how to know when a value in a db changes

[James Bird]

This is a general question on linking objects in a Shiny app to a database table. What is the best approach to ensure the object in shiny is updated when the table in the db is modified? If we build a reactive table in shiny based on a db table, does it know when the db table changes?

e.g.
getPTI4Journal <- function() {
out <- tryCatch(
{
sqlquery <- "select PTI1,PTI2,PTI4,MODELSELECTED,VALUE, NOTE, CHANGED from DF_JOURNALPTI4"
jdbcConnection <- getJdbcConn()
#Reading from Oracle
dbGetQuery(jdbcConnection, sqlquery)

},

finally={
dbDisconnect(jdbcConnection)
}
) 

[Joe Cheng]

You can use the reactivePoll function for this.

http://shiny.rstudio.com/reference/shiny/latest/reactivePoll.html

Your checkFunction would be some kind of cheap-to-execute expression that would change when the data has changed, for example the result of the query "select MAX(CHANGED) from DF_JOURNALPTI4" (I'm just assuming that CHANGED is a timestamp field).

The valueFunction would be the query to get the actual data you want (in this case, getPTI4Journal).

You also provide a polling interval. reactivePoll will automatically execute your checkFunction once per polling interval, and if the returned value changes, it will execute the full valueFunction.

The return value of reactivePoll is a reactive expression (e.g. you call it like a function to retrieve its value) that represents the most recent valueFunction result.

I hope that makes sense...

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[James Bird]

Thanks Joe!