[mtdewcmu]

It looks like .git/logs contains the history. It looks like the file format is a space-separated list, with the format "$parentcommitsha1 $newcommitsha1 ... $commitmessage". That's fairly comprehensible. What are the SHA-1 sums of? Are they of the entire snapshot, or the delta? I went into objects/ and ran `sha1sum $objfile`, and the sum did not match the file name. So that remains obscure. `file $objfile` could not identify the format; it gave nonsense.

Thanks for the help.

>One of those meta-data items is "Parent Commit," so if you change one item in history, it changes the SHA-1 sum of all subsequent items (because at the very least they all need to be re-parented).

What sequence of operations would change a history item in that way?

[pyre]

> It looks like .git/logs contains the history. It looks like the file format is a space-separated list, with the format "$parentcommitsha1 $newcommitsha1 ... $commitmessage". That's fairly comprehensible.

I've never looked at .git/logs, but it looks like that is used by the `git reflog` command. It's basically a history (or log) of every commit that a particular reference has pointed to[1]. For example, I cloned the git source code:

  user@host ~/src/git % cat .git/logs/HEAD
  0000000000000000000000000000000000000000 d7aced95cd681b761468635f8d2a8b82d7ed26fd First Last <[e-mail]> 1387237920 -0500	clone: from https://github.com/git/git.git

  user@host ~/src/git % git reflog
  d7aced9 HEAD@{0}: clone: from https://github.com/git/git.git

Note: `HEAD` is a reference to the current branch. E.g.:

  ~/src/git $ cat .git/HEAD
  ref: refs/heads/master

  ~/src/git $ cat .git/refs/heads/master
  d7aced95cd681b761468635f8d2a8b82d7ed26fd

It's also of note that branches are referred to as 'references' too, hence storing them under `.git/refs/`.

> What are the SHA-1 sums of? Are they of the entire snapshot, or the delta? I went into objects/ and ran `sha1sum $objfile`, and the sum did not match the file name. So that remains obscure.

See: http://stackoverflow.com/questions/5290444/why-does-git-hash...

[1]: Since the local repository was created. This information does not sync between local and remote.

[mtdewcmu]

>I've never looked at .git/logs, but it looks like that is used by the `git reflog` command. It's basically a history (or log) of every commit that a particular reference has pointed to[1]. For example, I cloned the git source code:

I think it's more or less the DAG represented as an adjacency list. I'd have to think a bit about why there is a separate log file for each branch. It seems that there's some redundancy in doing that, and I'm wondering what the advantages and disadvantages are of splitting the history up in that way.

>It's also of note that branches are referred to as 'references' too, hence storing them under `.git/refs/`.

I've developed a loathing of excessive hierarchies/trees, so I'd rather see them flattened in a single directory. But that makes sense.

>See: http://stackoverflow.com/questions/5290444/why-does-git-hash....

That's a good link. What's in an object? If an object corresponds to a commit, then it must aggregate data about changes to multiple files.

[pyre]

> I think it's more or less the DAG represented as an adjacency list. I'd have to think a bit about why there is a separate log file for each branch. It seems that there's some redundancy in doing that, and I'm wondering what the advantages and disadvantages are of splitting the history up in that way.

Think of each branch as a pointer. Then realize that you can make that pointer point anywhere on the DAG, even to parts of the DAG that have no connection to each other. The `reflog` is a (local, non-comprehensive) history of where that pointer has pointed. That's why there is a separate log for each branch. I guess that technically they could have a single log file and add another field to specify the branch, but using the same directory tree structure as under .git/refs/ makes the mental model simpler (and probably a performance improvement not to have to parse the reflog for every branch just to see the reflog for one branch).

> I've developed a loathing of excessive hierarchies/trees, so I'd rather see them flattened in a single directory. But that makes sense.

I'm not sure what branches living under .git/refs has to do with excessive hierarchies/trees. There are enough things stored in the .git directory, that if you mashed them all together it wouldn't make any sense.

> What's in an object?

If you really care to dive deeper, you can check objects here: https://github.com/git/git/blob/master/object.h

You can get a shorter version towards the bottom of the git manpage (e.g. `man git`):

  IDENTIFIER TERMINOLOGY
         <object>
             Indicates the object name for any
             type of object.
  
         <blob>
             Indicates a blob object name.
  
         <tree>
             Indicates a tree object name.
  
         <commit>
             Indicates a commit object name.
  
         <tree-ish>
             Indicates a tree, commit or tag
             object name. A command that takes a
             <tree-ish> argument ultimately wants
             to operate on a <tree> object but
             automatically dereferences <commit>
             and <tag> objects that point at a
             <tree>.
  
         <commit-ish>
             Indicates a commit or tag object
             name. A command that takes a
             <commit-ish> argument ultimately
             wants to operate on a <commit> object
             but automatically dereferences <tag>
             objects that point at a <commit>.
  
         <type>
             Indicates that an object type is
             required. Currently one of: blob,
             tree, commit, or tag.
  
         <file>
             Indicates a filename - almost always
             relative to the root of the tree
             structure GIT_INDEX_FILE describes.

[mtdewcmu]

I noticed that there is no delta compression until objects get incorporated into a pack.

>Think of each branch as a pointer. Then realize that you can make that pointer point anywhere on the DAG, even to parts of the DAG that have no connection to each other. The `reflog` is a (local, non-comprehensive) history of where that pointer has pointed.

I got that branches were pointers. Now that I'm aware that the DAG is fully represented inside objects, I can see that what's inside logs/ is actually just logs. Each log corresponds to a subgraph of the full DAG. Getting history from a log would be more efficient than from the objects themselves, because to get it from objects, you'd have to dereference a lot of object references.

>I'm not sure what branches living under .git/refs has to do with excessive hierarchies/trees. There are enough things stored in the .git directory, that if you mashed them all together it wouldn't make any sense.

Having to descend through layers of subdirectories makes things harder. I'd reduce the depth of the directory tree to the absolute minimum. It's hard to tell if this is the minimum without knowing exactly what all the implementation constraints might have been.

I can see that the real meat of this system is the object store. It's useful to know about `git cat-file` for inspecting it.

[pyre]

> Each log corresponds to a subgraph of the full DAG

I don't have the time to keep up this conversation, but this assertion is wrong. It is not a subgraph. It is a history of the values that the pointer was pointing to (e.g. "Pointer <branch_name> changed from pointing to value AAA to value BBB due to action XXX"). That is basically what all of those entries are. 'AAA' and 'BBB' maybe be in completely unconnected sections of the DAG.

If you create a new repository and add a couple of commits, then yes the reflog files will look like a history, but only because the branch pointer has traversed the DAG from start to end with no deviations.

For example you can have a DAG like this:

   A - B - C - D - E

   X - Y - Z

If you change the branch pointer to move from B to Z, this is not a subgraph. Well, I guess technically you could call it sgraph of the history of the branch pointer, but it in no way corresponds to the DAG other than that all of the pointer values exist within the DAG. For example the following operations:

  git clone
  git reset --hard Z
  git reset --hard X

Would create a graph like this (assuming that master pointed to E when you cloned):

  E - Z - X

Notice that this really don't correspond to the DAG other than the fact that those objects exist in the DAG.

Note:

- All of this information is only contained within the .git/logs files. None of it is stored in the objects themselves.

[mtdewcmu]

I'm guessing that the reason each branch has its own history is probably related to the goal of only appending new entries at the end of things. Since any branch can be under development, they need their own files. It sort of makes sense.

[pyre]

I still think that you're a little confused. The reflog is a "history of where this branch has pointed since the repository was created/cloned." If I clone a repository with a history of 100 commits on the 'master' branch, the reflog for the 'master' branch will only have one entry. You can completely delete the `.git/logs` and still run `git log` successfully.

Here's an example:

  $ git clone blah
  
  DAG:
  
    A - B - C - D - E
        \
         Z - X - Y
  
  
  Branches:
  
   master => E
   topic/new-feature => Y
  
  
  reflog:
  
    master
      E - clone from blah
  
    topic/new-feature
      Y - clone from blah

Notice how cloning a repository with an existing DAG doesn't populate the reflog. It just give it a single entry saying that the branch was updated from 'nothing' to whatever commit it was pointing to remotely.

Now let's change where 'master' is pointing:

  $ git reset master C
  
  
  DAG:
  
    A - B - C - D - E
        \
         Z - X - Y
  
  
  Branches:
  
   master => C
   topic/new-feature => Y
  
  
  reflog:
  
    master
      E - clone from blah
      C - reset to C
  
    topic/new-feature
      Y - clone from blah

Notice how the reflog is a history of the values that the branch was referencing, but is not the history as what you get when you run 'git log'. After the reset, 'git log master' would show you commits A, B and C, but A and B are nowhere in the reflog.

[mtdewcmu]

I see. I started reading the internals chapter at[1]. This free book seems better than the O'Reilly book, which I bought.

So the DAG is actually stored inside objects. The contents of the objects directory could be described by a relational schema, and I think that would make it easier for a lot of people to understand (myself included):

  Blob
  - sha1hash (primary key)
  - contents (blob)

  Tree
  - sha1hash (primary key)

  TreeEntry
  - treeid (foreign key into Tree)
  - mode (mode of blob/subtree)
  - type ("blob" or "tree")
  - objectid (foreign key into Tree or Blob)
  - name

  Commit
  - sha1hash (primary key)
  - tree (foreign key into Tree)
  - parent (foreign key into Commit)
  - author
  - committer
  - comment

The tree entries are actually denormalized and stored as a list inside the tree. You could represent this more accurately with XML. But who likes XML?

[1] http://git-scm.com/book