The scroll timeout is set by default to 5 minutes when using the _reindex API. It is possible to change this value when using the java client but not when using the rest API directly.
5 minutes is quite high but if the indexation is slow we can still reach the timeout and fail the entire reindex due to a context missing in the scroll. Since _update_by_query and _delete_by_query can change the default value in the rest API, we should also handle this parameter in _reindex.