RFC-0115: Build Types

RFC-0115: Build Types
  • System

A canonical definition of product build types.

Gerrit change
Date submitted (year-month-day)2021-07-06
Date reviewed (year-month-day)2021-07-21


This document describes the different product build types for Fuchsia: eng, userdebug, and, user.

A build type in Fuchsia refers to build time and runtime settings for a product configuration:

  • Build time settings include packages, tools, signing configuration, and flags that define system behavior.

  • Runtime settings include flags passed to the running kernel or software modulating runtime behavior based on the build type.

The intent of this document is to formally ratify the design decisions that have been made for Fuchsia products that are already using these build types. It also serves as a formal definition for the different build types in the Fuchsia platform.


Fuchsia defines several build types for use by a diverse set of users that include developers, testers, and end-users. Each of these user groups has certain expectations about the behavior of a running Fuchsia system.

A specific build type codifies and guarantees certain Fuchsia system behavior for different use cases such as development or a product shipped to end-users.

Furthermore, build time decisions, such as inclusion of a certain package flavor can be made based on the product build type. See the Package Flavors section for more details and an example.

Thus, the different build types provide the necessary flexibility for the Fuchsia platform and its users supporting the full lifecycle of a Fuchsia product.


This section outlines the current common convention of build types on Fuchsia. It details the different attributes of each type and the runtime constraints where applicable.

A constraint is also kept in mind around the number of build types that are defined due the complexity and implementation cost. See the Drawbacks section for more details.

For the purposes of Fuchsia's current requirements, there are three build types:

  • user
  • userdebug
  • eng

The build types are further explained in the Definitions section below.

Additional build types will require a proposal that goes through the RFC process and is approved by the Fuchsia Engineering Council (FEC).


The primary goals for the design are:

  • Formalize a common convention for build types that supports developer needs as well as final product shipping requirements.
  • Build types are implemented to be as similar as possible to avoid additional divergence in the behavior of the product.
  • Build types are released and qualified with the same criteria and cadence.



The user build type is used in production devices that are shipped to end-users.

For user builds, the platform enforces the highest security guarantees at runtime and only includes necessary components for the product during build time.


The userdebug build type behaves the same as user by default but allows for additional debug and instrumentation behavior.

The userdebug builds are primarily used in development devices for end-user product testing and qualification.


The eng build type is primarily for developers and testers and boots to the full product experience as defined by the product session.

The eng builds of various product configurations are available in-tree, and as prebuilt system images alongside the Fuchsia SDK.

Product Configuration

The build types are defined as product specific configurations. To maintain similarity across build types, the configurations inherit and include from the core product of Fuchsia and selectively add or remove packages and configurations.

The build type relationships can be summarized as such:

Build types. Notes below.

Figure 1: Build type as product configurations.


  • The eng build type inherits core and can include additional packages that are specific to the product.

  • The user build type directly inherits core but removes any extra packages, debug capabilities, and other instrumentation properties.

  • The userdebug then inherits user and is only slightly different to enable additional capabilities required for debugging and instrumentation.

  • The reverse inheritance between userdebug and user ensures that any changes to user that are required for the end-user product are automatically reflected in userdebug builds.

  • The design roughly reflects the product cycle, with eng being the first product configuration defined that is built on top of the core Fuchsia product. Then, when the product goes further along its lifecycle, userdebug and user are defined to reflect those use cases and requirements.


The section below documents the implementation details for each build type as organized in the major areas of the platform.


Zircon is the core platform that powers Fuchsia and is composed of the kernel and a small set of userspace services, drivers, and libraries.

There are several different assert-like mechanism used throughout the code base. There are kernel and usermode asserts which are enabled or disabled depending on the build type. As an example, ZX_ASSERT is always on for all build types and must be cheaper to avoid impacting performance.

In another example, assert_level is 2 in eng builds and enables all asserts while this is 0 in userdebug and user builds, which disables standard C assert() and ZX_DEBUG_ASSERT.

The following table outlines the asserts in the kernel:

Attribute eng userdebug, user
ASSERT on on
assert() on off

Recovery Partition

A Fuchsia end-user product uses an A/B/R update and recovery scheme.

In summary, the system updates via an Over-The-Air (OTA) mechanism, leverages the A/B update scheme in which two copies of the system are present(one active and the other inactive). This ensures that a system has a fallback in case of a failed update. If however, the fallback is not able to boot, the system uses the recovery image stored in the recovery partition in an attempt to recover the system.

Each product can define their own recovery image. By default, in eng builds, zedboot is used. Since zedboot is used for paving a Fuchsia system, it makes available a debug port and other facilities to recover the system.

In userdebug and user a different recovery image is used that is minimal and meets security requirements of an end-user product.

Attribute eng userdebug, user
Recovery (R) zedboot minimal recovery image

Packages and Updates

Auto Update of Packages

In the eng build type, auto_update_packages is enabled by default. Thus, when resolving a component (i.e., via running the component), an attempt to update the component's package is first made through fuchsia.pkg.PackageResolver.

In day-to-day development, this facility is useful to ensure the system is always running the latest components from the developer's environment.

For userdebug and user builds this feature is disabled. Any changes to the components and packages must be done through a System Update, which changes packages in the base system and triggers a reboot.

Attribute eng userdebug, user
auto_update_packages true false

Package Flavors

Package flavors are used to specify an eng flavor of a package that then includes an eng flavor of the component. The package is then included in the product configuration that defines the eng build type.

For example:

package_flavor_selections += [
    name = "my_component_pkg"
    flavor = "eng"

Similarly, a user flavor of the package is included in the user build for the product.

System Update

Fuchsia uses an Over-The-Air (OTA) update mechanism for operating system updates. There are two entry points, the omaha-client or the system-update-checker components:

Attribute eng userdebug, user
Auto System OTA false true
Update Checker system-update-checker omaha-client
Omaha Configuration n/a defined

Omaha Configuration defines a server side configuration for performing system updates which is suitable for end-user builds such as userdebug and user.

The Omaha configuration contains information that is stored in the VBMeta to ensure the full root of trust of the software is verified before execution.

Package Sets, Runnable Components, and Allowlists

Fuchsia boards and products definitions augment three lists of dependencies, Base, Cache and Universe. Thus, the packages that are defined within these sets determine how they are managed in Fuchsia.

For example, changes to packages in the base set are only possible via a system OTA update. Thus, requires a reboot of the system.

Since the universe set allows for access to the entire Fuchsia software set, it is not allowed in user builds.

Attribute eng userdebug user
Available Sets base, cache, universe base, cache, universe base
Allowlist all packages allowed base + allowlist only base

An allowlist is a policy that determines which component can be run (based on the component URL) or which component can access certain services at runtime. This is useful to ensure user build, for example, does not run a development component or service meant for eng builds.

In another example, the userdebug build allows for a limited set of tools to run for inspecting the system such as iquery to inspect properties of the running components.

In user, only the software required for the running of the product is allowed. In the current design, this software is restricted to what is included in the base set.

Software Sources

Fuchsia software is delivered through packages that are hosted in a software repository. These software sources must be known to Fuchsia and are verified cryptographically through a chain of trust.

For eng builds, there are no restrictions to which software sources can be added to the system at runtime.

For userdebug builds, software sources can only be added when the target system is connected via a direct interface to the developer's host workstation.

The software sources in user builds are fixed and unmodifiable.

Debug, Logs, and Shell

eng builds enable full access to the Fuchsia system and make available all debug ports and logs. user builds restrict all access and logs while userdebug looks to enable only limited access for debugging purposes.

Attribute eng userdebug user
ssh enabled enabled disabled
serial enabled r/o bootloader and kernel r/o in bootloader
debug_agent enabled disabled disabled
k commands enabled disabled disabled
ffx runtime enabled enabled disabled

Crash Reporting

Crash reporting services are available and provided by the fuchsia.feedback API. By default, crash upload is disabled for eng builds. For user and userdebug builds, user consent must be explicitly provided to enable crash report upload.

Crash reports are still captured and stored in the local crash report database by default for all builds.

Attribute eng userdebug, user
Crash reporting enabled enabled
Crash upload disabled allowed on user consent

Crash uploads includes system and kernel log files, and component inspect data to help with issue triage. These data files can contain Personal Identifiable Information (PII) and are scrubbed before upload.


The Fuchsia platform provides a service to log, collect, and analyze metrics. This service is provided by fuchsia.metrics. The two main pillars of Cobalt are protecting user privacy and providing high-quality, aggregate metrics to serve the system and component software developers' needs.

Attribute eng userdebug, user
Metrics collection enabled enabled
Metrics upload enabled allowed on user consent

Verified Execution

Verified execution is a central design of Fuchsia security and ensures that all executed code is trustworthy. Trustworthiness is recursively asserted through the verification of hashes and the verification of digital signatures from trustworthy entities, beginning with an immutable trust anchor.

Verified Boot

Verified boot is a phase of verified execution in which the bootloader validates that the Fuchsia zbi is trusted for execution by the bootloader.

Local builds (whether eng, userdebug, or user) use an in-tree development key, while builds produced for release are signed by more secure keys obtained from a secure key management service.

Prebuilt System Images

Prebuilt eng images are available in the Fuchsia SDK while userdebug and user builds are downloadable directly from Fuchsia Global Integration builders.

Build Time Optimizations

General build optimizations flags for the Fuchsia platform are kept the same across all build types to maintain alignment across the builds.

In eng builds, is_debug is set to true which sets the default flags to debug instead of speed as set in user and userdebug builds. The flags are carried and used in various configurations in the global BUILD.gn file.

For components, eng package flavors can include components that are compiled using special flags. For example, DCHECK assertions are often enabled in developer builds and bot configurations to help catch unexpected issues in C++ components.


It is understood that eng builds may induce performance penalties when compared to userdebug builds, though the actual penalty varies by product and board type. The primary impact comes from enabling extra assertions in the platform. This is tested using /zircon/system/ulib/perftest.

In components, eng flavors that enable DCHECK also will result in a significant performance impact when these components are heavily used during benchmarking and testing.

There is very little performance difference between userdebug and user builds. When benchmarking performance, prefer a userdebug build over an eng build. There are drawbacks to this approach which are noted in the Drawbacks section below.

Security considerations

Security implications were heavily considered in the design of Fuchsia build types. The following approaches are used throughout to enforce highest level of security guarantees for all users:

  • Leveraging verified execution to establish a full verified trust chain from boot with separate signing keys for cryptographic signatures.

  • Leveraging allowlist security policies in the Component Framework to ensure only the allowed components are resolved and executed.

  • Only including the necessary packages and components for end-user builds.

  • Disabling all access ports and facilities in end-user builds.

Privacy considerations

Privacy considerations were also heavily considered in the design of Fuchsia build types. All logs on end-user builds are scrubbed for PII and user consent MUST be provided to upload crash, logs, and metrics.


The build types will be further documented in Fuchsia > Concepts > Build System so there is a canonical reference for the different attributes and behaviors as they apply to products in Fuchsia.



The simplest and most straightforward way to test the behavior of different build types is by leveraging the Fuchsia Emulator.

Emulating the build types with the emulator allows the developer to compare and test different behaviors of the platform and applications. The downside is testing certain areas of the system such as system updates or verified boot properties is currently not possible in an emulator environment.

Release Process

Fuchsia leverages a CI (Continuous Integration) and CQ (Commit Queue) pipeline that ensures code changes are built and tested before a CL (changelist) has landed and during final integration when the CL has merged.

  • A CQ pipeline, builds and tests a CL across different product configurations and environments before the CL lands.

  • A CI pipeline, builds and tests code that have been checked in at the same integration commit, across different product configurations.

Since build types are also implemented as product configurations, all build type configurations are built, tested, and released at the same time via the same CI/CQ pipeline.

Note that only the eng build type product configurations are tested as part of CI/CQ. However, user and userdebug are still part of the continuous build process.


Testing frameworks and facilities are available in the universe set in eng and userdebug builds. These services are not enabled by default but are allowlisted for resolution and execution at runtime.

For user builds, testing facilities are not enabled nor allowed by security policies.

For example, Fuchsia has the SL4F framework for end-to-end tests and test_runner for Component Framework testing.

For automated testing use cases, the above frameworks can be used. But if additional packages or additional tools are required, the use of eng builds is recommended.

Drawbacks, alternatives, and unknowns

The design documented in this RFC has been implemented and reflects the current state of Fuchsia product build types.

By leveraging product configurations to implement build types, existing implementations could be leveraged and re-used. The primary drawback is the lack of scalability and flexibility to this approach. For example, for each new product, a minimum of three product configurations matching the three build types is required. There is cost associated with this approach in the redundancy of the implementation and the extra resources required in the Fuchsia infrastructure to spin up builders for these new configurations.

An additional drawback includes testing of userdebug builds. Since userdebug builds include specific flavors of packages, different system update configurations, and do not enable debug flags, they are desirable for running end-to-end tests. But due to security restrictions, it is not possible to run SL4F in official userdebug builds.

As an alternative, the Standalone System Assembly provides the ability to modify assembled images and it can be leveraged to assemble local userdebug builds that allow access to test frameworks and are less restrictive.

In the future, we should leverage a more scalable approach that allows eng, userdebug, and user build type profiles to be applied across the different product configurations. This avoids a linear scaling problem in which new products require multiple product configurations to accommodate build types.

It is also worth noting the current implementation blends product and platform, which is not the long term desire of the Fuchsia platform. As discussed, it is advisable to consider a more flexible design in the future.

Prior art and references

The Android operating system:

  • Defines eng, userdebug, and user builds.
  • Published guidelines for best practices in modifying userdebug builds.