> ## Documentation Index > Fetch the complete documentation index at: https://docs.prefect.io/llms.txt > Use this file to discover all available pages before exploring further. # db # `prefect.server.events.ordering.db` ## Classes ### `CausalOrdering` **Methods:** #### `event_has_been_seen` ```python theme={null} event_has_been_seen(self, event: Union[UUID, Event]) -> bool ``` #### `forget_follower` ```python theme={null} forget_follower(self, db: PrefectDBInterface, follower: ReceivedEvent) -> None ``` Forget that this event is waiting on another event to arrive #### `get_followers` ```python theme={null} get_followers(self, db: PrefectDBInterface, leader: ReceivedEvent) -> List[ReceivedEvent] ``` Returns events that were waiting on this leader event to arrive #### `get_lost_followers` ```python theme={null} get_lost_followers(self, db: PrefectDBInterface) -> List[ReceivedEvent] ``` Returns events that were waiting on a leader event that never arrived #### `preceding_event_confirmed` ```python theme={null} preceding_event_confirmed(self, handler: event_handler, event: ReceivedEvent, depth: int = 0) ``` Events may optionally declare that they logically follow another event, so that we can preserve important event orderings in the face of unreliable delivery and ordering of messages from the queues. This function keeps track of the ID of each event that this shard has successfully processed going back to the PRECEDING\_EVENT\_LOOKBACK period. If an event arrives that must follow another one, confirm that we have recently seen and processed that event before proceeding. event (ReceivedEvent): The event to be processed. This object should include metadata indicating if and what event it follows. depth (int, optional): The current recursion depth, used to prevent infinite recursion due to cyclic dependencies between events. Defaults to 0. Raises EventArrivedEarly if the current event shouldn't be processed yet. #### `record_event_as_seen` ```python theme={null} record_event_as_seen(self, event: ReceivedEvent) -> None ``` #### `record_follower` ```python theme={null} record_follower(self, db: PrefectDBInterface, event: ReceivedEvent) -> None ``` Remember that this event is waiting on another event to arrive