Almost everything that exists on a Fuchsia system is a Fuchsia package. Whether it is immediately apparent or not almost everything you see on Fuchsia lives in a package. This document will cover the basics of a package-driven workflow where you build a package and push it to a Fuchsia device that is reachable via IP from your development host.
Pre-requisites and overview
The host and target must be able to communicate over IP. In particular it must be possible to SSH from the development host to the target device, and the target device must be able to connect via TCP to the development host on port 8083. The SSH connection is used to issue commands to the target device.
The development host will run a simple, static file, HTTP server that makes the updates available to the target. This HTTP server is part of the Fuchsia source code and built automatically.
The target is instructed to look for changes on the development host via a couple of commands that are run manually. When the update system on the target sees these changes it will fetch the new software from the HTTP server running on the host. The new software will be available until the target is rebooted.
To build a package containing the required code, a package type build rule is used. If one of these needs to be created for the target package, consult the reference page for this.
Once an appropriate build rule is available the target package can be
re-generated by running
Fuchsia comes with a tool
far to work with package archives. After the
completion of a Fuchsia build the
far program can be used to
create, list, dump or extract package archives:
$ fx host-tool far
Usage: far <command> ...
where <command> is create, list, cat, extract, or extract-file.
For example, to list the contents of a package, observe that its
$ fx host-tool far list --archive=out/default/obj/third_party/sbase/sed/meta.far
meta/contents file maps the user-facing file names of a package to the
Merkle root of those files, allowing to see which files belong to it:
$ fx host-tool far cat --archive=out/default/obj/third_party/sbase/sed/meta.far --file=meta/contents
Connecting host and target
The Fuchsia source contains a simple HTTP server that serves static files. The build generates and serves a TUF file tree.
The update agent on the target does not initially know where to look for
updates. To connect the agent on the target to the HTTP server running on the
development host, it must be told the IP address of the development host. The
host HTTP server is started and the update agent is configured by calling
fx serve will run the update server and is often what people use.
-v is recommended because the command will print more output, which may assist
with debugging. If the host connects successfully to the target you will see the
Ready to push packages! in the shell on your host.
The update agent on the target will remain configured until it is repaved or persistent data is lost. The host will attempt to reconfigure the update agent when the target is rebooted.
Triggering package updates
Packages in Fuchsia are not "installed", they are cached on an as needed basis. There are two collections of packages on a Fuchsia system:
basepackage set is a group of software critical to proper system function that must remain congruent. The Fuchsia build system assigns packages to
basewhen they are provided to
fx setusing the
--with-baseflag. This set of software can only be updated by performing a whole system update, typically referred to as OTA, described below. This is updated using
ephemeral software Packages included in the
universepackage set are ephemeral software, with updates delivered on demand. The Fuchsia build system assigns packages to
universewhen they are provided to
fx setusing the
--withflag. Ephemeral software updates to the latest available version whenever the package URL is resolved.
Triggering an OTA
Sometimes there may be many packages changed or the kernel may change or there may be changes in the system package. To get kernel changes or changes in the system package an OTA or pave is required as base packages are immutable for the runtime of a system. An OTA update will usually be faster than paving or flashing the device.
fx ota asks the target device to perform an update from any of
the update sources available to it. To OTA update a build made on the dev host to
a target on the same LAN, first build the system you want. If
fx serve [-v]
isn't already running, start it so the target can use the development host as an
update source. The
-v option will show more information about the files the
target is requesting from the host. If the
-v flag was used there should
be a flurry of output as the target retrieves all the new files. Following
completion of the OTA the device will reboot.
Just the commands
fx serve -v(to run the update server for both build-push and ota)
fx test <component-url>(to build and run tests)
fx ota(to trigger a full system update and reboot)
Issues and considerations
You can fill up your disk
Every update pushed is stored in the content-addressed file system, blobfs. Following a reboot the updated packages may not be available because the index that locates them in blobfs is only held in RAM. The system currently does not garbage collect inaccessible or no-longer-used packages (having garbage to collect is a recent innovation!), but will eventually.
fx gc will reboot the target device and then evict all old
ephemeral software from the device, freeing up space.
Restarting without rebooting
If the package being updated hosts a service managed by Fuchsia that service
may need to be restarted. Rebooting is undesirable both because it is slow and
because the package will revert to the version paved on the device. Typically
a user can terminate one or more running components on the system, either by
asking the component to terminate gracefully, or by forcefully stopping the
ffx component stop <component-moniker>. Upon reconnection to
the component services, or by invocation via
ffx component start or
new versions available in the package server will be cached before launch.
Packaging code outside the Fuchsia tree
Packaging and pushing code that lives outside the Fuchsia tree is possible, but will require more work. The Fuchsia package format is quite simple. It consists of a metadata file describing the package contents, which is described in more detail in the Fuchsia package documentation. The metadata file is added to a TUF file tree and each of the contents are named after their Merkle root hash and put in a directory at the root of the TUF file tree called 'blobs'.