Google is committed to advancing racial equity for Black communities. See how.

Software delivery

Fuchsia software is delivered on demand to the system through packages. This is a critical component to Fuchsia's core design principles of security and updatability. Packages can be updated independently and delivered on demand, like a web page. This enables a vulnerability patch to be pushed to all Fuchsia products at once without the need for individual product coordination.

A package is not a single archive or image file, but rather a tree of Binary Large Objects (BLOBs) rooted in a single Fuchsia Archive (.far) known as the package metadata. The BLOBs inside Fuchsia packages are content-addressable, meaning they are referenced using a hash of their contents. The content-address of the package metadata is known as the package hash.

The package metadata (meta.far) contains a meta/ directory with at least the following two items:

  • meta/package: JSON file containing the package's identity information such as name and version.
  • meta/contents: A map of the human-readable file names in a package to their content-addresses.

package metadata and associated content blobs

If two or more packages share the same content (such as a library dependency, or font resource), their metadata will point to the same content-address for that resource. This enables Fuchsia to optimize package distribution and storage by avoiding the need to fetch and save a content BLOB that already exists.

Hosting and serving packages

Packages are hosted in repositories based on The Update Framework (TUF). This framework is a specification designed to enable secure delivery of software updates. TUF repositories secure updates through signed metadata attached to records that are verifiable against known trusted public and private keys. This means that any HTTP server can serve a TUF repository without the need for transport-level security, including a developer's workstation!

Packages within a repository are identified by a URL with the fuchsia-pkg scheme:

  • repo-hostname: Hostname of a trusted package repository, such as
  • pkg-name: Unique identifier for the package in this repository.
  • resource-path: Resource contained within the package, such as a component manifest.

package development workflow

Requests for software on a Fuchsia device are handled by the package resolver. The package resolver determines if the system already has the package cached locally. If not, the resolver fetches the package metadata from the repository and updates the necessary content BLOBs.

Storing packages

On the device, package BLOBs are stored in a content-addressable filesystem optimized for write-once, read-often files called blobfs. This allows them to be de-duplicated across all packages and cryptographically verified using their hash. Fuchsia runs the pkg-cache service on top of blobfs to facilitate package management.

package cache relies on blobfs

The pkg-cache layer tracks which packages in the system are currently active. Packages are not explicitly installed or removed in Fuchsia. Software is delivered on demand and likewise space can be reclaimed from packages that are no longer active through periodic garbage collection. When pkg-cache triggers garbage collection to reclaim space, content BLOBs not referenced by any active package are deleted.

Exercise: Packages

So far in this codelab, you've been experiencing on demand software delivery to your device and you probably didn't even know it! In this exercise, you'll peel back the covers and see the details of how packages are delivered and stored on a Fuchsia device.

Restart the emulator

Close any emulator instances you currently have open. Start a new FEMU instance with networking support:

fx vdl start -N --start-package-server

When startup is complete, the emulator prints the following message and opens a shell prompt:

To support fx tools on emulator, please run "fx set-device fuchsia-5254-0063-5e7a"

Examine the package server

When the emulator device launches, the device launcher also starts a local package server that will be used to deliver packages.

Inspect the logs from your terminal window where the emulator was launched. You should find output indicating that a package server is running. Note the port number of the server:

[info] pm: Package server serving ..., at port 54189, pid: ...
[pm serve] serving ... at http://[::]:54189
[pm serve] 200 /

Open a browser to http://localhost:[server-port]. This loads an HTML page listing all the packages currently available in the package repository. Each one of these are packages that can be delivered to the device.

Monitor package loading

Packages are resolved and loaded on demand by a Fuchsia device. Take a look at this in action with the bouncing_ball example package.

From the device shell prompt, you can confirm whether a known package is currently on the device:

pkgctl pkg-status fuchsia-pkg://
Package in registered TUF repo: yes (merkle=ef65e2ed...)
Package on disk: no

Open a new terminal and begin streaming the device logs for pkg-resolver:

fx log --only pkg-resolver

This shows all the instances where a package was loaded from the package server.

From the device shell prompt, attempt to resolve the package:

pkgctl resolve fuchsia-pkg://

Notice the new lines added to the log output for pkg-resolver:

[core/pkg-resolver][pkg-resolver][I] Fetching blobs for fuchsia-pkg://google3-devhost/bouncing_ball: [
[core/pkg-resolver][pkg-resolver][I] resolved fuchsia-pkg:// as fuchsia-pkg://google3-devhost/bouncing_ball to 4ca324998ae9679241c74d2d9d9779fe86c79e2fa1f1627d941a37e987215895 with TUF

From the device shell prompt, check the package status again on the device:

pkgctl pkg-status fuchsia-pkg://
Package in registered TUF repo: yes (merkle=ef65e2ed...)
Package on disk: yes (path=/pkgfs/versions/ef65e2ed...)

Fuchsia resolved the package and loaded it from the local TUF repository on demand!

Explore package metadata

Now that the bouncing_ball package has successfully been resolved, you can explore the package contents. Once resolved, the package is referenced on the target device using its content-address.

From the device shell prompt, use the pkgctl get-hash command to determine the package hash for bouncing_ball:

pkgctl get-hash fuchsia-pkg://

Provide the full package hash to the pkgctl open command to view the package contents:

pkgctl open ef65e2ed...
opening ef65e2ed... with the selectors []
package contents:

This lists the package metadata and each of the content BLOBs in the package. You can bin/ entries for executables, lib/ entries for shared library dependencies, additional metadata and resources.

What's Next?

Congratulations! You now have a better understanding of what makes Fuchsia unique and the goals driving this new platform's design.

In the next module, you'll learn more about the Fuchsia open source project and the tools used to build and customize the system:

Building Fuchsia