## Important Concepts

The following sections explain key concepts used internally in PackageKit.

## Package ID

One important idea is the `package_id`. This is the `name;version;arch;data` in a single string and is meant to represent a single package. This is important when multiple versions of a package are installed and only the correct one is removed.

The `package_id` is parsed and checked carefully in the helper code. The package arch and data is optional, but 3 `;`'s must be present. For instance, `gnome-keyring-manager;2.18.0;;` is valid but `gnome-keyring-manager;2.18.0` is not. The data field is used for the repository name.

The data field for an installed package must be `installed` as this is used to identify which packages are installable or installed in the client tools.

The data field for an non-installed local package must be `local` as this signifies a repository name is not available and that package resides locally on the client system.

For example:

- `csup;20060318-5;x86_64;local`: for locally available package file.

- `csup;20060318-5;x86_64;fedora-devel`: for package that is not installed and can be downladed from the Fedora development repostory.

- `csup;20060318-5;x86_64;installed`: for locally installed package

Situation | Value | Description
--- | --- | ---
Searching | `installed` | If installed
| `available` | If available to install
Getting Updates | `low` | If update is of low severity
| `normal` | If update is of normal severity
| `important` | If update is very important
| `security` | If the update is security sensitive
Installing/Updating/Removing | `downloading` | If we are downloading this package
| `updating` | If we are updating this package
| `installing` | If we are installing this package
| `removing` | If we are removing this package
Otherwise | `unknown` | If we cannot use any other option

The backend must ensure that the package_id only matches on one single package. A single package_id must be enough to uniquely identify a single object in any repository used on the system.