Best practices in automated database queries in Shiny - r

Question

As I am new to developing with Shiny, I am interested in the best practices for automated database queries. At the time of writing there are a number of different sources with different information.
If I am to query my postgres database every 10 minutes as in the example below, I want to make sure that there are no issues with a) closing the connection on session exit and b) not being able to connect due to too many open connections. My dashboard will in the future have at most a dozen users at one time.
Having done some research, I am convinced that the best way to do this is not necessarily to use a pool but to use the "one connection per query" method documented by Shiny here
Is using reactivePoll() as I have below the correct way to implement a query that will refresh the rendered table every 10 minutes? The database I will be querying will definitely return different data with every call. Does that mean that checkFunc and valueFunc should be the same or can checkFunc be left as an empty function altogether ?
library(shiny)
library(DBI)
args <- list(
drv = dbDriver("PostgreSQL"),
dbname = "shinydemo",
host = "shiny-demo.csa7qlmguqrf.us-east-1.rds.amazonaws.com",
username = "guest",
password = "guest"
)
ui <- fluidPage(
textInput("ID", "Enter your ID:", "5"),
tableOutput("tbl"),
numericInput("nrows", "How many cities to show?", 10),
plotOutput("popPlot")
)
server <- function(input, output, session) {
output$tbl <- renderTable({
conn <- do.call(DBI::dbConnect, args)
on.exit(DBI::dbDisconnect(conn))
sql <- "SELECT * FROM City WHERE ID = ?id;"
query <- sqlInterpolate(conn, sql, id = input$ID)
data <- reactivePoll(10000, session,
checkFunc = function() {}
valueFunc = function() {
dbGetQuery(conn, query)
})
})
}
shinyApp(ui, server)

Answer 1

I recommend creating your db connection 'conn' out of any output objects.
args <- list(
drv = dbDriver("PostgreSQL"),
dbname = "shinydemo",
host = "shiny-demo.csa7qlmguqrf.us-east-1.rds.amazonaws.com",
username = "guest",
password = "guest"
)
conn <- do.call(DBI::dbConnect, args)
it could be a global environment object, like the 'args' list in your sample code, or inside the server function, queries within rendered output objects will all access the same 'conn' db connection. In my experience, disconnect was not necessary to include, after the Rsession with the Shiny app is closed the database disconnects too.