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). The root of the tree is a BLOB called "meta.far" which contains metadata for the package, including a "meta/contents" file which contains references to the rest of the BLOBs. The BLOBs inside Fuchsia packages are content-addressed, meaning they are referenced using a hash of their contents. The content address of the meta.far itself is known as the package hash.
The 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.
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
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.
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 meta.far from the repository and updates the necessary content BLOBs.
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
service on top of
blobfs to facilitate package management.
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.
pkg-cache triggers garbage collection to reclaim space, content BLOBs
not referenced by any active package are deleted.
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
Run the following command to close any emulator instances you currently have open:
ffx emu stop --all
Start a new emulator instance:
ffx emu start --headless
When startup is complete, the emulator prints the following message and returns:
Logging to "$HOME/.local/share/Fuchsia/ffx/emu/instances/fuchsia-emulator/emulator.log" Waiting for Fuchsia to start (up to 60 seconds)........ Emulator is ready.
Start a local package server
Run the following command to start a package server and enable the emulator to load software packages:
The command prints output similar to the following, indicating the server is running and has successfully registered the emulator as a target device:
[serve-updates] Discovery... [serve-updates] Device up [serve-updates] Registering devhost as update source [serve-updates] Ready to push packages! [serve-updates] Target uptime: 139 [pm auto] adding client: [fe80::5888:cea3:7557:7384%qemu]:46126 [pm auto] client count: 1
Examine the package server
fx serve-updates command runs a local package server used to deliver
packages to the target devices. By default, this server runs at on port 8083.
Open a browser to
http://localhost:8083. 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:
fx shell pkgctl pkg-status fuchsia-pkg://fuchsia.com/bouncing_ball
Package in registered TUF repo: yes (merkle=ef65e2ed...) Package on disk: no
Open a new terminal and begin streaming the device logs for
ffx log --filter 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:
fx shell pkgctl resolve fuchsia-pkg://fuchsia.com/bouncing_ball
Notice the new lines added to the log output for
[core/pkg-resolver][pkg-resolver][I] Fetching blobs for fuchsia-pkg://google3-devhost/bouncing_ball: [ e57c05aa909bcb38ca452d31abfbf9cc1d099751c9cd644b4d40cbf64e2af48b, ] [core/pkg-resolver][pkg-resolver][I] resolved fuchsia-pkg://fuchsia.com/bouncing_ball as fuchsia-pkg://google3-devhost/bouncing_ball to 4ca324998ae9679241c74d2d9d9779fe86c79e2fa1f1627d941a37e987215895 with TUF
From the device shell prompt, check the package status again on the device:
fx shell pkgctl pkg-status fuchsia-pkg://fuchsia.com/bouncing_ball
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
fx shell pkgctl get-hash fuchsia-pkg://fuchsia.com/bouncing_ball
The command returns the unique package hash:
Provide the full package hash to the
pkgctl open command to view the package
fx shell pkgctl open ef65e2ed...
opening ef65e2ed... with the selectors  package contents: /bin/bouncing_ball /lib/ld.so.1 /lib/libasync-default.so /lib/libbackend_fuchsia_globals.so /lib/libc++.so.2 /lib/libc++abi.so.1 /lib/libfdio.so /lib/libsyslog.so /lib/libunwind.so.1 /meta/bouncing_ball.cmx /meta/contents /meta/package
This lists the package metadata and each of the content BLOBs in the package.
bin/ entries for executables,
lib/ entries for shared library
dependencies, additional metadata and resources.
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: