Keep alive function for PostgreSQL connection in Shiny app?

Question

I have a shiny app that utilizes RPostgreSQL to connect to database & queries data ad-hoc.

# Initialize environment to hold SQL parameters:
library(RPostgreSQL)
if (!exists('.sql')) .sql <- new.env()
.sql$cxn <- dbConnect(PostgreSQL(), host = "localhost", dbname = "testdb", user = "myuser", password = "Passw0rd!", port = 5432)

This code runs when the app initializes, but after some time, the connection is terminated by the server:

> dbGetQuery(.sql$cxn, "SELECT 1")
Error in postgresqlExecStatement(conn, statement, ...) : 
  RS-DBI driver: (could not run statement: no connection to the server
)

If I simply call:

.sql$cxn <- dbConnect(PostgreSQL(), host = "localhost", dbname = "testdb", user = "myuser", password = "Passw0rd!", port = 5432)

again, it creates a second connection: (and I do not want that)

> dbListConnections(PostgreSQL())
[[1]]
<PostgreSQLConnection:(10601,4)> 

[[2]]
<PostgreSQLConnection:(10601,5)> 

(Eventually, the maximum number of connections is reached and you cannot create another: https://groups.google.com/forum/#!topic/shiny-discuss/0VjQc2a6z3M )

I want to create a function that connects to my PostgreSQL database (if it hasn't been already), and keeps the connection open (by SELECT'ing 1) if it has:

getConnection <- function(.host, .user, .pw) {
  tryCatch({
    if (!exists('cxn', where = .sql)) {
      .sql$cxn <- dbConnect(PostgreSQL(), host = .host, dbname = "testdb", user = .user, password = .pw, port = 5432)
    } else {
      dbGetQuery(.sql$cxn, "SELECT 1")
    }
  }, warning = function(w) {
    NULL  # placeholder for warnings
  }, error = function(e) {
    print(e)
    cat("Looks like PostgreSQL connection died. Let's try to reconnect...\n")
    invisible(lapply(dbListConnections(PostgreSQL()), dbDisconnect))  # Close all db connections
.sql$cxn <<- dbConnect(PostgreSQL(), host = .host, dbname = "testdb", user = .user, password = .pw, port = 5432)
  }, finally = {
    return(.sql$cxn)
  })
}

getConnection(.host = "localhost", .user = "myuser", .pw = "Passw0rd!")

EDIT 2016/03/12: Not sure why, but the function I wrote above doesn't seem to be working properly...

When I call it, I get:

> getConnection(.host = awsrds$host, .user = awsrds$username, .pw = awsrds$password)
Error in postgresqlExecStatement(conn, statement, ...) : 
  RS-DBI driver: (could not run statement: no connection to the server
)
<PostgreSQLConnection:(12495,0)> 

In particular, this part dbGetQuery(.sql$cxn, "SELECT 1") returns:

Error in postgresqlExecStatement(conn, statement, ...) : 
  RS-DBI driver: (could not run statement: no connection to the server
)
NULL
Warning message:
In postgresqlQuickSQL(conn, statement, ...) :
  Could not create executeSELECT 1

And the class of this output is NULL (as opposed to an error?).

Any ideas what I'm doing wrong? Thanks!

Answer 1

I believe you can solve the error by including a check in your connection function. This is what I did for mysql...should work the same in your case I believe. The key is checking if SELECT 1 == 'try-error'

connectionFunction <- function() {
  if (!exists("connectionName", where = .GlobalEnv)) {
      connectionName <<- dbConnect(MySQL(), default.file = mysqlconf, dbname = "dbName")
  } else if (class(try(dbGetQuery(connectionName, "SELECT 1"))) == "try-error") {
      dbDisconnect(connectionName)
      connectionName <<- dbConnect(MySQL(), default.file = mysqlconf, dbName = "dbName")
  }
  return(connectionName)

}