That is what the internet is about, that's what the interchain is about, that's what physics requires you to do if you want a system to scale.
Put this way, it should be obvious: are we going to replace the entire internet with something that has the power of a cell phone? To ask the question is to know the answer: of course not. Even when we do admit blockchain'y systems into our system, we're going to have to have many of them communicating with each other.
Blockchains are just machines that many people/agents run. That's it.
Some of these are encoded with some nice default programming to do some useful things, but all of them can be done in non-blockchain systems because communicating islands of sequential processes is the generalization. You might still want a blockchain, ie you might want multiple parties running one of those machines as a shared abstract machine, but how you configure that blockchain from there might depend on your trust and integrity requirements.
What do I think of blockchains?
I've covered a wide variety of perspectives of "what is a blockchain" in this article.
On the worse end of things are the parts involving hand-wavey confusion about decentralization, mistaken ideas of them being tied to cryptocurrencies, marketing hype, cultural assumptions, and some real, but not intrinsic, cultural problems.
In the middle, I am particularly keen on highlighting the similarity between the term "blockchain" and the term "roguelike", how both of them might boil down to some key ideas or not, but more importantly they're both a rough family of ideas that diverge from one highly influential source (Bitcoin and Rogue respectively). This is also the source of much of the "shouting past each other", because many people are referring to different components that they view as essential or inessential. Many of these pieces may be useful or harmful in isolation, in small amounts, in large amounts, but much of the arguing (and posturing) involves highlighting different things.
On the better end of things is a revelation, that blockchains are just another way of abstracting a computer so that multiple parties can run it. The particular decisions and use cases layered on top of this fundamental design are highly variant.
Having made the waters clear again, we could muddy them. A friend once tried to convince me that all computers are technically blockchains, that blockchains are the generalization of computing, and the case of a solo computer is merely one where a blockchain is run only by one party and no transaction history or old state is kept around. Maybe, but I don't think this is very useful. You can go in either direction, and I think the time travel and Terminal Phase section maybe makes that clear to me, but I'm not so sure how it lands with others I suppose. But a term tends to be useful in terms of what it introduces, and calling everything a blockchain seems to make the term even less useful than it already is. While a blockchain could be one or more parties running a sequential machine as the generalization, I suggest we stick to two or more.
Blockchains are not magic pixie dust, putting something on a blockchain does not make it work better or more decentralized... indeed, what a blockchain really does is converging (or re-centralizing) a machine from a decentralized set of computers. And it always does so with some cost, some set of overhead... but what those costs and overhead are varies depending on what the configuration decisions are. Those decisions should always stem from some careful thinking about what those trust and integrity needs are... one of the more frustrating things about blockchains being a technology of great hype and low understanding is that such care is much less common than it should be.
Having a blockchain, as a convergent machine, can be useful. But how that abstracted convergent machine is arranged can diverge dramatically; if we aren't talking about the same choices, we might shout past each other. Still, it may be an unfair ask to request that those without a deep technical background go into technical specifics, and I recognize that, and in a sense there can be some amount gained from speaking towards broad-sweeping, fuzzy sets and the patterns they seem to be carrying. A gut-sense assertion from a set of loosely observed behaviors can be a useful starting point. But to get at the root of what those gut senses actually map to, we will have to be specific, and we should encourage that specificity where we can (without being rude about it) and help others see those components as well.
But ultimately, as convergent machines, blockchains will not operate alone. I think the system that will hook them all together should be CapTP. But no matter the underlying protocol abstraction, blockchains are just abstract machines on the network.
Having finally disentangled what blockchains are, I think soon I would like to move onto what cryptocurrencies are. Knowing that they are not necessarily tied to blockchains opens us up to considering an ecosystem, even an interoperable and exchangeable one, of varying cryptographically based financial instruments, and the different roles and uses they might play. But that is another post of its own, for whenever I can get to it, I suppose.
ADDENDUM: After writing this post, I had several conversations with several blockchain-oriented people. Each of them roughly seemed to agree that Bitcoin was roughly the prototypical "blockchain", but each of them also seemed to highlight different things they thought were "essential" to what a "blockchain" is: some kinds of consensus algorithms being better than others, that kinds of social arrangements are enabled, whether transferrable assets are encoded on the chain, etc. To start with, I feel like this does confirm some of the premise of this post, that Bitcoin is the starting point, but like Rogue and "roguelikes", "blockchains" are an exploration space stemming from a particular influential technical piece.
However my friend Kate Sills (who also gave me a much better definition for "smart contracts", added above) highlighted something that I hadn't talked about much in my article so far, which I do agree deserves expansion. Kate said: "I do think there is something huge missing from your piece. Bitcoin is amazing because it aligns incentives among actors who otherwise have no goals in common."
I agree that there's something important here, and this definition of "blockchain" maybe does explain why while from a computer science perspective, perhaps signed git trees do resemble blockchains, they don't seem to fit within the realm of what most people are thinking about... while git might be a tool used by several people with aligned incentives, it is not generally itself the layer of incentive-alignment.