Node apps

Node.js has the package.json format and npm or yarn to assist in packaging and distributing applications to other developers. However, these are not user-oriented tools. Nor do they offer a solution for notifying users of available updates.

These tools offer no way to guarantee your app be run with a compatible version of Node. While the package.json format has the engines option, it only warns when an incompatibility is detected.

Snaps address these gaps while building upon the work you’ve already done to teach Node how to package your app.

What benefits do snaps bring to Node.js apps?

  • Snaps are easy to discover and install. Millions of users can browse and install snaps graphically in the Snap Store or from the command-line.
  • Snaps install and run the same across Linux. They bundle the exact version of Node required and all of your app’s dependencies, be they Node modules or other applications.
  • Snaps automatically update to the latest version. Four times a day, users’ systems will check for new versions and upgrade in the background.
  • Upgrades are not disruptive. Because upgrades are not in-place, users can keep your app open as it’s upgraded in the background.
  • Upgrades are safe. If your app fails to upgrade, users automatically roll back to the previous revision.

Do you have twenty minutes to get started?

Ready to get started? By the end of this guide you’ll understand how to make a snap of your Node app that can be published in the Snap Store, showcasing it to millions of Linux users.

Diving in

Snaps are defined in a single YAML file placed in the root of your project. The wethr example shows the entire snapcraft.yaml file for an existing project. Don’t worry, we’ll break this down.

name: wethr
version: git
summary: Command line weather tool.
description: >
  Get current weather:-
    $ wethr
  Get current weather in metric units
    $ wethr --metric
  Get current weather in imperial units
    $ wethr --imperial

confinement: devmode

parts:
  wethr:
    plugin: nodejs
    source: .

apps:
  wethr:
    command: wethr

Metadata

The snapcraft.yaml file starts with a small amount of human-readable metadata, which usually can be lifted from the GitHub description or project README.md. This data is used in the presentation of your app in the Snap Store.

name: wethr
version: git
summary: Command line weather tool.
description: >
  Get current weather:-
    $ wethr
  Get current weather in metric units
    $ wethr --metric
  Get current weather in imperial units
    $ wethr --imperial

The name must be unique in the Snap Store. Valid snap names consist of lower-case alphanumeric characters and hyphens. They cannot be all numbers. They also cannot start or end with a hyphen.

By specifying git for the version, the current git tag or commit will be used as the version string. Versions carry no semantic meaning in snaps.

The summary can not exceed 79 characters. You can use a chevron ‘>’ in the description key to declare a multi-line description.

Security model

The next section describes the level of confinement applied to your app.

confinement: devmode

Snaps are containerised to ensure more predictable application behaviour and greater security. Unlike other container systems, the shape of this confinement can be changed through a set of interfaces. These are declarations that tell the system to give permission for a specific task, such as accessing a webcam or binding to a network port.

It’s best to start a snap with the confinement in warning mode, rather than strictly applied. This is indicated through the devmode keyword. When a snap is in devmode, runtime confinement violations will be allowed but reported. These can be reviewed by running journalctl -xe.

Because devmode is only intended for development, snaps must be set to strict confinement before they can be published as “stable” in the Snap Store. Once an app is working well in devmode, you can review confinement violations, add appropriate interfaces, and switch to strict confinement.

Parts

Parts define what sources are needed to assemble your app. Parts can be anything: programs, libraries, or other needed assets. We’ll deal with libraries and other assets later, so for now we just have one part: the wethr source code.

parts:
  wethr:
    plugin: nodejs
    source: .

The nodejs plugin builds upon the work you’ve already done to describe your app’s dependencies in your package.json. It will automatically include these in your snap.

It will also include Node 6. This can be changed with the node-engine keyword.

Apps

Apps are the commands you want to expose to users and any background services your application provides. Each key under apps is the command name that should be made available on users’ systems.

The command specifies the path to the binary to be run. This is resolved relative to the root of your snap contents and automatically searches in the usr/sbin, usr/bin, sbin, and bin subdirectories of your snap.

apps:
  wethr:
    command: wethr

If your command name matches the snap name, users will be able run the command directly. If the names differ, then apps are prefixed with the snap name (wethr.command-name, for example). This is to avoid conflicting with apps defined by other installed snaps.

You can request an alias on the Snapcraft forum if your command name and snap name do not match but you don’t want your command prefixed. These aliases are set up automatically when your snap is installed from the Snap Store.

Further customisations

Here are all the Node plugin-specific keywords:

- node-packages:
  (list)
  A list of dependencies to fetch using npm.
- node-engine:
  (string)
  The version of nodejs you want the snap to run on.
- npm-run:
  (list)
  A list of targets to `npm run`.
  These targets will be run in order, after `npm install`

Next steps

That’s it. You now have a snapcraft.yaml file which describes how the core of your app is assembled, presented, and run.

Continue on to learn how you can bundle your app’s dependencies, such as system libraries, into this snap so your app is portable across Linux distributions.

Last updated 22 days ago. Help improve this document in the forum.