pub trait Data {
    pub fn add_txnid(
        &self,
        user_id: &UserId,
        device_id: Option<&DeviceId>,
        txn_id: &TransactionId,
        data: &[u8],
    ) -> Result<()>;

    pub fn existing_txnid(
        &self,
        user_id: &UserId,
        device_id: Option<&DeviceId>,
        txn_id: &TransactionId,
    ) -> Result<Option<Vec<u8>>>;
}