Synchronization overview

This section describes the general operation of a fully automatic synchronization process. The objective of this section is to discuss the fundamental concepts of synchronization and to define PI APS terminology used in this manual. This section intentionally omits some details of the actual implementation of the PI APS synchronization process, which are presented in section Synchronization in PI APS.

Each interface instance has an associated set of PI points. The term existing points is used in this manual as a synonym for the set of PI points that are associated with the interface instance. Also, each interface instance has access to the set of tags in a data source. These two sets are represented by the two ellipses in the following set diagram.

Point Categories

This figure represents the PI points and data source tags for an interface instance at the start of a synchronization scan. The fundamental activity of synchronization is to identify the points in each of the subsets in this figure and take appropriate action.

The intersection of the set of PI points with the set of data source tags is shown in green (1) in the figure. The subset of PI points in the intersection contains the points that have valid associations with data source tags. Conversely, the subset of data source tags in the intersection contains the tags that have PI points configured to collect data from them.

The attributes of the PI points in the intersection can be synchronized with the corresponding data source tags. PI APS refers to the specific group of attributes that can be synchronized for a particular interface as the syncable attributes.

The subset of PI points that do not have valid associations with data source tags is represented by the blue (2) area in the figure. Assuming that these PI points were once configured for valid data source tags, the associated data source tags have been deleted. In any case, the PI points need to be removed from the interface instance because they are not operational. These points are called deleted points. When an interface instance is "in sync", this subset is empty.

PI APS refers to the group of attributes that identifies the data source tag associated with a PI point as the key attributes for the interface. The interface-specific key attributes are used to determine the subset of PI points that have valid data source tags, which is the intersection of PI points with data source tags in the figure. In the first phase of a synchronization scan, PI Server is queried for a list of the points associated with the interface instance. For each point, the key attributes are obtained from the PI Server. The key attributes are used to call the programming interface for the data source to obtain the current attributes of the data source tag. If the request for the tag attributes succeeds, the synchronizable attributes for the PI point are obtained and compared with the corresponding data source attributes. For any attributes that are different, the PI point attribute is changed to the data source attribute value.

If the call to the data source programming interface returns an indication that no data source tag corresponds to the key attribute values, the PI point is removed from the interface. Note that the objective for deleted points is to stop the interface from using them. Deleting the point in PI Server is one way to accomplish this. This option might be acceptable for PI COM Connector points where deleting a PI point simply breaks the mapping to an external historian system with no other effect on its archived data. For classic interface points, however, deleting the PI point effectively destroys the archived data for the point. Frequently, the archived data has value and needs to be retained (even though no new data will be collected), so the PI point cannot be deleted. Two other alternatives can be used to logically delete PI points for classic interface instances:

• Change the PointSource and/or InterfaceID attributes to break the association between the point and interface instance.

• Set the Scan attribute to 0 (off).

After automatic synchronization, the deleted points subset is empty. It is much less likely that the intersection of the PI points set and data source tags set is empty.

The data source tags that do not have PI points configured for them are represented by the yellow (3+4) area in the figure. Potentially, PI points can be created for each tag represented by the yellow area in the figure. However, the data source might contain tags that you want PI APS to ignore because they are irrelevant to your environment or you do not want to historize them in PI Server. The plain yellow area (3) in the figure represents the tags that are significant to you; the PI points that can be created for these tags are called available points. The hashed yellow area (4) in the figure represents the tags that you want to ignore, and PI APS provides a mechanism for you to designate the points that can be created for these tags as hidden points. In effect, hiding a tag removes it from the set of available points. Available and hidden points are points that can potentially be created for the interface instance but do not exist as actual PI points.

The second phase of a synchronization scan identifies the available points for the interface instance and creates PI points for them. The programming interface for the data source must provide a method for enumerating all tags in the data source for this phase to be implemented. That is, automatic point creation cannot be performed without a method for enumerating all tags in the data source. The second phase begins by creating a list that contains the key attributes of data source tags that are excluded from the set of available points. As can be seen in the figure, the excluded tags correspond to the existing and hidden points. The exclusion list is first loaded with the key attributes for all existing points. Then, the key attributes for all designated hidden points are added to the exclusion list. Next, the data source programming interface is used to enumerate all tags. For each tag found on the data source, the key attributes are compared with the entries in the exclusion list. If the key attributes for a data source tag are not found in the exclusion list, the tag is an available point and a new PI point is created. PI APS refers to the specific group of attributes that are assigned values when a point is created for a particular interface as the known attributes. The known attributes include the syncable attributes and attributes that can be assigned values when a point is created but cannot be changed subsequently. For example, the key attributes are always in the known attributes but usually not in the syncable attributes (because changing a key attribute causes a PI point to reference a different data source tag).

Following automatic synchronization, the available points subset is empty. Each available point is re-categorized as an existing point when actual point creation occurs.