Types of update mechanism

In this section, I will describe three approaches to applying software updates: symmetric, or A/B, image update; asymmetric image update, also known as recovery mode update; and finally, atomic file update.

Symmetric image update

In this scheme, there are two copies of the operating system, each comprising the Linux kernel, root filesystem, and system applications. They are labeled as A and B in the following diagram:

Figure 10.2 – symmetric image update

Symmetric image updates work as follows:

1. The bootloader has a flag that indicates which image it should load. Initially, the flag is set to A, so the bootloader loads OS image A.
2. To install an update, the updater application, which is part of the operating system, overwrites OS image B.
3. When complete, the updater changes the boot flag to B and reboots.
4. Now the bootloader will load the new operating system.
5. When a further update is installed...