Disconnected startup overview

Disconnected startup allows the interface to start from a local cache file with or without a valid connection to Data Archive. This prevents data loss when an interface starts up without a connection to Data Archive. Disconnected startup should only be configured and enabled after you have configured the interface and tested your configurations. Buffering must be configured.

Disconnected startup uses the following processes for an initial startup:

1. The interface connects to Data Archive.

2. The interface delays startup and creates two local cache files. One file contains PI point configuration and the other contains digital state information needed to start the interface.

3. The interface synchronizes the cache files to Data Archive.

4. The interface service begins data collection.

5. The interface collects and buffers data collected from the data source when no connection to Data Archive is available.

Subsequent disconnected startups use the following processes:

6. The interface starts and checks for the local cache files.

7. If the cache files are missing or unusable, the interface must recreate them using the initial startup process.

8. The interface loads points from the cache file instead of Data Archive.

9. The interface service begins data collection.

10. The interface collects and buffers data collected from the data source when no connection to Data Archive is available.

Using a local cache for startup has the additional benefit that the interface starts collecting data more quickly by loading points from the local cache, which is faster than loading points across the network. For example, an interface with 50,000 associated PI points may take several minutes to retrieve all the PI points from Data Archive. The disconnected startup feature can reduce the time to load the same 50,000 PI points from the cache file to approximately 10 seconds. Results will vary depending on the system architecture, network latency, and work load on Data Archive.

Point modification and synchronization

Interfaces using the disconnected startup configuration:

• Load points designated for the interface from locally stored cache files.

• Retrieve point information from Data Archive during normal updates or during point synchronization.

• After losing the Data Archive connection and reconnecting, the interface synchronizes interface point configuration data with that of Data Archive.

For example, an interface started using the disconnected startup configuration loses its connection to Data Archive due to a network failure. While the interface connection to Data Archive is down, the administrator modifies the point database. When the connection to Data Archive is restored, the interface synchronizes its point cache to match the current point configuration on Data Archive. Without restarting the interface, points that were created on Data Archive while disconnected are added to the interface. Similarly, edited points are updated in the interface and deleted points are removed from the interface. Avoid running the interface in disconnected startup mode while making major point modifications to Data Archive, because major point modifications can take a long time to sync with the disconnected startup file.

Point modification messages

Messages about point modifications that occurred while the interface was running but disconnected from Data Archive are logged to the PI Message Log file after the interface reestablishes a connection, including information about point creation, editing, or deletion. These messages appear once per point modification.

In some cases, an error message may be generated because data is being sent to PI points that no longer exist on Data Archive. Typically, the interface logs one message for a given error code only once per point that is in error. These informational and error messages are normal and should require no action by the user.

For more information, see Disconnected startup messages.