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[翻译完成][tech]: 20240105 Do we think of git commits as diffs, snapshots, and-or histories

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[#]: url: " "
Do we think of git commits as diffs, snapshots, and/or histories?
我们认为 git 提交是差异、快照还是历史记录？
======
Hello! I’ve been extremely slowly trying to figure how to explain every core concept in Git (commits! branches! remotes! the staging area!) and commits have been surprisingly tricky.
大家好！我一直在相当缓慢地尝试弄清楚如何解释 git 中的各个核心理念（提交！分支！远程！暂存区！），而提交却出奇地棘手。
Understanding how git commits are implemented feels pretty straightforward to me (those are facts! I can look it up!), but it’s been much harder to figure out how other people think about commits. So like I’ve been doing a lot recently, I went on Mastodon and started asking some questions.
要明白 git 提交是如何实现的对我来说相当简单（这些都是确定的！我可以直接查看！），但是要弄清楚别人是怎么看待提交的却相当困难。所以，就像我最近一直在做的那样，我上了 Mastodon 并提了一些问题。
### how do people think about Git commits?
### 大家是怎么看待 Git 提交的？
I did a [highly unscientific poll][1] on Mastodon about how people think about Git commits: is it a snapshot? is it a diff? is it a list of every previous commit? (Of course it’s legitimate to think about it as all three, but I was curious about the _primary_ way people think about Git commits). Here it is:
我进行了一个 [非常不科学的调查][1]，询问大家是怎么看待 Git 提交的：是快照？是差异？还是所有之前提交的列表？（当然，把它看作这三者都是合理的，但我很好奇人们的 _主要_ 观点）。这是调查结果：
![][2]
The results were:
结果是：
* 51% diff
* 42% snapshot
* 4% history of every previous commit
* 3% “other”
* 51% 差异
* 42% 快照
* 4% 所有之前的提交的历史记录
* 3% “其他”
我很惊讶差异和快照两个选项的比例如此接近。人们还提出了一些有趣但相互矛盾的观点，比如 “在我看来，提交是一个差异，但我认为它在实现上是一个快照” 和 “在我看来，提交是一个快照，但我认为它在实现上是一个差异”。我们稍后会讨论一下提交的实际实现方式。
I was really surprised that it was so evenly split between diffs and snapshots. People also made some interesting kind of contradictory statements like “in my mind a commit is a diff, but I think it’s actually implemented as a snapshot” and “in my mind a commit is a snapshot, but I think it’s actually implemented as a diff”. We’ll talk more about how a commit is actually implemented later in the post.
在我们进一步讨论之前：我们的说 “一个差异” 或 “一个快照” 都是什么意思？
Before we go any further: when we say “a diff” or “a snapshot”, what does that mean?
### 什么是差异？
### what’s a diff?
What I mean by a diff is probably obvious: it’s what you get when you run `git show COMMIT_ID`. For example here’s a typo fix from rbspy:
我想说的差异可能相当明显：差异就是你在运行 `git show COMMIT_ID` 时得到的东西。例如，这是一个 rbspy 项目中的拼写错误修复：
```
diff --git a/src/ui/summary.rs b/src/ui/summary.rs
index 5c4ff9c..3ce9b3b 100644
--- a/src/ui/summary.rs
@ -52,35 +50,33 @@ What I mean by a diff is probably obvious: it’s what you get when you run `git
let actual = String::from_utf8(buf).expect("summary output not utf8");
assert_eq!(actual, expected, "Unexpected summary output");
}
```
You can see it on GitHub here: <https://github.com/rbspy/rbspy/commit/24ad81d2439f9e63dd91cc1126ca1bb5d3a4da5b>
你可以在 GitHub 上看到它： <https://github.com/rbspy/rbspy/commit/24ad81d2439f9e63dd91cc1126ca1bb5d3a4da5b>
### what’s a snapshot?
### 什么是快照？
When I say “a snapshot”, what I mean is “all the files that you get when you run `git checkout COMMIT_ID`”.
当我说 “一个快照” 时，我的意思是 “当你运行 `git checkout COMMIT_ID` 时得到的所有文件”。
Git often calls the list of files for a commit a “tree” (as in “directory tree”), and you can see all of the files for the above example commit here on GitHub:
Git 通常将提交的文件列表称为 “树”，你可以在 GitHub 上看到上述提交的所有文件：
<https://github.com/rbspy/rbspy/tree/24ad81d2439f9e63dd91cc1126ca1bb5d3a4da5b> (it’s `/tree/` instead of `/commit/`)
<https://github.com/rbspy/rbspy/tree/24ad81d2439f9e63dd91cc1126ca1bb5d3a4da5b> (它是 `/tree/` 而不是 `/commit/`)
### is “how Git implements it” really the right way to explain it?
### Git 实现提交的方式真的是正确的解释方式吗？
Probably the most common piece of advice I hear related to learning Git is “just learn how Git represents things internally, and everything will make sense”. I obviously find this perspective extremely appealing (if you’ve spent any time reading this blog, you know I _love_ thinking about how things are implemented internally).
我最常听到的关于学习 Git 的建议大概是 “只要学会 Git 在内部是如何表示事物的，一切都会变得清晰明了”。我显然非常喜欢这种观点（如果你花了一些时间阅读这个博客，你就会知道我 _喜欢_ 思考事物在内部是如何实现的）。
But as a strategy for teaching Git, it hasn’t been as successful as I’d hoped! Often I’ve eagerly started explaining “okay, so git commits are snapshots with a pointer to their parent, and then a branch is a pointer to a commit, and…“, but the person I’m trying to help will tell me that they didn’t really find that explanation that useful at all and they still don’t get it. So I’ve been considering other options.
但是作为一个学习 Git 的方法，它并没有我希望的那么成功！通常我会兴奋地开始解释 “好的，所以 git 提交是一个快照，它有一个指向它的父提交的指针，然后一个分支是一个指向提交的指针，然后...”，但是我试图帮助的人会告诉我，他们并没有真正发现这个解释有多有用，他们仍然不明白。所以我一直在考虑其他的选择。
Let’s talk about the internals a bit anyway though.
但是让我们还是先谈谈内部实现吧。
### how git represents commits internally: snapshots
### Git 是如何在内部表示提交的：快照
Internally, git represents commits as snapshots (it stores the “tree” of the current version of every file). I wrote about this in [In a git repository, where do your files live?][3], but here’s a very quick summary of what the internal format looks like.
在内部，git 将提交表示为快照（它存储每个文件当前版本的 “树”）。我在 [在一个 git 仓库中，你的文件在哪里？][3] 中写过这个，但下面是一个非常快速的内部格式概述。
Here’s how a commit is represented:
这是一个提交的表示方式：
```
$ git cat-file -p 24ad81d2439f9e63dd91cc1126ca1bb5d3a4da5b
tree e197a79bef523842c91ee06fa19a51446975ec35
parent 26707359cdf0c2db66eb1216bf7ff00eac782f65
@ -88,13 +84,11 @@ Here’s how a commit is represented:
committer Adam Jensen <adam@acj.sh> 1672104890 -0500
Fix typo in expectation message
```
and here’s what we get when we look at this tree object: a list of every file / subdirectory in the repository’s root directory as of that commit:
以及，当我们查看这个树对象时，我们会看到这个提交中仓库根目录下每个文件/子目录的列表：
```
$ git cat-file -p e197a79bef523842c91ee06fa19a51446975ec35
040000 tree 2fcc102acd27df8f24ddc3867b6756ac554b33ef .cargo
040000 tree 7714769e97c483edb052ea14e7500735c04713eb .github
@ -104,97 +98,82 @@ and here’s what we get when we look at this tree object: a list of every file
100644 blob 9fac1017cb65883554f821914fac3fb713008a34 CONTRIBUTORS.md
100644 blob b009175dbcbc186fb8066344c0e899c3104f43e5 Cargo.lock
100644 blob 94b87cd2940697288e4f18530c5933f3110b405b Cargo.toml
```
What this means is that checking out a Git commit is always fast: it’s just as easy for Git to check out a commit from yesterday as it is to check out a commit from 1 million commits ago. Git never has to replay 10000 diffs to figure out the current state or anything, because commits just aren’t stored as diffs.
这意味着检出一个 Git 提交总是很快的：对 Git 来说，检出昨天的提交和检出 100 万个提交之前的提交一样容易。Git 永远不需要重新应用 10000 个差异来确定当前状态，因为提交根本就不是以差异的形式存储的。
### snapshots are compressed using packfiles
### 快照使用 packfiles 进行压缩
I just said that Git commits are snapshots, but when someone says “I think of git commits as a snapshot, but I think internally they’re actually diffs”, that’s actually kind of true too! Git commits are not represented as diffs in the sense you’re probably used to (they’re not represented on disk as a diff from the previous commit), but the basic intuition that if you’re editing a 10,000 lines 500 times, it would be inefficient to store 500 copies of that file is right.
我刚刚提到了 Git 提交是一个快照，但是，当有人说 “在我看来，提交是一个快照，但我认为它在实现上是一个差异” 时，这实际上也是有点道理的！Git 提交并不是以你可能习惯的差异的形式表示的（它们不是以与上一个提交的差异的形式存储在磁盘上的），但是，如果你编辑 500 次同样的 10000 行代码，如果要存储 500 个文件副本，那将是非常低效的，这个基本的直觉是正确的。
Git does have a way of storing files as differences from other ways. This is called “packfiles” and periodically git will do a garbage collection and compress your data into packfiles to save disk space. When you `git clone` a repository git will also compress the data.
Git 有一个将文件以差异的形式存储的方法。这被称为 “packfiles”，git 会定期进行垃圾回收，将你的数据压缩成 packfiles 以节省磁盘空间。当你 `git clone` 一个仓库时，git 也会压缩数据。
I don’t have space for a full explanation of how packfiles work in this post (Aditya Mukerjee’s [Unpacking Git packfiles][4] is my favourite writeup of how they work). But here’s a quick summary of my understanding of how deltas work and how they’re different from diffs:
我这里没有足够的空间来完整地解释 packfiles 是如何工作的（Aditya Mukerjee 的 [解压 Git packfiles][4] 是我最喜欢的解释它们是如何工作的文章）。但是这里是我对 delta 是如何工作的以及它们与差异的不同之处的简要总结：
* Objects are stored as a reference to an “original file”, plus a “delta”
* the delta has a bunch of instructions like “read bytes 0 to 100, then insert bytes ‘hello there’, then read bytes 120 to 200”. It cobbles together bytes from the original plus new text. So there’s no notion of “deletions”, just copies and additions.
* There’s only 1 layer of deltas: git only has look at 2 things to decompress a file, the original and the delta. It doesn’t need to look at 100 diffs or anything.
* The “original file” isn’t necessarily from the previous commit, it could be anything. Maybe it could even be from a later commit? I’m not sure about that.
* There’s no “right” algorithm for how to compute deltas, Git just has some approximate heuristics
* 对象是以对 “原始文件” 的引用加上一个 “delta” 存储的
* delta 是一系列例如 “读取第 0 到 100 字节，然后插入字节 ‘hello there’，然后读取第 120 到 200 字节” 的指令。它从原始文件中拼凑出新的文本。所以没有 “删除” 的概念，只有复制和添加。
* delta 只会有 1 层：git 只需要查看 2 个东西来解压缩一个文件，原始文件和 delta。它不需要查看 100 个差异或任何其他东西。
* ”原始文件” 不一定来自上一个提交，它可以是任何东西。也许它甚至可以来自一个更晚的提交？我不确定。
* 没有一个 “正确的” 算法来计算 delta，Git 只是有一些近似的启发式算法
### 当你查看差异时，实际上发生了一些奇怪的事情
当我们运行 `git show SOME_COMMIT` 来查看提交的差异时，实际上发生的事情有点反直觉。我的理解是：
### what actually happens when you do a diff is kind of weird
1. git 会在 packfiles 中查找并应用 delta 来重建该提交和其父提交的树。
2. git 会对两个目录树进行差异比较。通常这很快，因为几乎所有的文件都是完全相同的，所以 git 大部分时间都只需要比较同一个文件的哈希值。
3. 最后 git 会展示差异
When I run `git show SOME_COMMIT` to look at the diff for a commit, what actually happens is kind of counterintuitive. My understanding is:
所以，git 会将 delta 转换为快照，然后计算差异。它感觉有点奇怪，因为它从一个类似差异的东西开始，最终得到另一个类似差异的东西，但是 delta 和差异实际上是完全不同的，所以这是说得通的。
1. git looks in the packfiles and applies deltas to reconstruct the tree for that commit and for its parent.
2. git diffs the two directory trees. Usually this is pretty fast because almost all of the files are exactly the same, so git can just compare the hashes of the identical files and do nothing almost all of the time.
3. finally git shows me the diff
### 一个 “错误的” git 的理解：提交是差异
我认为一个相当常见的，对 git 的 “错误” 的理解是：
* 提交是以基于上一个提交的差异的形式存储的（加上指向父提交的指针和作者和消息）。
* 要获取提交的当前状态，Git 需要从头开始重新应用所有之前的提交。
So it takes deltas, turns them into a snapshot, and then calculates a diff. It feels a little weird because it starts with a diff-like-thing and ends up with another diff-like-thing, but the deltas and diffs are actually totally different so it makes sense.
这个理解当然是错误的（在现实中，提交是以快照的形式存储的，差异是从这些快照计算出来的），但是对我来说它似乎非常有用而且有意义！在考虑合并提交时会有一点奇怪，但是或许我们可以说这只是基于合并提交的第一个父提交的差异。
### a “wrong” mental model for git: commits are diffs
我认为这个错误的理解有的时候非常有用，而且对于日常 Git 使用来说它似乎并没有什么问题。我真的很喜欢它将我们最常使用的东西（差异）作为最基本的元素——它对我来说非常直观。
I think a pretty common “wrong” mental model for Git is:
我也一直在思考一些其他有用但 “错误” 的 Git 理解，比如：
* commits are stored as diffs from the previous commit (plus a pointer to the parent commit(s) and an author and message).
* to get the current state for a commit, Git starts at the beginning and replays all the previous commits
* 提交信息可以被编辑（实际上不是这样的，你只是复制了一个相同的提交然后给了它一个新的信息，旧的提交仍然存在）
* 提交可以被移动到一个不同的基础上（类似地，它们被复制了）
我认为有一系列非常有意义的、 “错误” 的对 Git 的理解，它们在很大程度上都受到 Git 用户界面的支持，并且在大多数情况下都不会产生什么问题。但是当你想要撤销一个更改或者出现问题时，它可能会变得混乱。
### 将提交视为差异的一些优势
This model is obviously not **true** (in real life, commits are stored as snapshots, and diffs are calculated from those snapshots), but it seems very useful and coherent to me! It gets a little weird with merge commits, but maybe you just say it’s stored as a diff from the first parent of the merge.
就算我知道在 Git 中提交是快照，我可能大部分时间也都将它们视为差异，因为：
I think wrong mental models are often extremely useful, and this one doesn’t seem very problematic to me for every day Git usage. I really like that it makes the thing that we deal with the most often (the diff) the most fundamental – it seems really intuitive to me.
* 大多时候我都在关注我正在做的 **更改** —— 如果我只是改变了一行代码，显然我主要是在考虑那一行代码而不是整个代码库的当前状态
* 当你选择一个 Git 提交或者使用 `git show` 时，你会看到差异，所以这只是我习惯看到的东西
* 我经常使用变基，它就是关于重新应用差异的
I’ve also been thinking about other “wrong” mental models you can have about Git which seem pretty useful like:
### 将提交视为快照的一些优势
* commit messages can be edited (they can’t really, actually you make a copy of the commit with a new message, and the old commit continues to exist)
* commits can be moved to have a different base (similarly, they’re copied)
但是我有时也会将提交视为快照，因为：
* 这种方式更容易理解 `git checkout COMMIT_ID` 在做什么（重新应用 10000 个提交的想法让我感到很有压力）
* 合并提交在我看来更像是快照，因为合并提交实际上可以是任何东西（它只是一个新的快照！）。它帮助我理解为什么在解决合并冲突时可以进行任意更改，以及为什么在解决冲突时要小心。
### 其他一些关于提交的理解
I feel like there’s a whole very coherent “wrong” set of ideas you can have about git that are pretty well supported by Git’s UI and not very problematic most of the time. I think it can get messy when you want to undo a change or when something goes wrong though.
Mastodon 的一些回复中还提到了：
### some advantages of “commit as diff”
* “额外的” 关于提交的信息，比如电子邮件、GitHub 拉取请求或者你和同事的对话
* 将“差异”视为一个“之前的状态 + 之后的状态”
* 以及，当然，很多人根据情况的不同以不同的方式看待提交
Personally even though I know that in Git commits are snapshots, I probably think of them as diffs most of the time, because:
### 目前就是这些了！
* most of the time I’m concerned with the **change** I’m making – if I’m just changing 1 line of code, obviously I’m mostly thinking about just that 1 line of code and not the entire current state of the codebase
* when you click on a Git commit on GitHub or use `git show`, you see the diff, so it’s just what I’m used to seeing
* I use rebase a lot, which is all about replaying diffs
对我来说，弄清楚人们对 git 的不同理解是非常困难的。人们会非常认真的去监管 “错误的” 理解，即使这些 “错误的” 理解通常非常有用，人们都不愿意分享他们的 “错误” 想法，因为他们害怕一些 Git 解释者会突然蹦出来向他们解释他们为什么是错误的。（这些 Git 解释者通常是出于善意的，但是无论如何它都会产生一种负面影响）这也使得弄清楚这些想法非常困难。
但是我学到了很多！我仍然不完全清楚我想如何谈论提交，但是我们最终会弄清楚的。
### some advantages of “commit as snapshot”
I also think about commits as snapshots sometimes though, because:
* it’s conceptually much easier to think about what `git checkout COMMIT_ID` is doing (the idea of replaying 10000 commits just feels stressful to me)
* merge commits kind of make more sense to me as snapshots, because the merged commit can actually be literally anything (it’s just a new snapshot!). It helps me understand why you can make arbitrary changes when you’re resolving a merge conflict, and why it’s so important to be careful about conflict resolution.
### some other ways to think about commits
Some folks in the Mastodon replies also mentioned:
* “extra” out-of-band information about the commit, like an email or a GitHub pull request or just a conversation you had with a coworker
* thinking about a diff as a “before state + after state”
* and of course, that lots of people think of commits in lots of different ways depending on the situation
### that’s all for now!
It’s been very difficult for me to get a sense of what different mental models people have for git. It’s especially tricky because people get really into policing “wrong” mental models even though those “wrong” models are often really useful, so folks are reluctant to share their “wrong” ideas for fear of some Git Explainer coming out of the woodwork to explain to them why they’re Wrong. (these Git Explainers are often well-intentioned, but it still has a chilling effect either way)
But I’ve been learning a lot! I still don’t feel totally clear about how I want to talk about commits, but we’ll get there eventually.
Thanks to Marco Rogers, Marie Flanagan, and everyone on Mastodon for talking to me about git commits.
感谢 Marco Rogers、Marie Flanagan 以及 Mastodon 上的所有人和我讨论 git 提交。
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