Can we consider git commits as diffs, snapshots, and/or histories?

2024-01-05 22:50:09

Hey! I’ve been extraordinarily slowly attempting to determine tips on how to clarify each core
idea in Git (commits! branches! remotes! the staging space!) and commits have
been surprisingly tough.

Understanding how git commits are applied feels fairly easy to
me (these are details! I can look it up!), nevertheless it’s been a lot tougher to determine
out how different individuals take into consideration commits. So like I’ve been doing loads
just lately, I went on Mastodon and began asking some questions.

how do individuals take into consideration Git commits?

I did a highly unscientific poll on Mastodon about how individuals take into consideration Git
commits: is it a snapshot? is it a diff? is it an inventory of each earlier commit?
(In fact it’s authentic to consider it as all three, however I used to be curious
concerning the main manner individuals take into consideration Git commits). Right here it’s:

The outcomes had been:

51% diff
42% snapshot
4% historical past of each earlier commit
3% “different”

I used to be actually stunned that it was so evenly break up between diffs and snapshots.
Individuals additionally made some attention-grabbing type of contradictory statements like “in my
thoughts a commit is a diff, however I feel it’s really applied as a snapshot”
and “in my thoughts a commit is a snapshot, however I feel it’s really applied
as a diff”. We’ll discuss extra about how a commit is definitely applied later in
the submit.

Earlier than we go any additional: once we say “a diff” or “a snapshot”, what does that
imply?

what’s a diff?

What I imply by a diff might be apparent: it’s what you get while you run git present COMMIT_ID. For instance right here’s a typo repair from rbspy:

diff --git a/src/ui/abstract.rs b/src/ui/abstract.rs
index 5c4ff9c..3ce9b3b 100644
--- a/src/ui/abstract.rs
+++ b/src/ui/abstract.rs
@@ -160,7 +160,7 @@ mod assessments {
 ";

         let mut buf: Vec<u8> = Vec::new();
-        stats.write(&mut buf).anticipate("Callgrind write failed");
+        stats.write(&mut buf).anticipate("abstract write failed");
         let precise = String::from_utf8(buf).anticipate("abstract output not utf8");
         assert_eq!(precise, anticipated, "Surprising abstract output");
     }

You possibly can see it on GitHub right here: https://github.com/rbspy/rbspy/commit/24ad81d2439f9e63dd91cc1126ca1bb5d3a4da5b

what’s a snapshot?

After I say “a snapshot”, what I imply is “all of the information that you simply get while you
run git checkout COMMIT_ID”.

Git usually calls the checklist of information for a commit a “tree” (as in “listing
tree”), and you may see the entire information for the above instance commit right here on
GitHub:

https://github.com/rbspy/rbspy/tree/24ad81d2439f9e63dd91cc1126ca1bb5d3a4da5b (it’s /tree/ as an alternative of /commit/)

is “how Git implements it” actually the correct technique to clarify it?

In all probability the most typical piece of recommendation I hear associated to studying Git is
“simply learn the way Git represents issues internally, and all the pieces will make
sense”. I clearly discover this angle extraordinarily interesting (when you’ve spent
any time studying this weblog, I love desirous about how issues are
applied internally).

However as a method for instructing Git, it hasn’t been as profitable as I’d hoped!
Usually I’ve eagerly began explaining “okay, so git commits are snapshots with
a pointer to their mother or father, after which a department is a pointer to a commit, and…“,
however the particular person I’m attempting to assist will inform me that they didn’t actually discover
that clarification that helpful in any respect and so they nonetheless don’t get it. So I’ve been
contemplating different choices.

Let’s discuss concerning the internals a bit anyway although.

how git represents commits internally: snapshots

Internally, git represents commits as snapshots (it shops the “tree” of the
present model of each file). I wrote about this in In a git repository, where do your files live?,
however right here’s a really fast abstract of what the interior format seems like.

Right here’s how a commit is represented:

$ git cat-file -p 24ad81d2439f9e63dd91cc1126ca1bb5d3a4da5b
tree e197a79bef523842c91ee06fa19a51446975ec35
mother or father 26707359cdf0c2db66eb1216bf7ff00eac782f65
writer Adam Jensen <adam@acj.sh> 1672104452 -0500
committer Adam Jensen <adam@acj.sh> 1672104890 -0500

Repair typo in expectation message

and right here’s what we get once we take a look at this tree object: an inventory of each file /
subdirectory within the repository’s root listing as of that commit:

$ git cat-file -p e197a79bef523842c91ee06fa19a51446975ec35
040000 tree 2fcc102acd27df8f24ddc3867b6756ac554b33ef	.cargo
040000 tree 7714769e97c483edb052ea14e7500735c04713eb	.github
100644 blob ebb410eb8266a8d6fbde8a9ffaf5db54a5fc979a	.gitignore
100644 blob fa1edfb73ce93054fe32d4eb35a5c4bee68c5bf5	ARCHITECTURE.md
100644 blob 9c1883ee31f4fa8b6546a7226754cfc84ada5726	CODE_OF_CONDUCT.md
100644 blob 9fac1017cb65883554f821914fac3fb713008a34	CONTRIBUTORS.md
100644 blob b009175dbcbc186fb8066344c0e899c3104f43e5	Cargo.lock
100644 blob 94b87cd2940697288e4f18530c5933f3110b405b	Cargo.toml

What this implies is that testing a Git commit is at all times quick: it’s simply as
simple for Git to take a look at a commit from yesterday as it’s to take a look at a
commit from 1 million commits in the past. Git by no means has to replay 10000 diffs to
determine the present state or something, as a result of commits simply aren’t saved as
diffs.

snapshots are compressed utilizing packfiles

I simply mentioned that Git commits are snapshots, however when somebody says “I consider
git commits as a snapshot, however I feel internally they’re really diffs”,
that’s really type of true too! Git commits usually are not represented as diffs in
the sense you’re most likely used to (they’re not represented on disk as a diff
from the earlier commit), however the fundamental instinct that when you’re modifying a
10,000 strains 500 instances, it could be inefficient to retailer 500 copies of that
file is correct.

Git does have a manner of storing information as variations from different methods. That is
referred to as “packfiles” and periodically git will do a rubbish assortment and
compress your knowledge into packfiles to avoid wasting disk area. Once you git clone a
repository git will even compress the information.

I don’t have area for a full clarification of how packfiles work on this submit
(Aditya Mukerjee’s Unpacking Git packfiles
is my favorite writeup of how they work). However right here’s a fast abstract of my
understanding of how deltas work and the way they’re totally different from diffs:

Objects are saved as a reference to an “unique file”, plus a “delta”
the delta has a bunch of directions like “learn bytes 0 to 100, then insert bytes ‘howdy there’, then learn bytes 120 to 200”. It cobbles collectively bytes from the unique plus new textual content. So there’s no notion of “deletions”, simply copies and additions.
There’s only one layer of deltas: git solely has take a look at 2 issues to decompress a file, the unique and the delta. It doesn’t want to have a look at 100 diffs or something.
The “unique file” isn’t essentially from the earlier commit, it might be something. Possibly it might even be from a later commit? I’m undecided about that.
There’s no “proper” algorithm for tips on how to compute deltas, Git simply has some approximate heuristics

what really occurs while you do a diff is type of bizarre

After I run git present SOME_COMMIT to have a look at the diff for a commit, what
really occurs is type of counterintuitive. My understanding is:

git seems within the packfiles and applies deltas to reconstruct the tree for that commit and for its mother or father.
git diffs the 2 listing bushes (the present commit’s tree, and the mother or father commit’s tree). Normally that is fairly quick as a result of virtually all of
the information are precisely the identical, so git can simply evaluate the hashes of the similar information and do nothing virtually the entire time.
lastly git exhibits me the diff

So it takes deltas, turns them right into a snapshot, after which calculates a diff. It
feels slightly bizarre as a result of it begins with a diff-like-thing and finally ends up with
one other diff-like-thing, however the deltas and diffs are literally completely
totally different so it is sensible.

That mentioned, the best way I consider it’s that git shops commits as snapshots and
packfiles are simply an implementation element to avoid wasting disk area and make clones
quicker. I’ve by no means really wanted to know the way packfiles work for any sensible
motive, nevertheless it does assist me perceive the way it’s doable for git commits to
be snapshots with out utilizing manner an excessive amount of disk area.

a “fallacious” psychological mannequin for git: commits are diffs

I feel a reasonably widespread “fallacious” psychological mannequin for Git is:

commits are saved as diffs from the earlier commit (plus a pointer to the mother or father commit(s) and an writer and message).
to get the present state for a commit, Git begins at first and
replays all of the earlier commits

This mannequin is clearly not true (in actual life, commits are saved as
snapshots, and diffs are calculated from these snapshots), nevertheless it appears very
helpful and coherent to me! It will get slightly bizarre with merge commits, however perhaps
you simply say it’s saved as a diff from the primary mother or father of the merge.

I feel fallacious psychological fashions are sometimes extraordinarily helpful, and this one doesn’t
appear very problematic to me for every single day Git utilization. I actually like that it
makes the factor that we take care of essentially the most usually (the diff) essentially the most
basic – it appears actually intuitive to me.

I’ve additionally been desirous about different “fallacious” psychological fashions you may have about
Git which appear fairly helpful like:

commit messages might be edited (they’ll’t actually, really you make a replica of the commit with a brand new message, and the outdated commit continues to exist)
commits might be moved to have a distinct base (equally, they’re copied)

I really feel like there’s an entire very coherent “fallacious” set of concepts you may have
about git which might be fairly properly supported by Git’s UI and never very problematic
more often than not. I feel it will probably get messy while you wish to undo a change or
when one thing goes fallacious although.

some benefits of “commit as diff”

Personally although I do know that in Git commits are snapshots, I most likely consider them as diffs more often than not, as a result of:

more often than not I’m involved with the change I’m making – if I’m simply
altering 1 line of code, clearly I’m principally desirous about simply that 1 line
of code and never your complete present state of the codebase
while you click on on a Git commit on GitHub or use git present, you see the diff, so it’s simply what I’m used to seeing
I exploit rebase loads, which is all about replaying diffs

some benefits of “commit as snapshot”

I additionally take into consideration commits as snapshots generally although, as a result of:

git usually will get confused about file strikes: generally if I transfer a file and edit
it, Git can’t acknowledge that it was moved and as an alternative will present it as
“deleted outdated.py, added new.py”. It’s because git solely shops snapshots, so
when it says “moved outdated.py -> new.py”, it’s simply guessing as a result of the
contents of outdated.py and new.py are comparable.
it’s conceptually a lot simpler to consider what git checkout COMMIT_ID is doing (the thought of replaying 10000 commits simply feels irritating to me)
merge commits type of make extra sense to me as snapshots, as a result of the merged
commit can really be actually something (it’s only a new snapshot!). It
helps me perceive why you can also make arbitrary modifications while you’re resolving
a merge battle, and why it’s so vital to watch out about battle
decision.

another methods to consider commits

Some people within the Mastodon replies additionally talked about:

“further” out-of-band details about the commit, like an electronic mail or a GitHub pull request or only a dialog you had with a coworker
desirous about a diff as a “earlier than state + after state”
and naturally, that a lot of individuals consider commits in a lot of other ways relying on the state of affairs

that’s all for now!

It’s been very troublesome for me to get a way of what totally different psychological fashions
individuals have for git. It’s particularly tough as a result of individuals get actually into
policing “fallacious” psychological fashions although these “fallacious” fashions are sometimes
actually helpful, so people are reluctant to share their “fallacious” concepts for worry of
some Git Explainer popping out of the woodwork to elucidate to them why they’re
Fallacious. (these Git Explainers are sometimes well-intentioned, nevertheless it nonetheless has a chilling impact both manner)

However I’ve been studying loads! I nonetheless don’t really feel completely clear about how I wish to
speak about commits, however we’ll get there ultimately.

Because of Marco Rogers, Marie Flanagan, and everybody on Mastodon for speaking to
me about git commits.

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