Switch between R Plumber Serializers

Question

#* @get /json
#* @serializer unboxedJSON
function() {
  return(iris)
}

#* @get /csv
#* @serializer csv list(type="text/plain; charset=UTF-8")
function() {
  return(iris)
}

#* @param type csv or json
#* @get /data
function(type = 'csv') {
  if (type == 'cvs') {
    #* @serializer csv list(type="text/plain; charset=UTF-8")
    return(iris)
  } else {
    #* @serializer unboxedJSON
    return(iris)
  }
}

The first 2 endpoints above each work fine, but the 3rd endpoint does not work as the #* @serializer unboxedJSON cannot be inside the function it seems. However, it would be great if I could somehow have a single endpoint which handles serializing before returning. plumber has plumber::serializer_csv and plumber::serializer_unboxed_json() and all of their serializers as functions, however I'm not sure how to use them inside the endpoint functions (or if this is even possible)

Thanks!

Why one endpoint? Serializers are on an endpoint basis. One serializers, one endpoint. Instead of passing a type parameters. Just create two endpoint /data/json and /data/csv. Both endpoint expression can use the same function. I mean, it is feasible but I do not see any reason to do it. — Bruno Tremblay, Oct 15 '20 at 21:12
If you really want to do down this route, I would use plumber fitlers, check for incoming headers (say Accept) for the type of data user wants returned and modify res$serializer according to the what the user asked. — Bruno Tremblay, Oct 15 '20 at 21:20

score 4 · Answer 1 · answered Mar 10 '21 at 20:29

You can create a custom serializer that changes the serialization format depending on the the response object. Here is an example. In this case, I attach an attribute to my response that provides the format that I want:

library(plumber)

#* @apiTitle Example Plumber API with Serializer to switch formats using object
#* attributes

serializer_switch <- function() {
  function(val, req, res, errorHandler) {
    tryCatch({
      format <- attr(val, "serialize_format")
      if (is.null(format) || format  == "json") {
        type <- "application/json"
        sfn <- jsonlite::toJSON
      } else if (format == "csv") {
        type <- "text/csv; charset=UTF-8"
        sfn <- readr::format_csv
      } else if (format == "rds") {
        type <- "application/rds"
        sfn <- function(x) base::serialize(x, NULL)
      }
      val <- sfn(val)
      res$setHeader("Content-Type", type)
      res$body <- val
      res$toResponse()
    }, error = function(err) {
      errorHandler(req, res, err)
    })
  }
}

register_serializer("switch", serializer_switch)

#* Return a data frame of random values
#* @param n size of data frame
#* @param format one of "json", "csv", or "rds"
#* @serializer switch
#* @get /random_df
function(n = 10, format = "json") {
  out <- data.frame(value = rnorm(n))
  attr(out, "serialize_format") <- format
  out
}

I was going crazy trying to implement it. Thank you very much for this clear and helpful example! — Adrià, Mar 29 '21 at 11:03
More and simpler options here: https://community.rstudio.com/t/switching-plumber-serialization-type-based-on-url-arguments/98535 — Noam Ross, Mar 30 '21 at 13:37

Switch between R Plumber Serializers

1 Answers1