Fuchsia packages

A Fuchsia package is a hierarchical collection of files that provides one or more programs, components or services to a Fuchsia system. A Fuchsia package is a term representing a unit of distribution, though unlike many other package systems, that unit is composed of parts and is not a single binary BLOB.

Some packages are present on a Fuchsia system at startup, and additional packages can be downloaded from a Fuchsia package server in BLOBs. The Fuchsia package server is an HTTP(S) server. These BLOBs are uniquely defined by a Merkle root. A BLOB is named after its content, using the Fuchsia Merkle Root algorithm. If two BLOBs have the same content, they have the same name. As a result, each BLOB has a unique identifier and is written to persistent storage using this Merkle root as a key. This process is done to eliminate duplication of BLOBs that may exist between packages. For example, a shared library which exists in multiple packages is only stored once on the device.

The package server serves as a root of trust as it validates the authenticity of each package.

Packages can also declare dependencies on named subpackage s, creating a hierarchy of nested packages. Build rules link a package with the build target of each subpackage. At build time, the package build tool records the subpackages in the parent package's metadata, mapping each subpackage name to its package hash (the BLOB id that identifies the subpackage). This ensures the list of subpackages and the internals of each subpackage cannot change without also changing the Merkle (package hash) of the parent.

Subpackages enable:

  • Encapsulated dependencies (packages are inherently "package trees")
  • Isolated /pkg directories (grouped components don't need to merge their files, libraries, and metadata into a single shared namespace)
  • Assured dependency resolution (system and build tools ensure subpackages always "travel with" their packages)

For more information on packaging components with their dependencies using Subpackages, see Subpackaging components.

Types of packages

The packages that comprise the Fuchsia operating system are categorized into three groups, which affect how they are managed:

Base packages

These are the packages that are part of the foundation of the Fuchsia operating system and are considered critical for security and the system. Resolving a package which is in base on a running Fuchsia system always returns the version that is on the device, and not a new version which may exist on a package server. However, base packages can be updated as part of the OTA process.

Since these packages are immutable for the runtime of a system, these packages must be updated with fx ota which triggers an over-the-air (OTA) update.

Cached packages

These are packages on the device which are not part of base. These packages exist when the device is flashed or paved, so these packages are usable if the device boots without a network connection. Cached packages are updated during the resolution process if a different package is available on the package server. These packages are not updated during a system update, but are updated ephermerally.

Fuchsia can also evict cached packages on a running system to free up resources based on runtime resource demands.

Universe packages

These are packages that exist on the package server, but not on the device.

Structure of a package

In most cases, a package in Fuchsia is a collection of BLOBs, which at a minimum contains one content-addressed BLOB named meta.far.

In Fuchsia, you build a package with the ffx package build command or the legacy pm tool, which both exist in the //tools/ directory of the Fuchsia IDK.

Essentially, a package is a tree of zero or more content-addressed items. A package contains the following:

meta.far

The package metadata archive, meta.far, contains metadata about a package, presented as the meta/ directory. meta.far has a merkleroot which in practical terms is also known as the merkleroot of a package.

The meta/ directory of a package contains at minimum two files:

  • meta/package

    The package identity file. This is a JSON file that contains the name and version of the package.

  • meta/contents

    The contents file. This file is created by the ffx package build command, (or the legacy pm update and pm build commands). This file maps the user-facing file names of a package to the Merkle root of those files.

If the package declares subpackages, the meta/ directory also contains:

  • meta/fuchsia.pkg/subpackages

    The subpackages file. This is a JSON file that contains the name and version of each declared subpackage. From the perspective of the parent package, the subpackage name is used as a relative package URL when resolving the subpackage.

    Package build tools traverse subpackage references (declared through build dependency declarations and package manifest files that reference other package manifest files for each subpackage) to compute the version (package hash) of each subpackage and generate the subpackages file.

Additionally, the meta/ directory can contain files such as a component manifest. For more information on component manifests, see Component manifests.

BLOBs outside of meta/

Most files of a package exist outside of the meta/directory and each are a BLOB.

For example, these files can be like the following:

  • bin/foo
  • lib/libfdio.so
  • data/mydata.db

Identification of a package

Every package in Fuchsia is identified by a package-url.

Absolute package URLs

An absolute Fuchsia package URL identifies a system-addressable package, without requiring any additional context, and looks like the following:

fuchsia-pkg://repository/package-name?hash=package-hash#resource-path

Fuchsia has different intereprations of fuchsia-pkg URL depending on which parts of the URL are present.

  • If the repository, package, and resource parts are present, then the URL identifies the indicated resource within the package.
  • If only the repository and package parts are present, then the URL identifies the indicated package itself.
  • If only the repository parts are present, then the URL identifies the indicated repository itself.

The package parts can express varying degrees of specificity. At minimum the package name must be present, optionally followed by the package hash.

If the package hash is missing, the package resolver fetches the resources from the newest revision of the package variant available to the client.

Relative package URLs

A relative Fuchsia package URL identifies a subpackage given previously loaded package (or subpackage) as "context". The repository and parent package are implicit, and the subpackage name is used to look up the package hash in the parent package's "meta/fuchsia.pkg/subpackages" file. (The package hash cannot be overridden). A relative package URL looks like the following:

package-name#resource-path

As with absolute package URLs, the resource path may or may not be included.