Part 1: Blockchain
Blockchain is the technology behind Bitcoin and other cybercurrencies.
That's about all anyone outside the software industry knows about it; that
and the fact that lots of people are claiming that it's going to transform
everything. (The financial industry, the Web, manufacturing supply chains,
identity, the music industry, ... the list goes on.) If you happen to be
in the software industry and have a moderately good idea of what
blockchain is, how it works, and what it can and can't do, you
may want to skip to Part 2.
Still with me? Here's the fifty-cent summary of blockchain. Blockchain
is a distributed, immutable ledger. Buzzword is a buzzword buzzword
buzzword? Blockchain is a chain of blocks? That's closer.
The purpose of a blockchain is to keep track of financial transactions
(that's the "ledger" part) and other data by making them public (that's
half of the "distributed" part), keeping them in blocks of data (that's
the "block" part) that can't be changed (that's the "immutable" part, and
it's a really good property for a ledger to have), are linked together by
hashes (that's the "chain" part, and we'll get to what hashes are in a
moment), with the integrity of that chain guaranteed by a large group of
people (that's the other half of the "distributed" part) called "miners"
Let's start in the middle: how can we link blocks of data together so
that they can't be changed? Let's start by making it so that any change
to a block, or to the order of those blocks, can be detected. Then, the
fact that everything is public makes the data impossible to change without
that change being glaringly obvious. We do that with hashes.
A hash function is something that takes a large block of data and turns it
into a very long sequence of bits (which we will sometimes refer to as a
"number", because any whole number can be represented by a sequence of
binary digits, and sometimes as a "hash", because the data has been
chopped up and mashed together like the corned beef hash you had for
breakfast). A good hash function has two important properties:
- It's irreversible. Starting with a hash, it is effectively impossible to
construct a block of data that will produce that hash. (It is
significantly easier to construct two blocks with the same hash, which
is why the security-conscious world moves to larger hashes from time to
- It's unpredictable. If two blocks of data differ anywhere, even by a
single bit, their hashes will be completely different.
Those two together mean that if two blocks have the same hash, they
contain the same data. If somebody sends you a block and a
hash, you can compare the hash of the block and if it matches, you can be
certain that the block hasn't been damaged or tampered with before it got
to you. And if they also cryptographically sign that hash, you
can be certain that they used the key that created that signature.
Now let's guarantee the integrity of the sequence of blocks by
chaining them together. Every block in the chain contains the hash of the
previous block. If block B follows block A in the chain, B's hash depends
in part on the hash of block A. If a villain tries to insert a forged
transaction into block A, its hash won't match the one in block B.
Now we get to the part that makes blockchain interesting: getting
everyone to agree on which transactions go into the next block. This is
done by publishing transactions where all of the miners can see
them. The miners then get to work with
shovels and pickaxes
, validating the transaction, putting it into
a block, and then running a contest to see which of them gets to add their
block to the chain and collect the associated reward. Winning the contest
requires doing a lot of computation. It's been estimated that
miners' computers collectively consume roughly the same amount of
electricity as Ireland.
There's more to it, but that's blockchain in a nutshell. I am
not going to say anything about what blockchain might be good for
besides keeping track of virtual money -- that's a whole other rabbit hole
that I'll save for another time. For now, the important thing is that
blockchain is a system for keeping track of financial transactions by
using a chain of blocks connected by hashes.
The need for miners to do work is what makes the virtual money they're mining
valuable, and makes it possible for everyone to agree on who owns how much
of it without anyone having to trust anyone else. It's all that work that
makes it possible to detect cheating. It also makes it expensive and
slow. The Ethereum blockchain can handle about ten transactions per
second. Visa handles about 10,000.
Part 2: The other blockchain
Meanwhile, in another part of cyberspace, software developers are using
another system based on hash chains to keep track of their software -- a
distributed version control system called
git. It's almost
completely different, except for the way it uses hashes. How different?
Well, for starters it's both free and fast, and you can use it at home.
And it has nothing to do with money -- it's a version control system.
If you've been with me for a while, you've probably figured out that I'm
extremely fond of git. This post is not an introduction to git
for non-programmers -- I'm working on that. However, if you managed to
get this far it does contain enough information to stand on its own,
Git doesn't use transactions and blocks; instead it uses "objects", but
just like blocks each object is identified by its hash. Instead of
keeping track of virtual money, it keeps track of files and their
histories. And just as blockchain keeps a complete history of everyone's
coins, git records the complete history of everyone's data.
Git uses several types of object, but the most fundamental one is called a
"blob", and consists of a file, its size, and the word "blob". For
example, here's how git idenifies one of my Songs for Saturday posts:
git hash-object 2019/01/05--s4s-welcome-to-acousticville.html
Everything you do with git starts with the
git command. In
this case we're using
git hash-object and giving it the
pathname of the file we want to hash. Hardly anyone needs to use the
hash-object subcommand; it's used mainly for testing and the
Git handles a directory (you may know directories as "folders" if
you aren't a programmer) by combining the names, metadata, and hashes of
all of its contents into a type of object called a "tree", and taking the
hash of the whole thing.
Here, by the way, is another place where git really differs from blockchain.
In a blockchain, all the effort of mining goes into making sure that every
block points to its one guaranteed-unique correct predecessor. In other
words, the blocks form a chain. Files and directories form a tree, with
the ordinary files as the leaves, and directories as branches. The
directory at the top is called the root. Top? Top. For some
reason software trees grow from the root down. After a while you get used
Actually, that's not quite accurate, because git stores each object in
exactly one place, and it's perfectly possible for the same file to be in
two different directories. This can be very useful -- if you
make a hundred copies of a file, git only has to store one of them. It's
also inaccurate because trees, called Merkle Trees are
used inside of blocks in a blockchain. But I digress.
Technically the hash links in both blockchains and git form a directed
acyclic graph -- that means that the links all point in one direction,
and there aren't any loops. In order to make a loop you'd have to predict
the hash of some later block, and you just can't do that. I have another post about why this is a good thing.
And that brings us to the things that make git, git: commits. ("Commit"
is used in the same sense, more or less, as it is in the phrase "commit
something to memory", or "commit to a plan of action". It has very little
to do with crime. Hashes are even more unique than fingerprints, and we
all know what criminals think about fingerprints. In cryptography, the
hash of a key is called its fingerprint.)
Anyway, when you're done making changes in a project, you type the command
... and git will make a new commit object which contains, among other
things, the time and date, your name and email address, maybe your
cryptographic signature, a brief description of what you did (git puts you
into your favorite text editor so you can enter this if you didn't put it
on the command line), the hash of the current root, and the hash of
the previous commit. Just like a blockchain.
Unlike earlier version control systems, git never has to compare files;
all it has to do is compare their hashes. This is fast -- git's
hashes are only 20 bytes long, no matter how big the files are or how many
are in a directory tree. And if the hashes of two trees are the
same, git doesn't have to look at any of the blobs in those trees to know
that they are all the same.
@ Blockchain 101 — only if you ‘know nothing’! – Hacker Noon
@ When do you need blockchain? Decision models. – Sebastien Meunier
@ Git - Git Objects
@ git ready » how git stores your data
@ Git/Internal structure - Wikibooks, open books for an open world
@ Why Singly-Linked Lists Win* | Stephen Savitzky
Another fine post from
The Computer Curmudgeon (also at