Question

We have a reporting app thats needs to update it’s charts as the data gets written to it’s corresponding table. (the report is based off just one table). Currently we just keep the last read sessionid + rowid (unique combo) in memory and a polling timer just does a select where rowid > what we have in memory (to get the latest rows added). Timer runs every second or so and the fast sql reader does it’s job well. So far so good. However I feel this is not optimal because sometimes there are pauses in the data writes due to the process by design. (user clicking the pause button on the system that writes data ..). Meanwhile our timer keeps hitting the db and does not get any new rows. No errors or anything. How is this situation normally handled. The app that writes the data is separate from the reporting app. The 2 apps run on different machines. Bottomline : How to get data into a c# app as and when it is written into a sql server table without polling unnecessarily. thank you

Answer 1

SQL Server has the capability to notify a waiting application for changes, see The Mysterious Notification. This is how SqlDependency works. But this will only work up to a certain threshold of data change rate. If your data changes too frequently then the cost of setting up a query notification just to be immediately invalidated by receiving the notification is too much. For really high end rates of changes the best place is to notify the application directly from the writer, usually achieved via some forms of a pub-sub infrastructure.

You could also attempt a mixed approach: pool for changes in your display application and only set up a query notification if there are no changes. This way you avoid the cost of constantly setting up Query Notifications when the rate of changes is high, but you also get the benefits of non-pooling once the writes settle down.