A grain's filesystem consists of read-only app data mounted at / and writable grain storage mounted at /var/. From Sandstorm's perspective, upgrading a grain to a new app version just means launching the grain with the new app version's read-only data, leaving the writable /var/ data untouched. The app is then responsible for detecting if any migrations are required and for performing them if so.

Some apps, such as WordPress, already have logic for automatically updating the database when the code changes. Other apps need to add some simple version-detection logic in their startup scripts.

Thanks for the explanation !