When this structure is used for reading Modified Values (isReadModified is set to TRUE), it reads the modified values, StatusCodes, timestamps, modification type, the user identifier, and the timestamp of the modification from the history database for the specified time domain for one or more HistoricalDataNodes . If there are multiple replaced values the Server shall return all of them. The updateType specifies what value is returned in the modification record. If the updateType is INSERT the value is the new value that was inserted. If the updateType is anything else the value is the old value that was changed. See 6.8 HistoryUpdateDetails parameter for details on what updateTypes are available.
The purpose of this function is to read values from history that have been Modified. The returnBounds parameter shall be set to FALSE for this case, otherwise the Server returns a Bad_InvalidArgument StatusCode.
The domain of the request is defined by startTime, endTime, and numValuesPerNode; at least two of these shall be specified. If endTime is less than startTime, or endTime and numValuesPerNode alone are specified, then the data shall be returned in reverse order with the later data coming first. If all three are specified then the call shall return up to numValuesPerNode results going from StartTime to EndTime, in either ascending or descending order depending on the relative values of StartTime and EndTime. If more than numValuesPerNode values exist within that time range for a given Node then only numValuesPerNode values per Node are returned along with a continuationPoint. When a continuationPoint is returned, a Client wanting the next numValuesPerNode values should call ReadRaw again with the continuationPoint set. If numValuesPerNode is 0 then all of the values in the range are returned. If the Server cannot return all modified values for a given timestamp in a single response then it shall return modified values with the same timestamp in subsequent calls.
If the request takes a long time to process then the Server can return partial results with a ContinuationPoint. This might be done if the request is going to take more time than the Client timeout hint. It may take longer than the Client timeout hint to retrieve any results. In this case the Server may return zero results with a ContinuationPoint that allows the Server to resume the calculation on the next Client HistoryRead call.
If a value has been modified multiple times then all values for the time are returned. This means that a timestamp can appear in the array more than once. The order of the returned values with the same timestamp should be from the most recent to oldest modification timestamp, if startTime is less than or equal to endTime. If endTime is less than startTime, then the order of the returned values will be from the oldest modification timestamp to the most recent. It is Server dependent whether multiple modifications are kept or only the most recent.
A Server does not have to create a modification record for data when it is first added to the historical collection. If it does then it shall set the ExtraData bit and the Client can read the modification record using a ReadModified call. If the data is subsequently modified the Server shall create a second modification record which is returned along with the original modification record whenever a Client uses the ReadModified call if the Server supports multiple modification records per timestamp.