Download and unzip the data from http://files.grouplens.org/datasets/movielens/ml-latest-small.zip into this directory. This should create a directory named ml-latest-small which has some CSV files inside of it.
You can then run the plumber API as follows:
plumber::run("plumber.R")$run(port=8000)
Then access http://localhost:8000/ to see the application.
Data from MovieLens https://grouplens.org/datasets/movielens/
F. Maxwell Harper and Joseph A. Konstan. 2015. The MovieLens Datasets: History and Context. ACM Transactions on Interactive Intelligent Systems (TiiS) 5, 4, Article 19 (December 2015), 19 pages. DOI=http://dx.doi.org/10.1145/2827872