pm - The Fuchsia Package Manager

The Fuchsia Package Manager is responsible for controlling and (locally) serving packages on a Fuchsia system. It is not responsible for indexing, updates or post-fetch operations that are handled by other systems.

pm is also a command line tool that can be used to perform some package related operations.


PM is the package manager command line interface. It is responsible for:

  • Demonstrating and exercising the pmd APIs.
  • Enabling developer oriented package functions such as:
    • Archiving packages
    • Installing packages
    • Listing packages
    • Removing packages
    • Validating packages
  • Summarizing the interface to Amber (?)

PM runs on several operating systems, although only a subset of operations are supported on platforms that do not have PMD.


PMD is the “Package Manager Daemon” and is responsible for:

  • Activating packages (making metadata locally available)
  • Serving packages (providing filesystem type access to packages)
  • Management (garbage collection of unused / unwanted packages)
  • Enumeration (seeing what packages and contents are locally available)

PMD only runs on Fuchsia.

Structure of a Fuchsia Package

A Fuchsia package is one or more collections of files that provide one or more programs, components or services for a Fuchsia system. Additional concerns such as revocation, attestations and distribution concerns are handled by the distribution system separately from pm, see Amber for more details.

A package is defined by a set of metadata that are stored in a directory at the top level of a Fuchsia package, as so:


Additional files may extend the specification in the future.


The metadata file contains a basic set of system, developer and user friendly data. Examples include human name, unique name, version, developer name, developer public key, and description.

TODO(raggi): provide exact format and structure of first gen metadata.


The contents file contains a strict and complete manifest of all files that are in the entire package. The names double as paths for packaging and paths for presentation at runtime. Associated with each path is a hash of the content using the merkle root strategy documented as below.

Merkle Root

Conventions for Fuchsia Packages

The Fuchsia Package System does not provide native support for inter-package dependencies. This is a considered restriction in order to avoid various challenges that are inherent to dependency management and versioning. This choice keeps the system extremely simple and secure. The solution for handling dependencies is therefore to embed all dependencies into a package. The features of the package manager, the package distribution and the Fuchsia storage systems then perform various levels of deduplication in order to make this approach transport, storage and runtime efficient.

Versions in the Fuchsia package system are open ended, but are largely considered to be linear. This is most relevant when integrating a third party package system such as Rubygems into the Fuchsia package system. By convention third party packages are integrated by adding the third party package version as a specific Fuchsia package. Concretely, third party package foo version 1.2.3 will conventionally be packaged as a Fuchsia package with the name foo-1.2.3. There may be additional naming conventions for language/runtime specific names in future. The goal is that if foo had contained a machine specific binary blob, the package can be rebuilt and subsequently updated without altering the “version” from the perspective of a higher level package manager.

Data dependencies and optional dependencies may be additional packages, but due to the explicit lack of a mechanism to manage specific versions of such things, it is expected that those cases will not be shared library objects. Instead they should be limited to progressive enhancement and safely loaded things that do not have strict version requirements. An example might be fonts, where additional font packages may be found and utilized by a font server. The discovery of additional fonts.

TODO(raggi): improve documentation

Updating Fuchsia Packages

When defining the contents of a Fuchsia package, it is worth considering how that package will be updated to a newer version. Essentially, to update to a newer version of a package, a Fuchsia system must determine which blobs referenced by the new package‘s meta/contents file are not present on the system and download them. Likewise, after updating a package, any blobs that are no longer referenced in the package’s meta/contents may be safely removed from the system, provided that no other package on the system references those blobs.

A major implication of this update model is that updates are performed at file granularity. If a Fuchsia package contains a single file with many assets, for example, updating a single asset within that file will require downloading the entire file. In this case, including the package assets as individual files would allow an update to download just the assets that have changed. This concept applies to binaries as well. To optimize package update download sizes, prefer dynamically linking to libraries instead of statically linking, especially if those dependencies change infrequently.

It is difficult to know how changes to a Fuchsia package will affect its update download size. Fortunately, pm has a few subcommands to help.

Package Snapshots

A Package Snapshot contains package and file metadata from a set of Fuchsia packages, and two package snapshots can be compared to simulate updating from one snapshot of packages to another.

Within the Platform Source Tree, a build automatically produces a package snapshot of all products and packages enabled by fx set. The automatically generated snapshot is stored in the output directory at obj/build/images/system.snapshot. Outside of the Platform Source Tree, snapshots can be built from a set of packages using fx snapshot.

To manually produce a package snapshot file:

  1. When building a Fuchsia package with pm build, pass in an -blobsfile argument to also generate a metadata file of all files within the package.

    Sample package metadata files are located at examples/snapshots/(source|target)/pkg_*.json

  2. Merge one or more package metadata files into a single package snapshot using pm snapshot. Packages within a snapshot can be tagged with arbitrary strings. When comparing two snapshots, these tags can be used to select or exclude certain packages.

    Using the package metadata files under examples/snapshots/source as an example:

    pm snapshot \
        -package "a=examples/snapshots/source/pkg_a.json" \
        -package "b#optional=examples/snapshots/source/pkg_b.json" \
        -output "examples/snapshots/source/packages.snapshot"


    Create a manifest file containing a package entry per line. Then, provide the manifest to pm snapshot:

    pm snapshot \
        -manifest "examples/snapshots/source/packages.manifest" \
        -output "examples/snapshots/source/packages.snapshot"

The contents of the package snapshot for the packages at examples/snapshots/source/pkg_*.json are included below. Notice how the package entries list a total of 7 files, but the blobs list only contains 6 files. The blob for lib/ is shared between a and b.

  "packages": {
    "a": {
      "files": {
        "bin/app": "aa02000000000000000000000000000000000000000000000000000000000000",
        "img/logo.png": "aa03000000000000000000000000000000000000000000000000000000000000",
        "lib/": "ab00000000000000000000000000000000000000000000000000000000000000",
        "meta/": "aa01000000000000000000000000000000000000000000000000000000000000"
    "b": {
      "files": {
        "bin/app": "bb02000000000000000000000000000000000000000000000000000000000000",
        "lib/": "ab00000000000000000000000000000000000000000000000000000000000000",
        "meta/": "bb01000000000000000000000000000000000000000000000000000000000000"
      "tags": [
  "blobs": {
    "aa01000000000000000000000000000000000000000000000000000000000000": {
      "size": 20480
    "aa02000000000000000000000000000000000000000000000000000000000000": {
      "size": 2686144
    "aa03000000000000000000000000000000000000000000000000000000000000": {
      "size": 1000000
    "ab00000000000000000000000000000000000000000000000000000000000000": {
      "size": 846896
    "bb01000000000000000000000000000000000000000000000000000000000000": {
      "size": 20480
    "bb02000000000000000000000000000000000000000000000000000000000000": {
      "size": 2686144

Using this package snapshot as a baseline, we can now compare it to another package snapshot (target) to show what needs to happen to update from source to target.

pm delta examples/snapshot/source/packages.snapshot examples/snapshot/target/packages.snapshot

Which produces the following output:

Source size: 6.9 MiB
Target size: 8.8 MiB
Discard size: 3.6 MiB
Keep size: 3.4 MiB
Download size: 5.5 MiB

Top 2 packages with largest update size:

Discard  Keep     Download  Name
2.6 MiB  827 KiB  4.5 MiB   b
                  4.5 MiB   - bin/app
                  20 KiB    - meta/
997 KiB  3.4 MiB  997 KiB   a
                  977 KiB   - img/logo.png
                  20 KiB    - meta/

Top 4 largest new blobs:

Download  Name
4.5 MiB   b/bin/app
977 KiB   a/img/logo.png
20 KiB    a/meta/
20 KiB    b/meta/

In the above example, b's application binary would be replaced with a new version, and a's logo asset would be updated as well. When a package is modified in any way, its meta FAR (represented by the “meta/” directory) would also change. “Discard size” represents the total size of all blobs that can be garbage collected after the update completes. “Keep size” represents the total size of all blobs that continue to be referenced by one or more packages after the update. Finally, “Download size” represents the total size of all blobs that need to be downloaded to complete the update.

Note that the total download size is not always the sum of each package's download size. If a blob that is shared between multiple packages (or even referenced more than once within a single package) is updated, it only needs to be downloaded once but will appear as an update to each package that references the blob. Similar concepts apply to the discard and keep sizes.

pm delta has various options to customize its output. See pm delta --help for more info. For example, pm delta can output its statistics in JSON format. This output format is intended to be parsed by tooling for further processing or rendering. An example of this output is at examples/snapshots/delta.json.