Re: About GIT Internals

From: Philip Oakley <philipoakley@iee.email>
Date: May 27, 2022, 14:40 UTC
Message-ID: <0201db28-d788-4458-e31d-c6cdedf5c9cf@iee.email>
In-Reply-To: <CACMKQb3exv13sYN5uEP_AG-JYu1rmVj4HDxjdw8_Y-+maJPwGg@mail.gmail.com>

Hi Aman, We try to keep all the cc's so every one can gain from the learning! comments in-line.

On 26/05/2022 15:17, Aman wrote:

> Hey Phillip.
>
> Thanks a lot for your email, and for sharing the book! This is great.

That was Eldon, thank you.. (https://lore.kernel.org/git/Yo68+kjAeP6tnduW@invalid/)

There is also the Git Magic 'book' from Stanford, with Ch8 covering the internals http://www-cs-students.stanford.edu/~blynn/gitmagic/ch08.html

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>
> Just a  follow up questions- if you don't mind:
>
> 1. I haven't had the experience of working with other (perhaps even
> older) version control systems, like subversion. So when refering to
> the "control" aspect,

The "control" aspect was from whoever was the 'manager' that limited access to the version system (i.e. acting like a museum curator), and deciding if your masterpiece was worthy of inclusion as a significant example of your craft, whether that was an engineering drawing or some software code.

>   you mean because with hashes we can verify

If you have a look at https://www.makeuk.org/insights/blogs/how-to-read-engineering-drawings-a-simple-guide and the part about the Title Block has a drawing (DWG) number (EEF-001-AM) that is used to reference it and, while it feels nice, the reference is rather arbitrary, (could someone else use that number? what's the next in the sequence? what happens when we reach EEF-999-AM? etc.).

So the computer hash (40 digits of 0-9a-f !) solves all those problems, it is unique (>40 card shuffle level), depends only on the content, computers like it. Yay. (Computers are great at perfect replication, so cost of manufacture tends to zero! Cost of design wanders in the other direction;-)

> the
> integrity of the files (like code) in git - there is no need for
> having a central authority to guarantee that's it's the right content
> files (which is great)?

And it means managers no longer worry about _your_ working copy - computers have digital storage space to spare. That wasn't the case when it was on paper, and we didn't have photocopies - have a look at 'blue prints' https://en.wikipedia.org/wiki/Blueprint (see the invention date!, I still remember the smell from the late 1970s)

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>
> On Thu, May 26, 2022 at 2:17 PM Philip Oakley <philipoakley@iee.email> wrote:
>> On 26/05/2022 00:34, git-vger@eldondev.com wrote:
>>> Hi Aman, responses inline below.
>>>
>>> On Wed, May 25, 2022 at 09:40:42PM +0530, Aman wrote:
>>>> Could someone please assist - in sharing some resources - which I
>>>> could go through, to better understand GIT software internals.
>>> There is an excellent free book at https://git-scm.com/book/en/v2 .
>>>
>>> Chapter 10 is about git internals. It is important to realize that,
>>> unlike many other version control systems, git works effectively on
>>> files locally on your computer, without any server or other shared
>>> resources to manage. Also, one good way to learn may be to form a
>>> question that you want to answer first. "How do I ...." or "what happens
>>> when I ....". Since git works locally, it is possible to create a git
>>> repo, look at the files contained in the .git directory, take action
>>> with git, and then look at the files again.
>>>
>>>
>> Another Git feature, compared to older version control systems, is that
>> it flips the 'control' aspect on its head. (who controls what you can
>> store?)
>>
>> It does this by using the hash (sha1, or sha256) values as a way of
>> users _checking_ that they have the right copy of a file or commit,
>> rather than needing special permissions to access (write/read) some
>> alleged 'master' copy (in the sense of a unique artefact) of the
>> particular version. Maintainers now check and authorise particular
>> versions much more easily.
>>
>> Hence Git _Distributes Control_ - you no longer need permission to keep
>> versioned copies of your work. This was, in my mind, a core element of
>> its success.
>>
>> There is other stuff about how Git splits the (file) content from it's
>> meta-data, so if say 10 files contain the same licence text, then it
>> only hold one copy of that text, with its own unique hash. Then has a
>> hierarchy (pyramid) of hashes of the meta-data to build up a whole
>> project's hash (the top level 'tree'), and the same hierarchy technique
>> is repeated for the project's history of commits.
>>
>> If you have a copy of the repository with the latest (same) hash then
>> you have a perfect copy, indistinguishable to the 'original'! Older
>> versioning systems did not have those guarantees, many were derived from
>> systems for versioning engineering and architectural drawings such as
>> those that were used for the RMS Titanic or Empire State Building.
>>
>> Philip
>>
>> PS it's worth checking out the distinction between having hash (a magic
>> id) of some text, and encrypting (a magic translation of) some text.
>>
>>