Introduction to A/B Seamless Updates

Android’s A/B (Seamless) System Updates, introduced with Android 7.0 Nougat, revolutionized how devices receive software updates. Unlike traditional A-only updates which require a dedicated recovery partition and downtime during installation, A/B updates allow for continuous operation of the device while an update is being downloaded and installed in the background. This mechanism significantly enhances user experience by minimizing downtime and reducing the risk of a bricked device due to update failures.

The core concept behind A/B updates is the presence of two identical sets of system partitions, typically referred to as ‘slot A’ and ‘slot B’. While the user is actively running the system from one slot (e.g., slot A), the update is downloaded and installed onto the inactive slot (slot B). Once the installation is complete, the user simply reboots the device, and it switches to the newly updated slot B. If any issue arises, the device can easily revert to the previous working slot A, offering a robust fallback mechanism.

The A/B Mechanism Deep Dive

At a more technical level, A/B updates involve partitioning schemes that duplicate critical system partitions. These include boot, system, vendor, product, and sometimes others, each with a slot suffix (e.g., boot_a, boot_b). The device’s bootloader, governed by the boot_control Hardware Abstraction Layer (HAL), determines which slot is active. When an update arrives, the Android OS uses the update_engine to apply the OTA package to the inactive slot.

This process typically involves delta updates, where only the changes between the current and target system versions are applied, saving bandwidth and time. The update engine reconstructs the new system on the inactive slot without interfering with the active system. Only upon a successful installation and reboot is the boot_control HAL instructed to switch the active slot, making the newly updated system operational.

Challenges for Rooted Devices

While A/B updates are a boon for average users, they pose unique challenges for the rooted community. Traditional methods of flashing custom ROMs, kernels, or Magisk often involve direct manipulation of system partitions, usually through a custom recovery like TWRP. With A/B partitions, the complexity increases:

• TWRP Compatibility: Not all TWRP versions fully support A/B partitioning schemes, making direct flashing to specific slots difficult or impossible.
• Magisk Integration: Magisk typically patches the boot.img of the active slot. When an A/B update applies to the inactive slot, the root is lost on the new slot. Re-rooting requires booting into the new system, then patching its boot.img.
• Direct OTA Flashing: Applying official OTAs directly often fails on rooted devices due to modifications. Even if it succeeds, it will overwrite the inactive slot, leading to a loss of root, and potentially causing issues if Magisk modules modified system files.

For these reasons, rooted users often prefer a ‘force-flashing’ method, manually extracting and flashing critical partitions to the inactive slot, providing greater control over the update process and allowing for subsequent re-rooting.

Prerequisites for Force-Flashing A/B Updates

Before proceeding, ensure you have the following:

• An Android device with an unlocked bootloader.
• ADB and Fastboot tools installed and configured on your computer.
• Python 3 installed on your computer.
• The full OTA package (ZIP file) for your specific device and region. This is crucial as delta updates can cause issues with manual flashing.
• The payload_dumper.py script, available from various GitHub repositories (e.g., Google’s own sources or popular Android modding forums). This script is used to extract individual partition images from the payload.bin file found within the OTA package.
• Magisk ZIP file for your desired root solution.

Step-by-Step Guide: Force-Flashing A/B Updates

Step 1: Determine Your Current Active Slot

First, identify which slot your device is currently booting from. This will tell you which slot is ‘active’ and, consequently, which slot is ‘inactive’ for the update.

adb shell getprop ro.boot.slot_suffix

The output will typically be _a or _b. If it’s _a, your inactive slot is _b, and vice-versa. We will flash the update to the inactive slot.

Step 2: Download and Extract `payload.bin`

Download the full OTA package (usually a ZIP file) for the desired Android version update. Once downloaded, extract its contents. Inside, you will find a file named payload.bin (and often payload_properties.txt). Place both these files in a convenient folder on your computer, preferably where your ADB/Fastboot executables are located.

Step 3: Extract Partitions from `payload.bin`

Use the payload_dumper.py script to extract individual partition images from payload.bin. Ensure the script is in the same directory as payload.bin. Open a command prompt or terminal in that directory and run:

python payload_dumper.py payload.bin

This command will extract various .img files (e.g., boot.img, system.img, vendor.img, product.img, dtbo.img) into a new subdirectory named extracted_payload or similar. These are the images we will flash.

Step 4: Boot into Fastboot Mode

Reboot your Android device into Fastboot mode. The method varies by device, but it often involves powering off and then holding down the Volume Down + Power buttons simultaneously. Connect your device to your computer via USB.

adb reboot bootloader

Step 5: Flash Extracted Images to the Inactive Slot

Now, we will flash the extracted images to the inactive slot determined in Step 1. Let’s assume your active slot is _a, making _b the inactive slot for flashing. You need to explicitly tell Fastboot to target the inactive slot by appending its suffix.

First, explicitly set the target slot to be active (this will be the slot you boot into after flashing and rebooting):

fastboot --set-active=b

Now, flash the images. Remember to specify the slot suffix for each partition. You must flash all critical partitions that were extracted, such as boot, system, vendor, product, and dtbo. Do NOT flash super.img; instead, flash the partitions that comprise super individually.

fastboot flash boot_b boot.imgfastboot flash dtbo_b dtbo.imgfastboot flash system_b system.imgfastboot flash vendor_b vendor.imgfastboot flash product_b product.img

Repeat the fastboot flash [partition_name]_[slot_suffix] [image_file.img] command for any other relevant partitions extracted by payload_dumper.py.

Step 6: Reboot to the Newly Flashed Slot

Once all necessary partitions have been flashed to the inactive slot, reboot your device. Because you used fastboot --set-active=b, your device should now boot into the updated slot.

fastboot reboot

The first boot after an update can take longer than usual. Be patient.

Step 7: Re-root with Magisk (Optional)

After successfully booting into the updated system, you will have lost root. To re-root:

1. If you have a custom recovery like TWRP (and it supports your A/B device and the new Android version), boot into it and flash the Magisk ZIP.
2. Alternatively, if you don’t use TWRP or it’s incompatible:
   • Extract the boot.img from the extracted_payload folder you created earlier.
   • Copy this boot.img to your device’s internal storage.
   • Install the Magisk app APK on your device.
   • Open Magisk, tap ‘Install’ next to Magisk, choose ‘Select and Patch a File’, and select the boot.img you copied.
   • Magisk will create a magisk_patched_[random_string].img file, usually in your device’s Download folder.
   • Copy this patched boot.img back to your computer.
   • Reboot your device into Fastboot mode.
   • Flash the patched boot.img to your currently active slot (e.g., fastboot flash boot_b magisk_patched.img).
   • Reboot your device: fastboot reboot.

Upon reboot, open the Magisk app to confirm root access.

Important Considerations and Troubleshooting

• Backup Everything: Always perform a full backup of your device before attempting any flashing procedures.
• Correct Files: Ensure you use the exact full OTA package for your device model and current software variant. Using incorrect files can lead to a hard brick.
• Partition Naming: Pay close attention to partition names and slot suffixes. A typo can flash an image to the wrong location.
• Factory Reset: While not always necessary, if you encounter persistent issues after an update, a factory reset might resolve them. This will wipe your user data.
• Super Partitions: Remember that modern Android devices often use dynamic partitions within a super partition. Do not attempt to flash a super.img directly; instead, flash the individual partitions (system, vendor, product, etc.) that reside within it. The payload_dumper.py script correctly extracts these individual images.

By following this detailed guide, you can confidently manage A/B updates on your rooted Android device, maintaining control over your system while benefiting from the latest software improvements.