Using {foreach} to Speed up Parameterized RMarkdown PDF Reports

Recently, I have been working on a project to find alternate methods for creating PDF score reports for assessments that have typically been made using Microsoft Access. As someone who has literally never had a fun time working in Access, I was thrilled to be assigned to this project, and was then quickly humbled by the task at hand.

It has been easy enough to create parameterized HTML reports for a handful of internal users, but when the audience is external at a scale of thousands of reports, the processing time in generating the reports quickly becomes a major consideration in operationalizing the solution.

Without any luck on StackOverflow or the RStudio Community, I took to #rstats Twitter 🐦 for some advice:

After tweaking code, unsuccessfully experimenting with cache = TRUE, and several cups of coffee ☕, I finally found a reasonable solution using the foreach and doParallel packages as suggested by a colleague, in reference to an email from 2018 from a different organization dealing with the same exact dilemma.

The first thing I learned that really helped understand why my PDF reports were being compiled so slowly is that R is single threaded by default. This is a new concept to me so I’m not going to even pretend to fully understand it, but I found this resource to be super helpful.

For this quick demo, we will need to make an RMarkdown file and an R file.

RMarkdown File: Report-Layout.rmd

The RMarkdown file could look something like this:

title: "Iris Demo"
output: pdf_document
  species: ""

	echo = FALSE,
	message = FALSE,
	warning = FALSE


df <- iris %>% 
  filter(Species == params$species)

Report for the species: params$species

df %>% 
  filter(Species == "setosa") %>% 
  geom_histogram() +
  ggtitle(paste0("Distribution of Sepal.Length for ", params$species))

The parameters for species will be passed through to generate a report for each species in iris.

When creating thousands of reports, this process takes quite a while, and luckily we can speed it up using foreach and doParallel.

R Script: Create Reports

The R script could look something like this:


species_names <- as.character(unique(iris$Species))

n_cores <- parallel::detectCores()
cluster <- parallel::makeCluster(n_cores-1)                 

foreach (i = seq_along(species_names), .combine = 'c') %dopar% {
                    params = list(species = species_names[i]),
                    output_file = paste0("Report for ", species_names[i])))

To generate reports, all we need to do is run the Create Reports.r script, and we’ll have three reports generated in a matter of seconds. As we increase the volume of reports, the multi-core processing enabled by foreach / doParallel will significantly cut down on the processing time.

I had intended for this to be longer but really just want to put this out there, and maybe revisit some day with a Part 2. For now, Molly and me are going to hike around Wissahickon with Donut 🐶 🍩

Our dog, Donut

Matt Roumaya
Matt Roumaya

Data Analyst in Philadelphia, PA