Difference between InstallUpdate and ApplyUpdate?

[br16643]

In the example script they both say "Perform Installation tasks here" what tasks would typically occur in the InstallUpdate action and what tasks would typically occur in the ApplyUpdate action?

[jw-msft]

Typically, Install is for copying bits and doing necessary setup. For example:

• decompress and/or decrypt the fs image payload
• verify signature of disk image
• write raw image to the target partition

Then, Apply typically attempts to commit/switch-over to the new disk image or use the new data/setup/package(s). For example, in an A/B system partition update scheme, Apply could be what:

• sets u-boot variable (e.g. via fw_setenv) to specify the new partition to try and boot into
• sets u-boot variable to track count of boot attempts (0 would be first attempt)
• returns a reboot result code to cause the agent initiate a reboot.

However, the separation of Install and Apply is mostly a historical layover from pre-0.8 agent where the update orchestration was driven by the service. For example, the older protocol would push down separate operation commands to the agent (download, install, apply), when the agent was a thin client that just did operations it was told to do.

The newer "goal state" protocol relies on the agent to do the orchestration. Currently, one can feel free to do everything in Install or Apply and just return an Install success or Apply success as appropriate to NOOP it. It's mainly being kept around for backwards compatibility and Apply could be deprecated in future revision (with later a breaking change to remove it) if there is enough feedback that it's more confusing than useful.