Device publisher is too busy log message

The Publisher component of the runtime handles the reading of device variables for Data event type triggers, Variable Group event type triggers and Data mappings.
Each of these items is configured with a frequency in milliseconds to read the device variable. The parameter for this is Priority (ms) and has a selection list of the frequency rate in milliseconds. For each instance of reading the device variable, the item's Condition is checked. If the condition has been met, the trigger is fired (executed) or the the data mapping is performed.

The Publisher keeps track of all of the device variables and their frequencies and attempts to read the device variables values by issuing an internal variable read command to the device driver that supports the device that the variable belongs to. Multiple references to a single device variable at the same or different frequencies are handled. The device drivers handle the communication to the physical devices using the commands and protocol supported by the physical devices.

Most device drivers interface with the physical devices with a demand read/demand write mechanism, meaning the driver must issue a command to the physical device to read a variable's value.

Some device drivers may support a publish/subscribe mechanism when communicating with the physical devices, and will be notified when the device variable changes.

The Publisher may not be able to read all of the device variables for a given frequency (the Priority (ms) parameter value) and get the responses from all of the device drivers for the variables before the next read cycle is scheduled to begin. When this happens, an Exception Logs message is generated and the Publisher continues with its reading of device variables. In other words, it is not a hard error that stops the Publisher.

Example Exception Log message

An Exception Logs message similar to this example will be logged:

Where the target interval (50 in this example) is the Priority (ms) value that could not be completed, and the actual interval (94 in this example) is how long it took to complete all of the device variable reads for the priority.

Determining the cause

Since the Publisher is handling multiple device variables for a given frequency, it is the total amount of time to read all device variables at the given frequency that could not be completed. This may not be indicating a single device is performing slower than normal, but may be caused by normal variance in device and network communication performance and load on the system.

The Administration -> System Variables tab displays values for the Publisher's function, and can be viewed to help understand the Publisher's status.

For example:

The values are grouped by an index that refers to the Priority (ms) parameter in the configured items. For example index 1 is for the first priority value, which is 50 milliseconds.

The values have the following description:

Value	Description
device_count	The number of devices that have variables that are being read for the given Publisher priority.
Iteration_count	The number of read cycles for the given Publisher priority.
last_time	The read time for the last read cycle for the given Publisher priority.
miss_count	The number of missed read cycles for the given Publisher priority.
miss_count_current	The number of missed read cycles for the given Publisher priority.
miss_percent	The percentage of missed read cycles compared to the total iteration count for the given Publisher priority.
reading_count	The number of missed read cycles for the given Publisher priority.
var_count	The number of variables that are being read for the given Publisher priority.

The miss_count, miss_count_current, and miss_percent values will give an indication that the Publisher is not able to read all of the variables for the given priority within the priority's read cycle.

• Review the priorities being used and the application logic as to why that priority is appropriate.
  
• Review the total number of variables (in different devices) and the normal performance for reading variables for each device. A device's read and write performance statistics are shown on the Device panel. For more information, see Defining, viewing, and controlling devices.
  
• Review the data event type triggers that are using the device variables. For more information, see Data.
  
• Review the data mappings that are using the device variables. For more information, see Defining, viewing, and controlling data mappings.
  
• Review the variable groups that are using the device variables. For more information, see Defining, viewing, and controlling variable groups.