diagrams and simulation

diagrams/plantuml/ab-storage-layout.puml

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+@startuml
+title A/B storage: logical areas and access responsibilities
+left to right direction
+skinparam shadowing false
+skinparam monochrome true
+
+actor "Installer" as installer
+actor "Boot control" as boot
+actor "OTA manager /\nconfirmation program" as manager
+actor "Running system\nand applications" as runtime
+
+rectangle "Device storage (logical areas, not a partition table)" {
+  rectangle "Shared boot components\nSeparate update and recovery method" as shared
+  database "Boot state\nNormal slot, trial target, attempts, confirmation" as state
+  rectangle "Slot A: retained system\nKernel + device tree + root filesystem" as A
+  rectangle "Slot B: update target\nKernel + device tree + root filesystem" as B
+  database "Persistent device data" as data {
+    rectangle "Identity and calibration\nRestricted access" as identity
+    rectangle "Configuration and application data\nMust support the retained version" as appdata
+    rectangle "OTA progress, failure blocks\nand pending reports" as progress
+  }
+  rectangle "Download and work area\nDisposable preparation files" as work
+}
+
+installer --> B : replace inactive slot
+installer --> work : use temporary files
+boot --> shared : execute
+boot --> state : read / save before Linux starts
+boot --> A : select matching components
+boot --> B : select matching components
+manager --> state : request trial / confirm\nthrough platform interface
+manager --> progress : read / save
+runtime --> appdata : read / write
+runtime --> identity : authorized access only
+
+legend bottom
+  A is running while B is prepared. Preserve A, boot state and device data.
+  Persistent device data survives replacement of either system slot.
+  Shared data is not rolled back by selecting A.
+  Both system versions must understand the recovery records they read.
+  Separate logical areas do not guarantee safe writes or whole-device recovery.
+endlegend
+@enduml