How to send RDS file to an R plumber API?

September 25, 2023

I work with the R plumber API. I’ve created a very simple api that receives an RDS file (a data.frame). On this received rds, I perform a few basic operations and then return this modified data.frame.
With swagger, I can graphically test whether my API is working and it’s not. I can’t get the RDS file to work. I tried to follow the example here How to send rds file to plumber API using httr package ? but without success

Here below is my code :

library(plumber)
library(dplyr)
library(magrittr)
library(data.table)

#* Get dataframe and create feature
#* @parser multi
#* Add rds parser
#* @parser rds
#* @param df:file
#* @post /dataframe
function(req, res, df) {
  str(df)
  return(df[,1])
}

I get this error with swagger UI :

>Solution :

Insert a browser() in there to see why your code is failing:

function(req, res, df) {
  browser()
  str(df)
  return(df[,1])
}

For me, I generated started with saveRDS(mtcars, file="mt.rds"), then uploaded it. From here,

Browse[2]> df
$mt.rds
                     mpg cyl  disp  hp drat    wt  qsec vs am gear carb
Mazda RX4           21.0   6 160.0 110 3.90 2.620 16.46  0  1    4    4
Mazda RX4 Wag       21.0   6 160.0 110 3.90 2.875 17.02  0  1    4    4
Datsun 710          22.8   4 108.0  93 3.85 2.320 18.61  1  1    4    1
Hornet 4 Drive      21.4   6 258.0 110 3.08 3.215 19.44  1  0    3    1
Hornet Sportabout   18.7   8 360.0 175 3.15 3.440 17.02  0  0    3    2
Valiant             18.1   6 225.0 105 2.76 3.460 20.22  1  0    3    1
Duster 360          14.3   8 360.0 245 3.21 3.570 15.84  0  0    3    4
Merc 240D           24.4   4 146.7  62 3.69 3.190 20.00  1  0    4    2
Merc 230            22.8   4 140.8  95 3.92 3.150 22.90  1  0    4    2
Merc 280            19.2   6 167.6 123 3.92 3.440 18.30  1  0    4    4
Merc 280C           17.8   6 167.6 123 3.92 3.440 18.90  1  0    4    4
Merc 450SE          16.4   8 275.8 180 3.07 4.070 17.40  0  0    3    3
Merc 450SL          17.3   8 275.8 180 3.07 3.730 17.60  0  0    3    3
Merc 450SLC         15.2   8 275.8 180 3.07 3.780 18.00  0  0    3    3
Cadillac Fleetwood  10.4   8 472.0 205 2.93 5.250 17.98  0  0    3    4
Lincoln Continental 10.4   8 460.0 215 3.00 5.424 17.82  0  0    3    4
Chrysler Imperial   14.7   8 440.0 230 3.23 5.345 17.42  0  0    3    4
Fiat 128            32.4   4  78.7  66 4.08 2.200 19.47  1  1    4    1
Honda Civic         30.4   4  75.7  52 4.93 1.615 18.52  1  1    4    2
Toyota Corolla      33.9   4  71.1  65 4.22 1.835 19.90  1  1    4    1
Toyota Corona       21.5   4 120.1  97 3.70 2.465 20.01  1  0    3    1
Dodge Challenger    15.5   8 318.0 150 2.76 3.520 16.87  0  0    3    2
AMC Javelin         15.2   8 304.0 150 3.15 3.435 17.30  0  0    3    2
Camaro Z28          13.3   8 350.0 245 3.73 3.840 15.41  0  0    3    4
Pontiac Firebird    19.2   8 400.0 175 3.08 3.845 17.05  0  0    3    2
Fiat X1-9           27.3   4  79.0  66 4.08 1.935 18.90  1  1    4    1
Porsche 914-2       26.0   4 120.3  91 4.43 2.140 16.70  0  1    5    2
Lotus Europa        30.4   4  95.1 113 3.77 1.513 16.90  1  1    5    2
Ford Pantera L      15.8   8 351.0 264 4.22 3.170 14.50  0  1    5    4
Ferrari Dino        19.7   6 145.0 175 3.62 2.770 15.50  0  1    5    6
Maserati Bora       15.0   8 301.0 335 3.54 3.570 14.60  0  1    5    8
Volvo 142E          21.4   4 121.0 109 4.11 2.780 18.60  1  1    4    2

So you need to use df[[1]] (or a name if you know it):

function(req, res, df) {
  return(df[[1]][,1])
}

With that running, I’m able to do this:

library(httr)
library(magrittr)
res <- 
  POST(
    "http://127.0.0.1:9999/dataframe",
    body = list(
      # send the file with mime type `"application/rds"` so the RDS parser is used
      df = upload_file("mt.rds", "application/rds")
    )
  ) %>%
  content()
str(res)
# List of 32
#  $ : int 21
#  $ : int 21
#  $ : num 22.8
#  $ : num 21.4
#  $ : num 18.7
#  $ : num 18.1
#  $ : num 14.3
#  $ : num 24.4
#  $ : num 22.8
#  $ : num 19.2
#  $ : num 17.8
#  $ : num 16.4
#  $ : num 17.3
#  $ : num 15.2
#  $ : num 10.4
#  $ : num 10.4
#  $ : num 14.7
#  $ : num 32.4
#  $ : num 30.4
#  $ : num 33.9
#  $ : num 21.5
#  $ : num 15.5
#  $ : num 15.2
#  $ : num 13.3
#  $ : num 19.2
#  $ : num 27.3
#  $ : int 26
#  $ : num 30.4
#  $ : num 15.8
#  $ : num 19.7
#  $ : int 15
#  $ : num 21.4