Implement call retries with httr::RETRY() function in API call (R) - r

Question

I use the UN Comtrade data API with R.
library(rjson)
get.Comtrade <- function(url="http://comtrade.un.org/api/get?"
,maxrec=50000
,type="C"
,freq="A"
,px="HS"
,ps="now"
,r
,p
,rg="all"
,cc="TOTAL"
,fmt="json"
)
{
string<- paste(url
,"max=",maxrec,"&" #maximum no. of records returned
,"type=",type,"&" #type of trade (c=commodities)
,"freq=",freq,"&" #frequency
,"px=",px,"&" #classification
,"ps=",ps,"&" #time period
,"r=",r,"&" #reporting area
,"p=",p,"&" #partner country
,"rg=",rg,"&" #trade flow
,"cc=",cc,"&" #classification code
,"fmt=",fmt #Format
,sep = ""
)
if(fmt == "csv") {
raw.data<- read.csv(string,header=TRUE)
return(list(validation=NULL, data=raw.data))
} else {
if(fmt == "json" ) {
raw.data<- fromJSON(file=string)
data<- raw.data$dataset
validation<- unlist(raw.data$validation, recursive=TRUE)
ndata<- NULL
if(length(data)> 0) {
var.names<- names(data[[1]])
data<- as.data.frame(t( sapply(data,rbind)))
ndata<- NULL
for(i in 1:ncol(data)){
data[sapply(data[,i],is.null),i]<- NA
ndata<- cbind(ndata, unlist(data[,i]))
}
ndata<- as.data.frame(ndata)
colnames(ndata)<- var.names
}
return(list(validation=validation,data =ndata))
}
}
}
However, sometimes it fails to connect server and I need to run the code several times to start working. Solution given here, to use Retry() function, which retries a request until it succeeds, seems attractive.
However, I have some difficulties implementing this function in the code given above. has anybody used it before and knows how to recode it?

Answer 1

An API call using httr::RETRY could look like the following:
library(httr)
library(jsonlite)
res <- RETRY(
verb = "GET",
url = "http://comtrade.un.org/",
path = "api/get",
encode = "json",
times = 3,
query = list(
max = 50000,
type = "C",
freq = "A",
px = "HS",
ps = "now",
r = 842,
p = "124,484",
rg = "all",
cc = "TOTAL",
fmt = "json"
)
)
# alternativ: returns dataset as a `list`:
# parsed_content <- content(res, as = "parsed")
# returns dataset as a `data.frame`:
json_content <- content(res, as = "text")
parsed_content <- parse_json(json_content, simplifyVector = TRUE)
parsed_content$validation
parsed_content$dataset
I'd suggest rewriting the get.Comtrade function using httr:
get.Comtrade <- function(verb = "GET",
url = "http://comtrade.un.org/",
path = "api/get",
encode = "json",
times = 3,
max = 50000,
type = "C",
freq = "A",
px = "HS",
ps = "now",
r,
p,
rg = "all",
cc = "TOTAL",
fmt = "json") {
res <- httr::RETRY(
verb = verb,
url = url,
path = path,
encode = encode,
times = times,
query = list(
max = max,
type = type,
freq = freq,
px = px,
ps = ps,
r = r,
p = p,
rg = rg,
cc = cc,
fmt = fmt
)
)
jsonlite::parse_json(content(res, as = "text"), simplifyVector = TRUE)
}
s1 <- get.Comtrade(r = "842", p = "124,484", times = 5)
print(s1)
Please see this and this for more information on library(httr).