Implementing A/B Updates

OEMs and SoC vendors who want to implement A/B system updates must ensure their bootloader implements the boot_control HAL and passes the correct parameters to the kernel.

Implementing the boot control HAL

A/B-capable bootloaders must implement the boot_control HAL at hardware/libhardware/include/hardware/boot_control.h. You can test implementations using the system/extras/bootctl utility and system/extras/tests/bootloader/.

You must also implement the state machine shown below:

Figure 1. Bootloader state machine

Setting up the kernel

To implement A/B system updates:

  1. Cherrypick the following kernel patch series (if needed):
  2. Ensure kernel command line arguments contain the following extra arguments:
    skip_initramfs rootwait ro init=/init root="/dev/dm-0 dm=system none ro,0 1 android-verity <public-key-id> <path-to-system-partition>"
    ... where the <public-key-id> value is the ID of the public key used to verify the verity table signature (for details, see dm-verity).
  3. Add the .X509 certificate containing the public key to the system keyring:
    1. Copy the .X509 certificate formatted in the .der format to the root of the kernel directory. If the .X509 certificate is formatted as a .pem file, use the following openssl command to convert from .pem to .der format:
      openssl x509 -in <x509-pem-certificate> -outform der -out <x509-der-certificate>
    2. Build the zImage to include the certificate as part of the system keyring. To verify,check the procfs entry (requires KEYS_CONFIG_DEBUG_PROC_KEYS to be enabled):
      angler:/# cat /proc/keys
      1c8a217e I------     1 perm 1f010000     0     0 asymmetri
      Android: 7e4333f9bba00adfe0ede979e28ed1920492b40f: X509.RSA 0492b40f []
      2d454e3e I------     1 perm 1f030000     0     0 keyring
      .system_keyring: 1/4
      Successful inclusion of the .X509 certificate indicates the presence of the public key in the system keyring (highlight denotes the public key ID).
    3. Replace the space with # and pass it as <public-key-id> in the kernel command line. For example, pass Android:#7e4333f9bba00adfe0ede979e28ed1920492b40f in place of <public-key-id>.

Setting build variables

A/B-capable bootloaders must meet the following build variable criteria:

Must define for A/B target
  • AB_OTA_UPDATER := true
      boot \
      system \
    and other partitions updated through update_engine (radio, bootloader, etc.)
      update_engine \
For an example, refer to /device/google/marlin/+/android-7.1.0_r1/ You can optionally conduct the post-install (but pre-reboot) dex2oat step described in Compiling.
Cannot define for A/B target
Optional for debug builds PRODUCT_PACKAGES_DEBUG += update_engine_client

Setting partitions (slots)

A/B devices do not need a recovery partition or cache partition because Android no longer uses these partitions. The data partition is now used for the downloaded OTA package, and the recovery image code is on the boot partition. All partitions that are A/B-ed should be named as follows (slots are always named a, b, etc.): boot_a, boot_b, system_a, system_b, vendor_a, vendor_b.


For non-A/B updates, the cache partition was used to store downloaded OTA packages and to stash blocks temporarily while applying updates. There was never a good way to size the cache partition: how large it needed to be depended on what updates you wanted to apply. The worst case would be a cache partition as large as the system image. With A/B updates there's no need to stash blocks (because you're always writing to a partition that isn't currently used) and with streaming A/B there's no need to download the whole OTA package before applying it.


The recovery RAM disk is now contained in the boot.img file. When going into recovery, the bootloader cannot put the skip_initramfs option on the kernel command line.

For non-A/B updates, the recovery partition contains the code used to apply updates. A/B updates are applied by update_engine running in the regular booted system image. There is still a recovery mode used to implement factory data reset and sideloading of update packages (which is where the name "recovery" came from). The code and data for recovery mode is stored in the regular boot partition in a ramdisk; to boot into the system image, the bootloader tells the kernel to skip the ramdisk (otherwise the device boots into recovery mode. Recovery mode is small (and much of it was already on the boot partition), so the boot partition doesn't increase in size.


The slotselect argument must be on the line for the A/B-ed partitions. For example:

<path-to-block-device>/vendor  /vendor  ext4  ro

No partition should be named vendor. Instead, partition vendor_a or vendor_b will be selected and mounted on the /vendor mount point.

Kernel slot arguments

The current slot suffix should be passed either through a specific device tree (DT) node (/firmware/android/slot_suffix) or through the androidboot.slot_suffix command line argument.

By default, fastboot flashes only slot a on an A/B device and sets the current slot to a. If the update package also contains images for slot b, fastboot flashes those images as well. Available options include:

  • --slot. Prompt fastboot to use slot b instead of slot a.
  • --set-active. Set the slot as active.
  • fastboot --help. Get details on commands.

If the bootloader implements fastboot, it should support the command set_active <slot> that sets the current active slot to the given slot (this must also clear the unbootable flag for that slot and reset the retry count to default values). The bootloader should also support the following variables:

  • has-slot:<partition-base-name-without-suffix>. Returns “yes” if the given partition supports slots, “no” otherwise.
  • current-slot. Returns the slot suffix that will be booted from next.
  • slot-count. Returns an integer representing the number of available slots. Currently, two slots are supported so this value is 2.
  • slot-successful:<slot-suffix>. Returns "yes" if the given slot has been marked as successfully booting, "no" otherwise.
  • slot-unbootable:<slot-suffix>. Returns “yes” if the given slot is marked as unbootable, "no" otherwise.
  • slot-retry-count. Number of retries remaining to attempt to boot the given slot.

To view all variables, run fastboot getvar all.

Generating OTA packages

The OTA package tools follow the same commands as the commands for non-A/B devices. The file must be generated by defining the build variables for the A/B target. The OTA package tools automatically identify and generate packages in the format for the A/B updater.


  • To generate a full OTA:
    ./build/tools/releasetools/ota_from_target_files \
  • To generate an incremental OTA:
    ./build/tools/releasetools/ota_from_target_files \
      -i \

Configuring partitions

The update_engine can update any pair of A/B partitions defined in the same disk. A pair of partitions has a common prefix (such as system or boot) and per-slot suffix (such as _a). The list of partitions for which the payload generator defines an update is configured by the AB_OTA_PARTITIONS make variable.

For example, if a pair of partitions bootloader_a and booloader_b are included (_a and _b are the slot suffixes), you can update these partitions by specifying the following on the product or board configuration:

  boot \
  system \

All partitions updated by update_engine must not be modified by the rest of the system. During incremental or delta updates, the binary data from the current slot is used to generate the data in the new slot. Any modification may cause the new slot data to fail verification during the update process, and therefore fail the update.

Configuring post-installation

You can configure the post-install step differently for each updated partition using a set of key-value pairs. To run a program located at /system/usr/bin/postinst in a new image, specify the path relative to the root of the filesystem in the system partition.

For example, usr/bin/postinst is system/usr/bin/postinst (if not using a RAM disk). Additionally, specify the filesystem type to pass to the mount(2) system call. Add the following to the product or device .mk files (if applicable):

  RUN_POSTINSTALL_system=true \
  POSTINSTALL_PATH_system=usr/bin/postinst \


For security reasons, system_server cannot use just-in-time (JIT) compilation. This means you must compile ahead of time odex files for system_server and its dependencies at a minimum; anything else is optional.

To compile apps in the background, you must add the following to the product's device configuration (in the product's

  1. Include the native components in the build to ensure compilation script and binaries are compiled and included in the system image.
      # A/B OTA dexopt package
      PRODUCT_PACKAGES += otapreopt_script
  2. Connect the compilation script to update_engine such that runs as a post-install step.
      # A/B OTA dexopt update_engine hookup
        RUN_POSTINSTALL_system=true \
        POSTINSTALL_PATH_system=system/bin/otapreopt_script \
        FILESYSTEM_TYPE_system=ext4 \

For help installing the preopted files in the unused second system partition, refer to First boot installation of DEX_PREOPT files.