What is a bare repository and why would I need one?

Question

This maybe has been answered, but I didn't find a good answer.
I come from centralized repositories, such as SVN, where usually you only perform checkouts, updates, commits, reverts, merges and not much more.

Git is driving me crazy. There are tons of commands, but the most difficult to understand is why many things work as they do.

According to "What is a bare git repository?":

Repositories created with git init --bare are called bare repos. They are structured a bit differently from working directories. First off, they contain no working or checked out copy of your source files.
…
A bare repository created with git init --bare is for… sharing. …developers will clone the shared bare repo, make changes locally in their working copies of the repo, then push back to the shared bare repo to make their changes available to other users.
– Jon Saints, http://www.saintsjd.com/2011/01/what-is-a-bare-git-repository/

However, from the accepted answer to "what's the difference between github repository and git bare repository?":

Git repos on GitHub are bare, like any remote repo to which you want to push to [sic].
– VonC, https://stackoverflow.com/a/20855207

However, in GitHub there are source files. I can see them. If I create a bare repository, there are no source files, only the contents for .git directory of a working repository.

How is this possible? What don't I understand?

Can you give an example about why I would need a bare repository and its motivation to work that way?

UPDATE

Edward Thomson's answer is, in part, what I wanted to know. Nevertheless, I will rephrase my question:

First link I posted states("What is a bare git repository?"):

they [bare repositories] contain no working or checked out copy of your source files.

VonC's answer:

Git repos on GitHub are bare

Both statements implies

Github has no working copy.

Edward Thomson says:

it renders the web page based on the data as you navigate through it - pulling the data directly out of the repo and out to your web browser, not writing it to a disk on the fileserver first

Somehow, a bare repository has to contain all data and source code. If not, it wouldn't be impossible to render anything, because I can see all source code updated (commited), all branches (with their respective source), the whole log of a repo, etc.

Is there the whole data of a repository always within .git directory (or in a bare repo), in some kind of format which is able to render all files at any time? Is this the reason of bare repository, while working copy only has the files at a given time?

Answer 1

Is there the whole data of a repository always within .git directory (or in a bare repo), in some kind of format which is able to render all files at any time?

Yes, those files and their complete history are stored in .git/packed-refs and .git/refs, and .git/objects.

When you clone a repo (bare or not), you always have the .git folder (or a folder with a .git extension for bare repo, by naming convention) with its Git administrative and control files. (see glossary)

Git can unpack at any time what it needs with git unpack-objects.

The trick is:

From a bare repo, you can query the logs (git log in a git bare repo works just fine: no need for a working tree), or list files in a bare repo.
Or show the content of a file from a bare repo.
That is how GitHub can render a page with files without having to checkout the full repo.

I don't know that GitHub does exactly that though, as the sheer number of repos forces GitHub engineering team to do all kind of optimization.
See for instance how they optimized cloning/fetching a repo.
With DGit, those bare repos are actually replicated across multiple servers.

Is this the reason of bare repository, while working copy only has the files at a given time?

For GitHub, maintaining a working tree would cost too much in disk space, and in update (when each user request a different branch). It is best to extract from the unique bare repo what you need to render a page.

In general (outside of GitHub constraint), a bare repo is used for pushing, in order to avoid having a working tree out of sync with what has just been pushed. See "but why do I need a bare repo?" for a concrete example.

That being said:

• since git 2.3 you could push to a non-bare repo (that would update the working tree accordingly)
• since git 2.4, you can "push-to-deploy" (ie, it works for unborn branch as well)

But that would not be possible for GitHub, which cannot maintain one (or server) working tree(s) for each repo it has to store.