doParallel “foreach” inconsistently inherits objects from parent environment: “Error in { : task 1 failed - ”could not find function…"

Question

I have a problem with foreach that I just can\'t figure out. The following code fails on two Windows computers I\'ve tried, but succeeds on three Linux computers, all runnin

Answer 1

@Tensibai is right. When trying to use doParallel on Windows, you have to "export" the functions that you want to use that are not in the current scope. In my experience, the way I've made this work is with the following (redacted) example.

format_number <- function(data) {
  # do stuff that requires stringr
}

format_date_time <- function(data) {
  # do stuff that requires stringr
}

add_direction_data <- function(data) {
  # do stuff that requires dplyr
}

parse_data <- function(data) {
  voice_start <- # vector of values
  voice_end <- # vector of values
  target_phone_numbers <- # vector of values
  parse_voice_block <- function(block_start, block_end, number) {
    # do stuff
  }

  number_of_cores <- parallel::detectCores() - 1
  clusters <- parallel::makeCluster(number_of_cores)
  doParallel::registerDoParallel(clusters)
  data_list <- foreach(i = 1:length(voice_start), .combine=list,
                       .multicombine=TRUE, 
                       .export = c("format_number", "format_date_time", "add_direction_data"), 
                       .packages = c("dplyr", "stringr")) %dopar% 
                       parse_voice_block(voice_start[i], voice_end[i], target_phone_numbers[i])
  doParallel::stopCluster(clusters)
  output <- plyr::rbind.fill(data_list)
}

Since the first three functions aren't included in my current environment, doParallel would ignore them when firing up the new instances of R, but it would know where to find parse_voice_block since it's within the current scope. In addition, you need to specify what packages should be loaded in each new instance of R. As Tensibai stated, this is because you're not running forking the process, but instead firing up multiple instances of R and running commands simultaneously.