Function check_ddl_changes 

async fn check_ddl_changes(
    boot_ts: Timestamp,
    persist_client: PersistClient,
    environment_id: EnvironmentId,
    deploy_generation: u64,
    catalog_metrics: Arc<Metrics>,
    bootstrap_args: BootstrapArgs,
    initial_user_items: &BTreeSet<CatalogItemId>,
    initial_user_replicas: &BTreeSet<ReplicaId>,
)

Restart in read-only mode when user items or replicas have been created or dropped, so bootstrap hydrates new objects and releases dropped resources.