Git (1) -- data model

since 2005 Since its birth in ,git Already popular in the open source world , Many of us also use it in our jobs . It's a great VCS Tools , It has many advantages , But easy to learn is not one of them . about git If you just memorize commands by rote, it won't be long before you forget , And recite it over and over again , No doubt it's very frustrating , in my opinion , Familiar use git The only way to even start to like it is to understand how it works inside .

git Commands are just abstractions of data storage , If you don't understand git How it works , No matter how much we remember or store in our notes git Command or skill we will still be dealing with git Confused about the use of . and git It uses abstract commands to expose the usage of its data structure .

So we should pay more attention to this article git The internal relationship of - Data model , Of course, this article will not deal with git Source code .

preparation

Initialize the repository

To explain the data model , We first need to initialize an empty... In our working directory git Warehouse

git init

git We will be informed that we have created one in the current directory .git Catalog , Let's take a look at this .git What does it look like .

$ tree .git/
.git
|-- HEAD
|-- config
|-- description
|-- hooks
|   |-- applypatch-msg.sample
|   |-- commit-msg.sample
|   |-- fsmonitor-watchman.sample
|   |-- post-update.sample
|   |-- pre-applypatch.sample
|   |-- pre-commit.sample
|   |-- pre-push.sample
|   |-- pre-rebase.sample
|   |-- pre-receive.sample
|   |-- prepare-commit-msg.sample
|   |-- update.sample
|-- info
|   |-- exclude
|-- objects
|   |-- info
|   |-- pack
|-- refs
    |-- heads
    |-- tags

8 directories, 15 files

Some of the files and directories look familiar , Now we mainly look at objects This directory , Now it's empty , But in a moment we'll change it .

To submit documents

First let's create one Main.java file

touch Main.java

And then type in part of the content

public class Main {
    public static void main(String[] args) {
        System.out.println("Hello World");
    }
}

And then in the same way prepare a README.md file

touch README.md

Enter the following into the file

this is my first java project!

Now? add also commit They went to the warehouse

git add .
git commit -m 'Initial Commit'

Model creation

Now it doesn't look special , Now let's go back and look at .git/objects There are already some subfolders and files in the directory

.git/objects
|-- 84
|   -- 705622ee44f2afbb21087ca7d81fda01fccded
|-- 95
|   -- fc1236534b6f73930367f02895467040f47d4a
|-- b0
|   -- 81e51f448387e72a3e3551ba8610eedc172e60
|-- f1
|   -- a8b89f50a2fd8287578daa2b0374adf3cad8aa
|-- info
|-- pack
6 directories, 4 files

It should be noted that the directory and file names on your computer are different from mine .

blob object The creation of

stay .git/objects We notice that the name of each directory is only 2 Character length ,Git Generate one for each object 40 Check sum of characters （SHA-1） Hash , The first two characters of the check sum are used as the directory name , in addition 38 Characters for file （ object ） name .
When we submit some documents ,git The first type of object created is blob object, In our case, there are two , every last blob object For every document we submit :

blob object Contains a snapshot of the file and has file checksums .

tree object The creation of

git Another kind of object created is tree object, In our case, there is only one , It contains a list of all the files in our project , It contains the... Assigned to them blob object The pointer to ( This is it. git How to combine documents with blob object Related to )

commit object The creation of

Last git Also created a commit object, The object has tree object The pointer to ( And some other information )

)

This time I'm looking at the following objects The structure of the directory is much clearer

.git/objects
|-- 84
|   -- 705622ee44f2afbb21087ca7d81fda01fccded
|-- 95
|   -- fc1236534b6f73930367f02895467040f47d4a
|-- b0
|   -- 81e51f448387e72a3e3551ba8610eedc172e60
|-- f1
|   -- a8b89f50a2fd8287578daa2b0374adf3cad8aa
|-- info
|-- pack

Verify the accuracy of the model

There's a model on it , But do you think I guessed this model by myself ? How do I know which is blob object? Which is tree object? Which is commit object What about ? And then it's time to witness miracles .

Use git log Command us to view our submission history

commit f1a8b89f50a2fd8287578daa2b0374adf3cad8aa (HEAD -> master)
Author: zhu.yang <[email protected]>
Date:   Tue Jan 8 10:12:06 2019 +0800
    Initial Commit

According to the naming convention we mentioned earlier , We can do it in objects Found in f1a8b89f50a2fd8287578daa2b0374adf3cad8aa This object .
We can't simply use cat command , Because these are not plain text files , But fortunately, git It provides us with a cat-file command

git cat-file commit f1a8b89f50a2fd8287578daa2b0374adf3cad8aa

You can get commit object The content in

tree 95fc1236534b6f73930367f02895467040f47d4a
author zhu.yang <[email protected]> 1546913526 +0800
committer zhu.yang <[email protected]> 1546913526 +0800
Initial Commit

You can see from above commit Point to tree object And we can use git ls-tree Command to check what's in it

git ls-tree 95fc1236534b6f73930367f02895467040f47d4a

As we said we expected , It includes pointing to blob object List of files

100644 blob 84705622ee44f2afbb21087ca7d81fda01fccded    Main.java
100644 blob b081e51f448387e72a3e3551ba8610eedc172e60    README.md

If you want to see Main.java The content in the is used in cat-file Command is enough

git cat-file blob 84705622ee44f2afbb21087ca7d81fda01fccded

We can see that it's coming back Main.java The content of the document

public class Main {
        public static void main(String[] args) {
                System.out.println("Hello World");
        }
}

That's what happens when we create and submit some files . At the same time, it also verifies the accuracy of our model .

Model changes when modifying files

Now let's revise main.java Submit it again

As we can see ,git In the form of a snapshot Main.java I've created a new one blob object, because README.md Not modified , So no new... Will be created for it blob object. and git Will reuse existing blob object.

Now? , When git Create a tree object when , Assigned to Main.java Of blob The pointer will be updated , And assigned to README.md Of blob The pointer will remain the same as in the previous commit tree .

)

In the end ,git Create a commit object And point to it tree object. There is also a pointer to its parent commit object ( Each submission has at least one parent submission in addition to the first submission )

So far we have known git How to deal with the addition and editing of files , The only thing left is how to deal with the deletion , Let's delete Main.java:

Notice the red line above , We found that deleting is also very simple , Just delete tree object Point to blob object The pointer of . In this case, we deleted... In the new submission Main.java, So our submitted tree objects no longer have a pointing representation Main.java Of blob object The pointer to .

How the model handles the folder

The data model we provide has an additional function -tree object Can be nested ( They can point to other tree objects ), You can think like this : Every blob object For a file , Each tree object represents a directory , So if we have nested directories , We have nested tree object.

Because the above figure has been drawn after submitting results for many times , If you draw the structure on the basis of the above, it will not be so clear , This time I reinitialize a repository to demonstrate , The existing data in the warehouse is as follows :

|-- README.md
`-- app
    `-- user.json

And then submit , Finally, you can see the following data model

Git Use blob object as well as tree object To reproduce the folder structure of the project . I'm sure you are right here git We have a more in-depth understanding of the data model , It's really simple , I believe in learning based on it Git It must be twice the result with half the effort .

summary

1. When creating a submission git Will the new blob object,tree object,commit object And it will form a link diagram
2. Nested tree object Used to represent a folder
3. git From reuse blob object
4. Except for the first submission , Each submission has a parent submission