Polling versus Subscription

OPC Expert enables users to ask servers for data either on a time-basis (polling) or by exception (subscription). Here is a brief description:

Polling: Client requests server for all the data, regardless of change
Exception: Client instructs server to send new data when the data changed and a time-period elapsed

The table below illustrates the difference between polling and subscription

Time	Value in OPC Server	Client Update in “Polling”	Client Update in “Subscription”
1	20	20	20
2	20	20
3	20	20
4	20	20
5	21	21	21
6	21	21
7	22	22	22

Subscription uses less disk space than polling

Notes

Subscription is also known by different names such as
- On Data Change
- Unsolicited updates
- Report by exception
- Asynchronous callbacks
- Etc.
OPC Expert’s typical behavior subscriptions whenever possible. Users should consider the following in their system architecture design
- Polling (synchronous)
  - Advantages: Some data calculations are easier to perform
  - Disadvantages
    - Uses more CPU cycles
    - Uses more network bandwidth
    - Uses more disk space when archiving
- Subscription (asynchronous report-by-exception)
  - Advantages
    - Uses less CPU cycles
    - Uses less network bandwidth
    - Uses less disk space when archiving
  - Disadvantages: Some data calculations are more difficult to perform
When running as an OPC client, OPC Expert supports both polling and subscriptions
When running as an OPC server, OPC Expert provides clients with both polling and subscription capabilities
OPC Expert supports the above in both OPC DA and UA