Full image updates are the simplest and work from any starting version. Delta updates are smaller but only move a device from one specific version to another. Adaptive updates ship the full bundle but let the device download only the blocks it lacks. Pick by bandwidth cost and how much version spread your fleet has.
Definitions
A full image replaces the whole root file system or application slot. A delta update carries the data needed to move from one specific version to the new version. An adaptive update, as the RAUC project uses the term, does not need to be installed on a specific previous version. It carries information that lets the installer pick a method using data already on the device, either from any previous version or from an interrupted attempt.
Comparison
| Property | Full image | Delta | Adaptive |
|---|---|---|---|
| Download size | Largest | Smallest | Reduced, depends on overlap |
| Needs a known starting version | No | Yes | No |
| Build and test effort | Low | One delta per source version | Low to medium |
| Fits a fleet with many old versions | Yes | Poorly | Yes |
| Works over constrained links | Poorly | Well | Reasonably |
RAUC states that adaptive updates are meant to be used together with HTTP streaming, so only the needed parts of the bundle are fetched, and that the bundle still contains the full information. Its documentation describes the approach as an optimization of download size, not a change to how bundles are created, distributed, and installed.
How version spread decides it
If every device is one release behind, deltas are efficient. If devices sit across eight releases because some are offline for months, you would need a delta for every source version, or a fallback to a full image for the stragglers. Adaptive updates avoid that matrix.
Decision steps
- Count how many releases are live in the field today.
- Measure the cost of a full download on your worst link.
- If versions are spread widely, prefer full or adaptive.
- If the fleet moves in lockstep and links are costly, consider deltas with a full image fallback.
- Whatever you choose, verify the signature on the final result, not just the transport.
Security angle
Smaller downloads do not change the trust requirement. The signed manifest or metadata must still cover what gets installed, and a device must refuse an older release unless you explicitly allow it. Keep the SBOM for each target release, since the fleet inventory depends on knowing exactly what each release contains.
Safeguard can scan the release SBOM and keep watching it after shipping, whichever update type you pick. Try gold.safeguard.sh/scan.
Mistakes to avoid
Do not choose deltas just because the download is small and then discover half the fleet is on a version you did not build a delta for. Do not skip testing the full image path, since it is your fallback when anything else fails. Do not forget storage: an update method that needs scratch space can fail on a device that is nearly full. Finally, measure real download cost in the field, not on the office network, before you decide that the complexity pays off.
Quick check
Is an adaptive update the same as a delta update?
No. A delta targets one specific previous version. An adaptive update works from whatever data is on the device.
Do smaller updates need less verification?
No. The result still has to be authenticated before it is installed.
Sources
Last reviewed: 19 October 2026.
