/// Returns the pdu from the outlier tree.
    pub fn get_outlier_pdu_json(&self, event_id: &EventId) -> Result<Option<CanonicalJsonObject>> {
        self.eventid_outlierpdu
            .get(event_id.as_bytes())?
            .map_or(Ok(None), |pdu| {
                serde_json::from_slice(&pdu).map_err(|_| Error::bad_database("Invalid PDU in db."))
            })
    }

    /// Returns the pdu from the outlier tree.
    pub fn get_pdu_outlier(&self, event_id: &EventId) -> Result<Option<PduEvent>> {
        self.eventid_outlierpdu
            .get(event_id.as_bytes())?
            .map_or(Ok(None), |pdu| {
                serde_json::from_slice(&pdu).map_err(|_| Error::bad_database("Invalid PDU in db."))
            })
    }

    /// Append the PDU as an outlier.
    #[tracing::instrument(skip(self, pdu))]
    pub fn add_pdu_outlier(&self, event_id: &EventId, pdu: &CanonicalJsonObject) -> Result<()> {
        self.eventid_outlierpdu.insert(
            event_id.as_bytes(),
            &serde_json::to_vec(&pdu).expect("CanonicalJsonObject is valid"),
        )
    }