Cobalt Evergreen Overview

Cobalt non-Evergreen vsEvergreen

What is Cobalt Evergreen?

Cobalt Evergreen is an end-to-end framework for cloud-based deployment of Cobalt updates without the need for supplemental Cobalt integration work on device platforms.

There are two configurations available:

  • Evergreen-Lite
    • Please read this document for general Evergreen details then see Evergreen-Lite specific configuration details in
  • Evergreen Full
    • Please continue reading below documentation for configuration details

Cobalt Evergreen Configurations

For a bit of background context, as the number of Cobalt devices in the field increases there is a growing proliferation of version fragmentation. Many of these devices are unable to take advantage of the benefits of Cobalt performance, security, and functional improvements as it is costly to integrate and port new versions of Cobalt. We recognized this issue, listened to feedback from the Cobalt community and as a result developed Cobalt Evergreen as a way to make updating Cobalt a much simpler process for everyone involved.

This relies on separating the Starboard(platform) and Cobalt(core) components of a Cobalt implementation into the following discrete components:

Google-built (on Google toolchain)

  • Cobalt Core
    • Pre-built shared library available for all supported architectures
  • Cobalt Updater
    • Part of Cobalt Core and used to query servers to check for and download updated Cobalt Core

Partner-built (on Partner toolchain)

  • Starboard
    • Platform-specific implementation
    • Contains system dependencies (e.g.,
  • Cobalt Loader (Loader App)
    • Selects the appropriate Cobalt core for usage
    • An ELF loader is used to load the Cobalt core and resolves symbols with Starboard APIs when Cobalt starts up in Evergreen mode

With this new Cobalt platform architecture, less engineering effort is necessary for a full Cobalt integration/deployment. The idea here is you should only need to implement Starboard one time and any Cobalt-level updates should only require platform testing. NOTE that certain new Cobalt features may require Starboard changes, so if you want to take advantage of some of these new features, Starboard changes may be necessary.

Main Benefits

  • More stable platform as there is less Cobalt version fragmentation
  • Little-to-no engineering effort necessary for Cobalt updates
  • Longer device lifetime due to more Cobalt updates
  • Less engineering work/accelerated timeline for Cobalt integration/deployment as Google builds the Cobalt components and partners are only responsible for the Starboard, loader_app, and crashpad_handler portion

New in Evergreen

  • Larger storage and system permissions requirements in order to update and store multiple Cobalt binaries
  • Access permissions to download binaries onto a device platform from Google servers
  • New loader_app and crashpad_handler components required to be built on platform toolchains
  • Additional testing/verification required to ensure new Cobalt releases work properly

How is Evergreen different from porting Cobalt previously?

There are minimal differences in switching to Evergreen as the Cobalt team has already done a majority of the work building the necessary components to support the Evergreen architecture. You will still be responsible for building the Starboard and platform-specific components as usual. Thereafter, switching to Evergreen is as simple as building a different configuration. Please see the Raspberry Pi 2 Evergreen reference port (Instructions) for an example.

Cobalt non-Evergreen vsEvergreen

Building Cobalt Evergreen Components

Cobalt Evergreen requires that there are two separate build configurations used due to the separation of the Cobalt core( and the platform-specific layers. As a result, you will have to run gen gn separately for each. This is required since the Cobalt core binary is built with the Google toolchain settings and the components in the platform-specific Starboard layer are built with partner toolchain configurations.

Cobalt Evergreen is built by a separate GN platform using the Google toolchain. For example:

$ gn gen out/evergreen-arm-softfp_qa --args="target_platform=\"evergreen-arm-softfp\" build_type=\"qa\" target_cpu=\"arm\" target_os=\"linux\" sb_api_version=14"
$ ninja -C out/evergreen-arm-softfp_qa cobalt_install

Which produces a shared library targeted for specific architecture, ABI and Starboard version.

The GN variable sb_is_evergreen is set to true when building

Finally, note that these instructions are for producing a local build of For submissions to YTS and deployments to users' devices, an official release build of should be downloaded from GitHub.

The partner port of Starboard is built with the partner’s toolchain and is linked into the loader_app which knows how to dynamically load, and the crashpad_handler which handles crashes.

$ gn gen out/<partner_port_name>_qa --args='target_platform="<partner_port_name>" build_type="qa" sb_api_version=14'
$ ninja -C out/<partner_port_name>_qa loader_app crashpad_handler

Partners should set sb_is_evergreen_compatible to true in their GN platform config. DO NOT set the sb_is_evergreen to true in your platform-specific configuration as it is used only by Cobalt when building with the Google toolchain.

Additionally, partners should install crash handlers as instructed in the Installing Crash Handlers for Cobalt guide.

The following additional Starboard interfaces are necessary to implement for Evergreen:

  • kSbSystemPathStorageDirectory
    • Dedicated location for storing Cobalt Evergreen-related binaries
    • This path must be writable and have at least 96MB of reserved space for Evergreen updates. Please see the “Platforms Requirements” section below for more details.
  • kSbMemoryMapProtectExec
    • Ensures mapped memory can be executed
    • Specifies that the platform can map executable memory
    • Defined in configuration_public.h

Only if necessary, create a customized SABI configuration for your architecture. Note, we do not anticipate that you will need to make a new configuration for your platform unless it is not one of our supported architectures:

  • x86_32
  • x86_64
  • arm32
  • arm64

If your target architecture falls outside the support list above, please reach out to us for guidance.

Adding Crash Handlers to Evergreen

What is an example for how this would help me?

Some common versions of Cobalt in the field may show a bug in the implementation of the CSS which can cause layout behavior to cause components to overlap and give users a poor user experience. A fix for this is identified and pushed to Cobalt open source ready for integration and deployment on devices.

Without Cobalt Evergreen:

Though a fix for this was made available in the latest Cobalt open source, affected devices in the field are not getting updated (e.g. due to engineering resources, timing, device end-of-life), users continue to have a poor experience and have negative sentiment against a device. In parallel, the web app team determines a workaround for this particular situation, but the workaround is obtuse and causes app bloat and technical debt from on-going maintenance of workarounds for various Cobalt versions.

With Cobalt Evergreen:

The Cobalt team can work with you to guide validation and deployment of a shared Cobalt library to all affected devices much more quickly without all the engineering effort required to deploy a new Cobalt build. With this simpler updating capability, device behavior will be more consistent and there is less technical debt from workarounds on the web application side. Additionally, users can benefit from the latest performance, security, and functional fixes.

Platform Requirements

Cobalt Evergreen currently supports the following

Target Architectures:

  • x86_32
  • x86_64
  • armv7 32
  • armv8 64

Supported Javascript Engines

  • V8

Additional reserved storage (96MB) is required for Evergreen binaries. We expect Evergreen implementations to have an initial Cobalt preloaded on the device and an additional reserved space for additional Cobalt update storage.

  • Initial Cobalt binary deployment - 64MB
  • Additional Cobalt update storage - 96MB
    • Required for 2 update slots under kSbSystemPathStorageDirectory

As Cobalt Evergreen is intended to be updated from Google Cloud architecture without the need for device FW updates, it is important that this can be done easily and securely on the target platform. There are a set of general minimum requirements to do so:

  • Writable access to the file system of a device platform to download Cobalt Evergreen binaries
  • Enough reserved storage for Cobalt updates
  • Platform supporting mmap API with writable memory (PROT_WRITE, PROT_EXEC) for loading in-memory and performing relocations for Cobalt Evergreen binaries

Building and Running Tests

The elf_loader_sandbox binary can be used to run tests in Evergreen mode. This is much more lightweight than the loader_app, and does not have any knowledge about installations or downloading updates.

The elf_loader_sandbox is run using two command line switches: --evergreen_library and --evergreen_content. These switches are the path to the shared library to be run and the path to that shared library's content. These paths should be relative to the content of the elf_loader_sandbox.

For example, if we wanted to run the NPLB set of tests and had the following directory tree,


we would use the following command to run NPLB:

.../elf_loader_sandbox --evergreen_library=app/nplb/lib/

Building tests is identical to how they are already built except that a different platform configuration must be used. The platform configuration should be an Evergreen platform configuration, and have a Starboard ABI file that matches the file used by the platform configuration used to build the elf_loader_sandbox.

For example, building these targets for the Raspberry Pi 2 would use the raspi-2 and evergreen-arm-hardfp platform configurations.

Verifying Platform Requirements

In order to verify the platform requirements you should run the nplb_evergreen_compat_tests. These tests ensure that the platform is configured appropriately for Evergreen.

To enable the test, set the sb_is_evergreen_compatible GN variable to true in the platform's configuration.gni. For more details please take a look at the Raspberry Pi 2 GN files.

There is a reference implementation available for Raspberry Pi 2 with instructions available here.

Verifying Crashpad Uploads

  1. Build the crashpad_database_util target and deploy it onto the device.
$ cobalt/build/ -p <partner_port_name> -c qa
$ ninja -C out/<partner_port_name>_qa crashpad_database_util
  1. Remove the existing state for crashpad as it throttles uploads to 1 per hour:
$ rm -rf <kSbSystemPathCacheDirectory>/crashpad_database/

  1. Launch Cobalt.
  2. Trigger crash by sending abort signal to the loader_app process:
$ kill -6 <pid>
  1. Verify the crash was uploaded through running crashpad_database_util on the device pointing it to the cache directory, where the crash data is stored.
$ crashpad_database_util -d <kSbSystemPathCacheDirectory>/crashpad_database/ --show-completed-reports --show-all-report-info
  Path: cobalt/cache/crashpad_database/completed/8c3af145-30a0-43c7-a3a5-0952dea230e4.dmp
  Remote ID: c9b14b489a895093
  Creation time: 2021-06-01 17:01:19 HDT
  Uploaded: true
  Last upload attempt time: 2021-06-01 17:01:19 HDT
  Upload attempts: 1

In this example the minidump was successfully uploaded because we see Uploaded: true.

Reference for crashpad_database_util

System Design

Cobalt EvergreenComponents

The above diagram is a high-level overview of the components in the Cobalt Evergreen architecture.

  • Partner-built represents components the Partner is responsible for implementing and building.

  • Cobalt-built represents components the Cobalt team is responsible for implementing and building.

Cobalt Evergreen Components

Cobalt Updater

This is a new component in the Cobalt Shared Library component that is built on top of the Starboard API. The purpose of this module is to check the update servers if there is a new version of the Cobalt Shared Library available for the target device. If a new version is available, the Cobalt updater will download, verify, and install the new package on the target platform. The new package can be used the next time Cobalt is started or it can be forced to update immediately and restart Cobalt as soon as the new package is available and verified on the platform. This behavior will take into account the suspend/resume logic on the target platform.

Functionally, the Cobalt Updater itself runs as a separate thread within the Cobalt process when Cobalt is running. This behavior depends on what the target platform allows.

For more detailed information on Cobalt Updater, please take a look here.

Google Update (Update Server)

We use Google Update as the infrastructure to manage the Cobalt Evergreen package install and update process. This has been heavily used across Google for quite some time and has the level of reliability required for Cobalt Evergreen. There are also other important features such as:

  • Fine grained device targeting
  • Rollout and rollback
  • Multiple update channels (e.g. production, testing, development)
  • Staged rollouts

For more detailed information on Google Update for Cobalt Evergreen, please take a look here.

Google Downloads (Download Server)

We use Google Downloads to manage the downloads available for Cobalt Evergreen. The Cobalt Updater will use Google Downloads in order to download available packages onto the target platform. We selected Google Downloads for this purpose due to its ability to scale across billions of devices as well as the flexibility to control download behavior for reliability.

For more detailed information on Google Downloads (Download Server) for Cobalt Evergreen, please take a look here.

Cobalt Evergreen Interfaces

Starboard ABI

The Starboard ABI was introduced to provide a single, consistent method for specifying the Starboard API version and the ABI. This specification ensures that any two binaries, built with the same Starboard ABI and with arbitrary toolchains, are compatible.

Note that Cobalt already provides default SABI files for the following architectures:

  • x86_32
  • x86_64
  • arm v7 (32-bit)
  • arm v8 (64-bit)

You should not need to make a new SABI file for your target platform unless it is not a currently supported architecture. We recommend that you do not make any SABI file changes. If you believe it is necessary to create a new SABI file for your target platform, please reach out to the Cobalt team to advise.

For more detailed information on the Starboard ABI for Cobalt Evergreen, please take a look here.

Installation Slots

Cobalt Evergreen provides support for maintaining multiple, separate versions of the Cobalt binary on a platform. These versions are stored in installation slots(i.e. known locations on disk), and are used to significantly improve the resilience and reliability of Cobalt updates.

All slot configurations assume the following:

  • 1 System Image Installation Slot (read-only)
  • 2+ Additional Installation Slot(s) (writable)

The number of installation slots available will be determined by the platform owner. 3 slots is the default configuration for Evergreen. There can be N installation slots configured with the only limitation being available storage.

Slot Configuration

NOTE: 3-slots is the DEFAULT configuration.

The number of installation slots is directly controlled using kMaxNumInstallations, defined in

It is worth noting that all slot configurations specify that the first installation slot (SLOT_0) will always be the read-only factory system image. This is permanently installed on the platform and is used as a fail-safe option. This is stored in the directory specified by kSbSystemPathContentDirectory under the app/cobalt subdirectory.

All of the other installation slots are located within the storage directory specified by kSbSystemPathStorageDirectory. This will vary depending on the platform.

For example, on the Raspberry Pi the kSbSystemPathStorageDirectory directory is /home/pi/.cobalt_storage, and the paths to all existing installation slots will be as follows:

/home/pi/<kSbSystemPathContentDirectory>/app/cobalt (system image installation SLOT_0) (read-only)
/home/pi/.cobalt_storage/installation_1 (SLOT_1)
/home/pi/.cobalt_storage/installation_2 (SLOT_2)
/home/pi/.cobalt_storage/installation_N (SLOT_N)

Where the most recent update is stored will alternate between the available writable slots. In the above example, this would be SLOT_1...SLOT_N.

Understanding Slot Structure

Slots are used to manage Cobalt Evergreen binaries with associated app metadata to select the appropriate Cobalt Evergreen binaries.

See the below structures for an example 3-slot configuration.

Structure for kSbSystemPathContentDirectory used for the read-only System Image required for all slot configurations:

├── content <--(kSbSystemPathContentDirectory)
│   └── fonts <--(kSbSystemPathFontDirectory, `standard` or `limit` configuration, to be explained below)
│   └── app
│       └── cobalt <--(SLOT_0)
│           ├── content <--(relative path defined in kSystemImageContentPath)
│           │   ├── fonts <--(`empty` configuration)
│           │   ├── (icu) <--(only present when it needs to be updated by Cobalt Update)
│           │   ├── licenses
│           │   ├── ssl
│           ├── lib
│           │   └── <--(System image version of
│           └── manifest.json
└── loader_app <--(Cobalt launcher binary)
└── crashpad_handler <--(Cobalt crash handler)

Structure for kSbSystemPathStorageDirectory used for future Cobalt Evergreen updates in an example 3-slot configuration:

├── .cobalt_storage <--(kSbSystemPathStorageDirectory)
    ├── cobalt_updater
    │   └── prefs_<APP_KEY>.json
    ├── installation_1 <--(SLOT_1 - currently unused)
    ├── installation_2 <--(SLOT_2 - contains new Cobalt version)
    │   ├── content
    │   │   ├── fonts <--(`empty` configuration)
    │   │   ├── (icu) <--(only present when it needs to be updated by Cobalt Update)
    │   │   ├── licenses
    │   │   ├── ssl
    │   ├── lib
    │   │   └── <--(SLOT_2 version of
    │   ├── manifest.fingerprint
    │   └── manifest.json <-- (Evergreen version information of under SLOT_2)
    ├── installation_store_<APP_KEY>.pb
    └── icu (default location shared by installation slots, to be explained below)

Note that after the Cobalt binary is loaded by the loader_app, kSbSystemPathContentDirectory points to the content directory of the running binary, as stated in Starboard Module Reference of system.h.

App metadata

Each Cobalt Evergreen application has a set of unique metadata to track slot selection. The following set of files are unique per application via a differentiating <APP_KEY> identifier, which is a Base64 hash appended to the filename.


You should NOT change any of these files and they are highlighted here just for reference.


The system font directory kSbSystemPathFontDirectory should be configured to point to either the system fonts on the device or the Cobalt standard (23MB) or the Cobalt limited (3.1MB) font packages. An easy way to use the Cobalt fonts is to set kSbSystemPathFontDirectory to point to kSbSystemPathContentDirectory/fonts and configure cobalt_font_package to standard or limited in your port.

Cobalt Evergreen (built by Google), will by default use the empty font package to minimize storage requirements. A separate cobalt_font_package variable is set to empty in the Evergreen platform.

On Raspberry Pi the Cobalt fonts are configured the following way:

empty set of fonts under:


standard or limited set of fonts under:


ICU Tables

The ICU table should be deployed under the kSbSystemPathStorageDirectory. This way all Cobalt Evergreen installations would be able to share the same tables. The current storage size for the ICU tables is 7MB.

On Raspberry Pi this is:


The Cobalt Evergreen package will not carry ICU tables by default but may add them in the future if needed. When the package has ICU tables they would be stored under the content location for the installation:


Handling Pending Updates

Pending updates will be picked up on the next application start, which means that on platforms that support suspending the platform should check loader_app::IsPendingRestart and call SbSystemRequestStop instead of suspending if there is a pending restart.

Please see for an example.

Multi-App Support

Evergreen can support multiple apps that share a Cobalt binary. This is a very common way to save space and keep all your Cobalt apps using the latest version of Cobalt. We understand that there are situations where updates are only needed for certain apps, so we have provided a way where Cobalt Updater and loader_app behavior can be easily configured on a per-app basis with simple command-line flags.

The configurable options for Cobalt Updater configuration are:

  • --evergreen_lite Use the System Image version of Cobalt under Slot_0 and turn off the updater for the specified application.
  • --disable_updater_module Stay on the current version of Cobalt that might be the system image or an installed update, and turn off the updater for the specified application.

Each app’s Cobalt Updater will perform an independent, regular check for new Cobalt Evergreen updates. Note that all apps will share the same set of slots, but each app will maintain metadata about which slots are “good” (working) or “bad” (error detected) and use the appropriate slot. Sharing slots allows Evergreen to download Cobalt updates a single time and be able to use it across all Evergreen-enabled apps.

To illustrate, a simple example:

  • Cobalt v5 - latest Cobalt Evergreen version


[APP_1] (currently using SLOT_1, using Cobalt v4)
[APP_2] (currently using SLOT_0, using Cobalt v3)
[APP_3] (currently using SLOT_0, using Cobalt v3)

Now remember, apps could share the same Cobalt binary. Let’s say APP_1 has detected an update available and downloads the latest update (Cobalt v5) into SLOT_2. The next time APP_2 runs, it may detect Cobalt v5 as well. It would then simply do a request_roll_forward operation to switch to SLOT_2 and does not have to download a new update since the latest is already available in an existing slot. In this case, APP_1 and APP_2 are now using the same Cobalt binaries in SLOT_2.

If APP_3 has not been launched, not run through a regular Cobalt Updater check, or launched with the --evergreen_lite/--disable_updater_module flag, it stays with its current configuration.


[APP_1] (currently using SLOT_2, using Cobalt v5)
[APP_2] (currently using SLOT_2, using Cobalt v5)
[APP_3] (currently using SLOT_0, using Cobalt v3)

Now that we have gone through an example scenario, we can cover some examples of how to configure Cobalt Updater behavior and loader_app configuration.

Some example configurations include:

# All Cobalt-based apps get Evergreen Updates [APP_1] (Cobalt Updater ENABLED) [APP_2] (Cobalt Updater ENABLED) [APP_3] (Cobalt Updater ENABLED) loader_app --url="<YOUR_APP_1_URL>" loader_app --url="<YOUR_APP_2_URL>" loader_app --url="<YOUR_APP_3_URL>" # Only APP_1 gets Evergreen Updates, APP_2 disables the updater and uses an alternate splash screen, APP_3 uses # the system image and disables the updater [APP_1] (Cobalt Updater ENABLED) [APP_2] (Cobalt Updater DISABLED) [APP_3] (System Image loaded, Cobalt Updater DISABLED) loader_app --url="<YOUR_APP_1_URL>" loader_app --url="<YOUR_APP_2_URL>" --disable_updater_module \ --fallback_splash_screen_url="/<PATH_TO_APP_2>/app_2_splash_screen.html" loader_app --url="<YOUR_APP_3_URL>" --evergreen_lite # APP_3 is a local app, wants Cobalt Updater disabled and stays on the system image, and uses an alternate content # directory (This configuration is common for System UI apps. APP_3 in this example.) [APP_1] (Cobalt Updater ENABLED) [APP_2] (Cobalt Updater ENABLED) [APP_3] (System Image loaded, Cobalt Updater DISABLED) loader_app --url="<YOUR_APP_1_URL>" loader_app --url="<YOUR_APP_2_URL>" loader_app --csp_mode=disable --allow_http --url="file:///<PATH_TO_APP_3>/index.html" --content="/<PATH_TO_APP_3>/content" --evergreen_lite

Please see for full list of available command-line flags.

Platform Security

As Cobalt binary packages (CRX format) are downloaded from the Google Downloads server, the verification of the Cobalt update package is critical to the reliability of Cobalt Evergreen. There are mechanisms in place to ensure that the binary is verified and a chain of trust is formed. The Cobalt Updater is responsible for downloading the available Cobalt update package and verifies that the package is authored by Cobalt(and not an imposter), before trying to install the downloaded package.

Understanding Verification

Cobalt Evergreen CRXVerification

In the above diagram, the Cobalt Updater downloads the update package if available, and parses the CRX header of the package for verification, before unpacking the whole package. A copy of the Cobalt public key is contained in the CRX header, so the updater retrieves the key and generates the hash of the key coming from the header of the package, say Key hash1.

At the same time, the updater has the hash of the Cobalt public key hard-coded locally, say Key hash2.

The updater compares Key hash1 with Key hash2. If they match, verification succeeds.


Can I host the binaries for Cobalt core myself to deploy on my devices?

Not at this time. All Cobalt updates will be deployed through Google infrastructure. We believe Google hosting the Cobalt core binaries allows us to ensure a high-level of reliability and monitoring in case issues arise.

What is the performance impact of switching to Cobalt Evergreen?

We expect performance to be similar to a standard non-Evergreen Cobalt port.

How can I ensure that Cobalt updates work well on our platform?

Google will work closely with device partners to ensure that the appropriate testing is in place to prevent regressions.

Will there be tests provided to verify functionality and detect regression?

Yes, there are tests available to help validate the implementation:

  • NPLB tests to ensure all necessary APIs are implemented
  • Cobalt Evergreen Test Plan to verify the functionality of all components and use cases

How can I be sure that Cobalt space requirements will not grow too large for my system resources?

The Cobalt team is focusing a large amount of effort to identify and integrate various methods to reduce the size of the Cobalt binary such as compression and using less fonts.

What if something goes wrong in the field? Can we rollback?

Yes, this is one of the benefits of Evergreen. We can initiate an update from the server side that addresses problems that were undetected during full testing. There are a formal set of guidelines to verify an updated binary deployed to the device to ensure that it will work properly with no regressions that partners should follow to ensure that there are no regressions. In addition, it is also critical to do your own testing to exercise platform-specific behavior.

How can I be sure that Cobalt Evergreen will be optimized for my platform?

Much of the optimization work remains in the Starboard layer and configuration so you should still expect good performance using Cobalt Evergreen. That being said, the Cobalt Evergreen configuration allows you to customize Cobalt features and settings as before.