git-lfs/docs/spec.md

# Git Media Specification

This is a general guide for Git Media clients.  Typically it should be
implemented by a command line `git-media` tool, but the details may be useful
for other tools.

## The Pointer

The core Git Media idea is that instead of writing large blobs to a Git repository,
only a pointer file is written.

```
# git-media
4d7a214614ab2935c943f9e0ff69d22eadbb8f32b1258daaa5e2ca24d17e2393
(ending \n)
```

The pointer file should be small (less than 100 bytes), and consist of only
ASCII characters.  Here's some ruby code to parse this out.

```
# data is a string of the content
# last full line contains the oid
return nil unless data.size < 100
lines = data.
  strip.      # strip ending whitespace
  split("\n") # split by line breaks

# We look for a comment line, and the phrase `git-media` somewhere
lines[0] =~ /# (.*git-media|external)/ && lines.last
```

Note: An early version of git-media used "external" instead of "git-media" in the
pointer file.  The regex in the code will match both.

That code returns the OID, which should be on the last line.  The OID is
generated from the SHA-256 signature of the file's contents.

## The Server

Git Media needs a URL endpoint to talk to a remote server.  A Git repository
can have different media endpoints for different remotes.  Here is the list
of rules that Git Media uses to determine a repository's Git Media server:

1. The `media.url` string.
2. The `remote.{name}.media` string.
3. Append `/info/media` to the remote URL.  Only works with HTTPS URLs.

Here's a sample Git config file with the optional remote and media configuration
options:

```
[core]
  repositoryformatversion = 0
[media]
  endpoint = "https://github.com/github/assets-team/info/media"
[remote "origin"]
  url = https://github.com/github/assets-team
  fetch = +refs/heads/*:refs/remotes/origin/*
  media = "https://github.com/github/assets-team/info/media"
```

Git Media uses `git credential` to fetch credentials for HTTPS requests.  Setup
a credential cache helper to save passwords for future users.

## Intercepting Git

Git Media uses the `clean` and `smudge` filters to decide which files use
Git Media.  The global filters can be set up with `git media init`:

```
$ git media init
```

The `clean` filter runs as files are added to repositories.  Git sends the
content of the file being added as STDIN, and expects the content to write
to Git as STDOUT.

* Stream binary content from STDIN to a temp file, while calculating its SHA-256
signature.
* Check for the file at `.git/media/{OID}`.
* If it does not exist:
  * Queue the OID to be uploaded.
  * Move the temp file to `.git/media/{OID}`.
* Delete the temp file.
* Write the pointer file to STDOUT.

Note that the `clean` filter does not push the file to the server.  Use the
`git media sync` command to do that.

The `smudge` filter runs as files are being checked out from the Git repository
to the working directory.  Git sends the content of the Git blob as STDIN, and
expects the content to write to the working directory as STDOUT.

* Read 100 bytes.
* If the content is ASCII and matches the pointer file format:
  * Look for the file in `.git/media/{OID}`.
  * If it's not there, download it from the server.
  * Read its contents to STDOUT
* Otherwise, simply pass the STDIN out through STDOUT.

The `.gitattributes` file controls when the filters run.  Here's a sample file
runs all mp3 and zip files through Git Media:

```
$ cat .gitattributes
*.mp3 filter=media -crlf
*.zip filter=media -crlf
```

Use the `git media path` command to view and add to `.gitattributes`.