The Inactive OTA Slot Protects the Running Image

The dangerous shortcut here was to collapse distributed state into one field. The actual problem was that An update should be able to fail during download or write without destroying the last working firmware.

The system behind this series is LOUP's production-style OTA control plane: per-device identity, explicit release objects, compatibility metadata, assignment state, heartbeat observation, A/B application slots, signed artifacts, first-boot validation, append-only events and staged rollout. The point is not the exact API shape. It is the state discipline required when server, device and bootloader can each be correct locally while disagreeing about the fleet globally.

The evidence for this case was specific: The OTA-capable design uses otadata plus ota_0 and ota_1; the new image is written to the inactive slot before boot selection changes. I keep that claim tied to this implementation and policy rather than presenting it as a universal OTA benchmark.

The result I retained was: A failed transfer no longer had to consume the known-good boot target.

The state I was actually debugging

operator -> release state -> device assignment (desired)
                           |
                           v
device heartbeat ----> running_release_id (observed)
       |                   |
       |                   v
       +---- desired manifest if eligible/compatible
                           |
                      inactive OTA slot
                           |
                      reboot pending verify
                           |
                 local self-test -> accept / rollback
                           |
                  release-scoped event + heartbeat

The device-side updater is a transaction across flash and reboot. Download, inactive-slot write, boot selection, pending verification and final acceptance are separate commit points with different power-loss behavior.

I treated this as a causality problem. The symptom was An update should be able to fail during download or write without destroying the last working firmware. The strongest evidence was The OTA-capable design uses otadata plus ota_0 and ota_1; the new image is written to the inactive slot before boot selection changes. The underlying mechanism was A/B OTA separates the currently trusted executable from the candidate being written. That made the tempting shortcut—Overwriting the only runnable app partition in place.—unsafe. The retained result was A failed transfer no longer had to consume the known-good boot target.

The practical rule was: Write candidates beside the running image, not on top of it. That rule is more durable than any one endpoint or database column because it defines which component is allowed to claim which truth.

Reconstructing the transition

running slot A
   -> download/write slot B
   -> select B
   -> reboot: PENDING_VERIFY
   -> local diagnostics
      -> ACCEPT => B becomes known-good
      -> REJECT/crash => bootloader returns to A

I used one question to keep the model honest: What would the database say if the device never came back?

For this case, the answer starts with the observed problem: An update should be able to fail during download or write without destroying the last working firmware. The control plane already had evidence that The OTA-capable design uses otadata plus ota_0 and ota_1; the new image is written to the inactive slot before boot selection changes. That evidence only becomes useful when it is attached to the correct transition. The underlying reason is A/B OTA separates the currently trusted executable from the candidate being written.

Now consider the counterfactual. Suppose the server keeps its desired state, but the device never reports the corresponding running state. Nothing should silently advance. Suppose the device reports a terminal-looking string that belongs to an older release. The new assignment should not inherit that causality. Suppose a release is cryptographically valid but persistent-state compatibility is wrong. Delivery still has to stop. These are all examples of locally reasonable facts that become globally wrong when their scope is lost.

The shortcut I rejected was Overwriting the only runnable app partition in place. It removes a state or validation step, but that apparent simplicity only pushes ambiguity into recovery. The retained result—A failed transfer no longer had to consume the known-good boot target.—keeps the ambiguity visible until a component with the right authority resolves it.

Implementation boundary

The device updater should behave like a small transactional engine. It verifies preconditions, streams into the inactive slot, validates artifact metadata, changes boot selection only after the write is complete, reboots into pending verification and reaches a local commit point with esp_ota_mark_app_valid_cancel_rollback(). Until that call, resets and validation failures must preserve a path back to the previous image.

The database model is part of the safety mechanism, not just storage. In this case, the key observation is The OTA-capable design uses otadata plus ota_0 and ota_1; the new image is written to the inactive slot before boot selection changes.. I would expose enough state to verify that observation without copying secrets or giant diagnostic payloads into the event stream.

The minimum useful operational record includes the device identifier, release identifier where relevant, previous and target versions, assignment state, boot/update state, and a sanitized result. For device-side acceptance I also want the generations that determine compatibility. These fields are not decoration: they let an incident review distinguish “server wanted release X,” “device downloaded release X,” “device booted release X,” and “device accepted release X.”

The unsafe alternative was Overwriting the only runnable app partition in place.. That alternative usually saves one field or one state transition, but it makes recovery ambiguous. When the system later fails, an operator has to infer what probably happened from timestamps and logs. I would rather spend a little more schema/API complexity up front and make the transition mechanically provable.

What changes when the fleet grows

At two devices, an engineer can remember almost everything. At five devices, informal state already becomes unreliable. At twenty devices, a shared secret, ambiguous version string or manual “I think that board updated” workflow becomes an incident generator.

I would keep the same model as the fleet grows and change the implementation around it: stronger key custody, richer cohorts, more formal release approvals, better metrics and eventually geographically independent control-plane recovery. I would not remove the distinctions between device identity, desired release, running release, release policy and rollback state. Those distinctions become more valuable with scale.

The mechanism remains the same: A/B OTA separates the currently trusted executable from the candidate being written. Distributed state does not disappear when more automation is added; automation simply makes incorrect state transitions happen faster if the model is weak.

The invariants I wanted before the transition

• candidate writes inactive slot
• boot target changes only after verified write
• pending image self-tests locally
• accept/rollback API is called explicitly
• active call and low battery can defer work

For The Inactive OTA Slot Protects the Running Image, the key mechanism is that A/B OTA separates the currently trusted executable from the candidate being written. If one of these invariants is unknown, the control plane should prefer a blocked or pending state over inventing convergence.

This is where production OTA diverges from a lab script. A lab script can assume the operator remembers which board is on which image. A fleet service has to make those assumptions explicit enough that another process can reject a dangerous transition automatically.

What the evidence proves—and what it does not

The evidence supports this narrow statement: A failed transfer no longer had to consume the known-good boot target.

It does not prove that every future firmware is safe, that every device will remain online, or that the server can infer unreported device state. OTA is distributed: the server knows assignment intent, the device knows what it is executing, and the bootloader knows pending/rollback state. No one component has perfect knowledge at every instant.

That is why A/B OTA separates the currently trusted executable from the candidate being written. The correct model tolerates temporary disagreement and waits for the observation that resolves it. It is better to show PENDING than to manufacture success from stale data.

The rule I kept

Write candidates beside the running image, not on top of it.

The result from this case was A failed transfer no longer had to consume the known-good boot target.

That is the core of production OTA for me. The hard problem is not moving a .bin file over HTTPS. The hard problem is preserving causal truth while identity, policy, persistent data, bootloader state, device observations and operator intent change at different times.

A successful update is therefore not “download returned 200.” It is a sequence of authorized, compatible and observable state transitions with a recovery path at every irreversible boundary.