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Compatibility Checks and Semantic Versioning
Semantic Versioning is a well-known way to structure software versions that conveys additional meaning with version changes. It is explained very comprehensively at https://semver.org/.
We use a version format Major.Minor.Patch with the following definition
- Major - is changed when the API is changed in a manner incompatible to former versions
- Minor - is changed whenever the API is changed in a backward-compatible manner
- Patch - is changed when making bug fixes the guarantee backward compatibility
The ATTinyDaemon has the ability to hard-reset the RPi and thus to potentially wreak havoc on the sd card installed in the RPi. This is something that we do not really want.
Thus the Daemon running on the RPi first checks the major version of the firmware running on the ATTiny and compares it to its own. If they differ then an error is written to the log file. The daemon still executes, but even though at runtime it only checks the register should_shutdown it still potentially writes the configuration values in the config file to the ATTiny. And with a incompatible API change this could lead to serious problems later on.