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Diffusion User Guide Article

Guide to Diffusion, the Phabricator repository browser.


Diffusion is a repository browser which allows you to explore source code in a Subversion, Git, or Mercurial repository. It is somewhat similar to software like Trac and GitWeb.

Diffusion can either import a read-only copy of repositories hosted somewhere else (for example, from GitHub, Bitbucket or existing hosting) or host repositories within Phabricator. Hosted repositories support a variety of triggers and access controls.

Diffusion is integrated with the other tools in the Phabricator suite. For instance:

  • when you commit Differential revisions to a tracked repository, they are automatically updated and linked to the corresponding commits;
  • you can add Herald rules to notify you about commits that match certain rules;
  • for hosted repositories, Herald can enforce granular access control rules;
  • in all the tools, commit names are automatically linked.

Adding Repositories

Repository administration is accomplished through Diffusion. You can use the web interface in Diffusion to import an external repository, or create a new hosted repository.

By default, you must be an administrator to create a new repository.

Repository Callsigns and Commit Names

Each repository is identified by a "callsign", which is a short uppercase string like "P" (for Phabricator) or "ARC" (for Arcanist).

Each repository must have a unique callsign. Callsigns must be unique within an install but do not need to be globally unique, so you are free to use the single-letter callsigns for brevity. For example, Facebook uses "E" for the Engineering repository, "O" for the Ops repository, "Y" for a Yum package repository, and so on, while Phabricator uses "P", "ARC", "PHU" for libphutil, and "J" for Javelin. Keeping callsigns brief will make them easier to use, and the use of one-character callsigns is recommended if they are reasonably evocative and you have no more than 26 tracked repositories.

The primary goal of callsigns is to namespace commits to SVN repositories: if you use multiple SVN repositories, each repository has a revision 1, revision 2, etc., so referring to them by number alone is ambiguous. However, even for Git they impart additional information to human readers and allow parsers to detect that something is a commit name with high probability (and allow distinguishing between multiple copies of a repository).

Diffusion uses this callsign and information about the commit itself to generate a commit name, like "rE12345" or "rP28146171ce1278f2375e3646a1e1ea3fd56fc5a3". The "r" stands for "revision". It is followed by the repository callsign, and then a VCS-specific commit identifier (for SVN, the commit number; for Git and Mercurial, the commit hash). When writing the name of a Git commit you may abbreviate the hash, but note that hash collisions are probable for short prefix lengths. See this post on the LKML for a historical explanation of Git's occasional internal use of 7-character hashes:


Because 7-character hashes are likely to collide for even moderately large repositories, Diffusion generally uses either a 16-character prefix (which makes collisions very unlikely) or the full 40-character hash (which makes collisions astronomically unlikely).

Running Diffusion Daemons

In most cases, it is sufficient to run:

phabricator/bin/ $ ./phd start

...to start the daemons. For a more in-depth explanation of phd and daemons, see Managing Daemons with phd.

NOTE: If you have an unusually large install with multiple web frontends, see notes in Managing Daemons with phd.

You can use the repository detail screen and the Daemon Console to monitor the daemons and their progress importing the repository. Small repositories should import quickly, while larger repositories may take some time. Commits should begin appearing in Diffusion within a few minutes for all but the largest repositories.

Next Steps