Historical Data Access
Historical Data Access addresses the handling of historical time series data. The AddressSpace of HDA Servers contains Historical Nodes representing the history of Variables and Properties. HDA Clients deal with historical data by accessing these Nodes with the HistoryRead and HistoryUpdate Services. There are several types of Historian servers. Some key types supported by OPC UA are:
- Simple trend data servers, providing little other than simple raw data storage. (Data would typically be the types of data available from a Data Access server.
- Complex data compression and analysis servers, providing data compression as well as raw data storage. They are capable of providing summary data or data analysis functions, such as average values, minimums and maximums etc. They can support data updates and history of the updates. They can support storage of annotations along with the actual historical data storage.
Note – Historical Access is specified in OPC UA Part 11, Aggregates are specified in OPC UA Part 13.
Historical Data Access Capabilities
The AddressSpace includes Variables that represent historized Variable values. Read access is provided via ReadHistory.
Simple trending packages that just desire values over a given time frame or they may produce complex reports that require data in multiple formats.
Aggregates are used to derive values from raw data over a defined time range. There are several defined aggregates as well as the ability for vendors to provide custom aggregates.
- Interpolated data is required.
- Raw historical data needs to be normalized before it can be used.
- Summarized trending and reporting.
Ability to modify existing historized Variable values or add historized Variable values that are not in time series order.
- Storage of data received late.
- Correction of incorrect value.
Ability to permanently remove historized Variable values.
- Removal of out of service data.
- Removal of redundant data.
Annotations are additional meta data (notes) that can be inserted into an archive at a specific time.
Record, alongside the data, abnormal conditions or readings such as when maintenance or recalibration was done.