Keep alive function for PostgreSQL connection in Shiny app? - r

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)
}