Tag configuration versioning

Each time there is a change to a tag property -- description, engineering unit, minimum/maximum range, etc. -- a new tag metadata instance may be created with a unique TagId. This allows AVEVA Historian to preserve the tag configuration history when different types of data is stored for the same tag over time.

Note: Because each new tag metadata instance consumes system resources, it's best to limit these types of updates. AVEVA Historian does not impose a strict limit on the number of metadata instances per tag, but recommends keeping that number under 200 to avoid performance degradation and instability.

Several different tag metadata instances can share the same tag name, resulting in several versions of the same tag. The most recently created tag metadata instance is known as the current version of that tag.

Here is an example:

You create a tag named MyTag. It stores 16-bit integer values from some device. A new tag metadata instance with a TagId -- 883DDAE3-E3F5-441C-A5FD-38AD97DEC070 -- gets created and then some data values get stored.

Several months later, the device is upgraded to generate 32-bit integer values. So, you reconfigure MyTag to store 32-bit integers. During that tag reconfiguration, a new tag metadata instance with another TagId -- FFA0E74C-12FD-49A6-8EBA-B30AFAEF55DA -- is created. When new 32-bit values are stored, they are associated with that new TagId.

Now although the current version of the MyTag references it as a 32-bit integer, the older 16-bit values are still accessible because they are associated with the older tag metadata instance preserved in the history.