How to retry a statement on error?

Question

How can I simply tell R to retry a statement a few times if it errors? E.g. I was hoping to do something like:

tryCatch(dbGetQuery(...),           # Query database     error = function(e) {         if (is.locking.error(e))    # If database is momentarily locked             retry(times = 3)        # retry dbGetQuery(...) 3 more times         else {             # Handle other errors         }     } ) 

Answer 1

I usually put the try block in a loop, and exit the loop when it no longer fails or the maximum number of attempts is reached.

some_function_that_may_fail <- function() {   if( runif(1) < .5 ) stop()   return(1) }  r <- NULL attempt <- 1 while( is.null(r) && attempt <= 3 ) {   attempt <- attempt + 1   try(     r <- some_function_that_may_fail()   ) }  

Answer 2

I wrote a quick function that allows you to easily retry an operating a configurable number of times, with a configurable wait between attempts:

library(futile.logger) library(utils)  retry <- function(expr, isError=function(x) "try-error" %in% class(x), maxErrors=5, sleep=0) {   attempts = 0   retval = try(eval(expr))   while (isError(retval)) {     attempts = attempts + 1     if (attempts >= maxErrors) {       msg = sprintf("retry: too many retries [[%s]]", capture.output(str(retval)))       flog.fatal(msg)       stop(msg)     } else {       msg = sprintf("retry: error in attempt %i/%i [[%s]]", attempts, maxErrors,                      capture.output(str(retval)))       flog.error(msg)       warning(msg)     }     if (sleep > 0) Sys.sleep(sleep)     retval = try(eval(expr))   }   return(retval) } 

So you can just write val = retry(func_that_might_fail(param1, param2), maxErrors=10, sleep=2) to retry calling that function with those parameters, give up after 10 errors, and sleep 2 seconds between attempts.

Also, you can redefine the meaning of what an error looks like by passing a different function as parameter isError, which by default will catch an error signaled with stop. This is useful if the function being called does something else on error, such as returning FALSE or NULL.

This is the alternative I've found so far that results in clearer, more readable code.

Hope this helps.