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Exploring the three-trees architecture

From: Git Essential Training

Video: Exploring the three-trees architecture

In this chapter we're going to examine a few key concepts in Git that will help you to better understand how it works, and the first of these is the three tree architecture that it uses. Let's begin by looking at a typical two-tree architecture. This is what a lot of other version control systems use. We have a repository and a working copy, and those are our two trees. Now we call them trees because they represent a file structure. All right, our working copy begins with the top of our project directory and below of that might be four or five different folders that have a few files in them, maybe a few more folders, each of those folders has a few more folders in it, and you can imagine that if you map that out, that each of those folders would then branch out like the branches of a tree.

Exploring the three-trees architecture

In this chapter we're going to examine a few key concepts in Git that will help you to better understand how it works, and the first of these is the three tree architecture that it uses. Let's begin by looking at a typical two-tree architecture. This is what a lot of other version control systems use. We have a repository and a working copy, and those are our two trees. Now we call them trees because they represent a file structure. All right, our working copy begins with the top of our project directory and below of that might be four or five different folders that have a few files in them, maybe a few more folders, each of those folders has a few more folders in it, and you can imagine that if you map that out, that each of those folders would then branch out like the branches of a tree.

It's really a directory tree whose trunk begins with the root of our project. Now the repository also has a set of files in it. And when we want to move files between the repository or the working copy, we check out copies--that's the term that we use--we check it out from the repository into our working directory, and when we finish making our changes we commit those changes back to the repository. Now the reason why there are two distinct trees is that these files don't have to be the same between them. If I check out copy from the repository, I make some changes into it, I save those changes on my hard drive.

Now those changes are saved, they are permanent, they are saved in my working copy, but they're not yet committed to the repository. So my working copy looks different from the repository. Both are saved, it's not like I haven't saved the files, I've done that. They just aren't saved and tracked in the version control repository. Now if the repository is a shared repository, and there are many people working from it, they may commit their changes to the repository. And if I haven't checked out a copy recently to get those changes, then my working copy doesn't have their changes.

So once again the repository and the working trees will not have the same information in them. So that's a typical two-tree architecture. Git however uses a three-tree architecture. It still has the repository and the working copies, but in between is another tree which is the staging index. Remember when we did our first commit in the last chapter, we didn't just do a commit, we did an add first. We added, then we committed, it was a two-step process. That add, added our files to the staging index, and then from there we committed to the repository.

Now it is possible to go ahead and just commit directly to the repository and skip that staging step. We'll learn how to do that later. But it's important that you understand that this is part of the architecture of Git, and it's a really nice feature. Because then what it means is that we can make changes to ten different files in our working copy. And then we can say, all right, I am ready to make a commit, but I don't want to commit all ten of those, I just want to commit five of these as one changed set. So what I am going to do is I am going to put those on the staging index, add them to the staging index, get those five files ready to go, and as soon as I am satisfied that they are ready, now I will commit those five files in one changed set to the repository.

The other five files are still saved in my working tree, but they never got added to the staging index or to the repository. They are sitting there waiting for me to make another commit, to stage those changes and then commit them to the repository. And of course we can pull things out of the repository in the same way. It's possible to pull them from the repository to the staging index, from the staging index to the working directory, usually that's not what we do. Usually we go ahead and pull them straight from the repository down to the working directory. And in the process the staging index will be updated too. We have our working copy, where we have our changes that we've made, and we've saved, and saved to our hard drive, but we have not yet committed them to the repository, we haven't told Git to make this a changed set and to track it.

Then we have the staging index, which is where we prepare things, we stage them for the commit, and then after they've been staged, we commit them to the repository so that they are permanently tracked and they now have a commit message attached to them.

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This video is part of

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Git Essential Training

89 video lessons · 30320 viewers

Kevin Skoglund
Author

 
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  1. 2m 46s
    1. Introduction
      1m 7s
    2. How to use the exercise files
      1m 39s
  2. 20m 24s
    1. Understanding version control
      4m 48s
    2. The history of Git
      7m 58s
    3. About distributed version control
      5m 4s
    4. Who should use Git?
      2m 34s
  3. 26m 12s
    1. Installing Git on a Mac
      3m 44s
    2. Installing Git on Windows
      5m 37s
    3. Installing Git on Linux
      1m 30s
    4. Configuring Git
      7m 29s
    5. Exploring Git auto-completion
      5m 35s
    6. Using Git help
      2m 17s
  4. 15m 49s
    1. Initializing a repository
      1m 58s
    2. Understanding where Git files are stored
      2m 34s
    3. Performing your first commit
      2m 4s
    4. Writing commit messages
      5m 22s
    5. Viewing the commit log
      3m 51s
  5. 17m 44s
    1. Exploring the three-trees architecture
      3m 57s
    2. The Git workflow
      3m 15s
    3. Using hash values (SHA-1)
      4m 7s
    4. Working with the HEAD pointer
      6m 25s
  6. 25m 52s
    1. Adding files
      5m 59s
    2. Editing files
      3m 56s
    3. Viewing changes with diff
      3m 35s
    4. Viewing only staged changes
      2m 28s
    5. Deleting files
      5m 29s
    6. Moving and renaming files
      4m 25s
  7. 19m 18s
    1. Introducing the Explore California web site
      2m 2s
    2. Initializing Git
      3m 48s
    3. Editing the support phone number
      6m 20s
    4. Editing the backpack file name and links
      7m 8s
  8. 38m 45s
    1. Undoing working directory changes
      3m 49s
    2. Unstaging files
      2m 37s
    3. Amending commits
      4m 50s
    4. Retrieving old versions
      4m 7s
    5. Reverting a commit
      3m 12s
    6. Using reset to undo commits
      3m 44s
    7. Demonstrating a soft reset
      4m 8s
    8. Demonstrating a mixed reset
      4m 7s
    9. Demonstrating a hard reset
      5m 8s
    10. Removing untracked files
      3m 3s
  9. 27m 22s
    1. Using .gitignore files
      8m 23s
    2. Understanding what to ignore
      4m 47s
    3. Ignoring files globally
      4m 49s
    4. Ignoring tracked files
      5m 26s
    5. Tracking empty directories
      3m 57s
  10. 26m 51s
    1. Referencing commits
      4m 52s
    2. Exploring tree listings
      3m 46s
    3. Getting more from the commit log
      7m 38s
    4. Viewing commits
      4m 4s
    5. Comparing commits
      6m 31s
  11. 39m 35s
    1. Branching overview
      4m 56s
    2. Viewing and creating branches
      2m 57s
    3. Switching branches
      2m 58s
    4. Creating and switching branches
      4m 53s
    5. Switching branches with uncommitted changes
      3m 26s
    6. Comparing branches
      4m 28s
    7. Renaming branches
      2m 28s
    8. Deleting branches
      4m 18s
    9. Configuring the command prompt to show the branch
      9m 11s
  12. 28m 32s
    1. Merging code
      3m 11s
    2. Using fast-forward merge vs. true merge
      6m 49s
    3. Merging conflicts
      7m 26s
    4. Resolving merge conflicts
      7m 5s
    5. Exploring strategies to reduce merge conflicts
      4m 1s
  13. 14m 34s
    1. Saving changes in the stash
      4m 5s
    2. Viewing stashed changes
      2m 39s
    3. Retrieving stashed changes
      4m 24s
    4. Deleting stashed changes
      3m 26s
  14. 1h 5m
    1. Using local and remote repositories
      6m 38s
    2. Setting up a GitHub account
      5m 39s
    3. Adding a remote repository
      4m 0s
    4. Creating a remote branch
      4m 3s
    5. Cloning a remote repository
      4m 26s
    6. Tracking remote branches
      4m 5s
    7. Pushing changes to a remote repository
      5m 8s
    8. Fetching changes from a remote repository
      5m 47s
    9. Merging in fetched changes
      4m 50s
    10. Checking out remote branches
      3m 22s
    11. Pushing to an updated remote branch
      2m 6s
    12. Deleting a remote branch
      3m 8s
    13. Enabling collaboration
      3m 40s
    14. A collaboration workflow
      8m 43s
  15. 16m 23s
    1. Setting up aliases for common commands
      5m 14s
    2. Using SSH keys for remote login
      2m 56s
    3. Exploring integrated development environments
      1m 4s
    4. Exploring graphical user interfaces
      4m 32s
    5. Understanding Git hosting
      2m 37s
  16. 55s
    1. Goodbye
      55s

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