Function migrate_component 

pub fn migrate_component(
    conn: &mut Connection,
    component: &str,
    migrations: &[&str],
) -> Result<()>

Like migrate, for one component of a database that several own (kernel spec §1): the kernel’s tables share hennery.db with the sessions module, which keeps user_version for itself. Each component’s version is a row of schema_versions, and a component newer than this binary is refused the same way. Each step reads the version inside its transaction, as migrate does.