git-lfs/docs/spec.md

Hawser Specification

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

The Pointer

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

version https://hawser.github.com/spec/v1
oid sha256:4d7a214614ab2935c943f9e0ff69d22eadbb8f32b1258daaa5e2ca24d17e2393
size 12345
(ending \n)

The pointer file should be small (less than 200 bytes), and consist of only ASCII characters. Libraries that generate this should write the file identically, so that different implementations write consistent pointers that translate to the same Git blob OID. This means:

• Use properties "version", "oid", and "size" in that order.
• Separate the property from its value with a single space.
• Oid has a "sha256:" prefix. No other hashing methods are currently supported for Hawser oids.
• Size is in bytes.

Note: Earlier versions only contained the OID, with a # comment above it. Here's some ruby code to parse older pointer files.

# 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

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

Hawser 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"

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

Intercepting Git

Hawser uses the clean and smudge filters to decide which files use Hawser. The global filters can be set up with git hawser init:

$ git hawser 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 hawser 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=hawser -crlf
*.zip filter=hawser -crlf

Use the git hawser path command to view and add to .gitattributes.