# Anatomy of an OSTree repository

## Core object types and data model

OSTree is deeply inspired by git; the core layer is a userspace content-addressed versioning filesystem. It is worth taking some time to familiarize yourself with Git Internals, as this section will assume some knowledge of how git works.

Its object types are similar to git; it has commit objects and content objects. Git has "tree" objects, whereas OSTree splits them into "dirtree" and "dirmeta" objects. But unlike git, OSTree's checksums are SHA256. And most crucially, its content objects include uid, gid, and extended attributes (but still no timestamps).

### Commit objects

A commit object contains metadata such as a timestamp, a log message, and most importantly, a reference to a dirtree/dirmeta pair of checksums which describe the root directory of the filesystem.

Also like git, each commit in OSTree can have a parent. It is designed to store a history of your binary builds, just like git stores a history of source control. However, OSTree also makes it easy to delete data, under the assumption that you can regenerate it from source code.

### Dirtree objects

A dirtree contains a sorted array of (filename, checksum) pairs for content objects, and a second sorted array of (filename, dirtree checksum, dirmeta checksum), which are subdirectories.

### Dirmeta objects

In git, tree objects contain the metadata such as permissions for their children. But OSTree splits this into a separate object to avoid duplicating extended attribute listings.

### Content objects

Unlike the first three object types which are metadata, designed to be `mmap()ed`, the content object has a separate internal header and payload sections. The header contains uid, gid, mode, and symbolic link target (for symlinks), as well as extended attributes. After the header, for regular files, the content follows.

## Repository types and locations

Also unlike git, an OSTree repository can be in one of two separate modes: `bare` and `archive-z2`. A bare repository is one where content files are just stored as regular files; it's designed to be the source of a "hardlink farm", where each operating system checkout is merely links into it. If you want to store files owned by e.g. root in this mode, you must run OSTree as root. In contrast, the `archive-z2` mode is designed for serving via plain HTTP. Like tar files, it can be read/written by non-root users.

On an OSTree-deployed system, the "system repository" is `/ostree/repo`. It can be read by any uid, but only written by root. Unless the `--repo` argument is given to the `ostree` command, it will operate on the system repository.

## Refs

Like git, OSTree uses "refs" to which are text files that point to particular commits (i.e. filesystem trees). For example, the gnome-ostree operating system creates trees named like `gnome-ostree/buildmaster/x86_64-runtime` and `gnome-ostree/buildmaster/x86_64-devel-debug`. These two refs point to two different generated filesystem trees. In this example, the "runtime" tree contains just enough to run a basic GNOME system, and "devel-debug" contains all of the developer tools.

The `ostree` supports a simple syntax using the carat `^` to refer to the parent of a given commit. For example, `gnome-ostree/buildmaster/x86_64-runtime^` refers to the previous build, and `gnome-ostree/buildmaster/x86_64-runtime^^` refers to the one before that.