Snaps in Ubuntu Core
Ubuntu Core is built from snaps, a secure, confined, dependency-free, cross-platform Linux packaging format.
Snaps are self-contained, which means they possibly include everything needed to run or use components from other snaps in a limited and controlled manner. They’re used by Ubuntu Core to both compose the image that’s run on a device, and to deliver consistent and reliable software updates, often to low-powered, inaccessible, and remotely administered embedded and IoT systems.
Whether it’s an update to a single device, a specific subset of devices, or a deployment of tens of thousands of devices, snaps enable Ubuntu Core to maintain and verify a system’s integrity:
Autonomous updates to any device: the update service needs to be a reliable and automatic process, catering for a predictable update cadence and an optional fine level of control over when and how updates are delivered
Incomplete and problematic update recovery: the inaccessible nature of many embedded device installations makes it imperative that updates are impregnable and capable of withstanding broken, blocked, partial and interrupted updates
Critical update provision: in specific circumstances when an ad-hoc or critical update needs to be made, the update system needs to give these priority and incorporate the update back into the regular update service
Unpredictable hardware and network conditions: in situations that can’t be easily modelled or predicted, any update system needs to have enough redundancy to handle roll-backs, network-free bootstrapping and autonomous re-provisioning
Types of snap
The snap packaging ecosystem consists of the following parts:
- snap is both the command line interface and the application package format
- snapd is the background service that manages and maintains your snaps
- snapcraft is the command and the framework used to build your own snaps
- Snap Store provides a place to upload your snaps, and for users to browse and install
Developers can publish snaps to the Snap Store or to their own private Brand Store. They take sole responsibility for update cadence and quality. While snaps are commonly known as an application packaging format, Ubuntu Core is built from several different types of snap:
kernel: contains the Linux kernel for a device
The kernel snap is selected with the model assertion describing the device which is produced and signed before the image is built. Once the image is built, the kernel snap may be updated but cannot be replaced by a completely different kernel snap.
gadget: defines device properties
The gadget snap is responsible for defining and manipulating the system properties and configuration which are specific to one or more devices that will usually look similar to one another from an implementation perspective. It is also responsible for shipping the device bootloader and bootloader assets. It is selected with the model assertion.
base: the runtime environment
The base snap provides the run-time environment with a minimal set of libraries that are common to most applications. Base snaps mirror Ubuntu LTS releases and include core20, built from Ubuntu 20.04 LTS, core18 based on Ubuntu 18.04 LTS, and core, based on Ubuntu 16.04 LTS. One of them, selected with the model assertion, also serves as the root file system for the Ubuntu Core system.
For historical reasons the core snap has a different specific type: core.
snapd: the snap daemon
The core16, core18 and core20 base snaps do not include the snap daemon (core, however, does). Instead, they package the daemon as an upgradeable snap of this type.
app: applications, daemons and tools
Packages applications, pulled from multiple upstream sources using diverse build systems. The snapd daemon is itself installed as a snap (except with an old core, where it’s included).
As with the core file system, snaps are presented to the system as read-only and are granted access to whatever resources they need through a set of explicit permissions, known as interfaces. Interfaces are implemented using well-tested Linux kernel confinement features.