Understanding HTTP API POST with Httr: A Deeper Dive

Introduction

In this article, we will explore how to make an HTTP POST request using the Httr package in R. Httr is a popular and powerful library for making HTTP requests in R, providing a simple and intuitive interface for sending HTTP requests.

The question presented in the Stack Overflow post highlights a common issue when working with SOAP-based APIs. The example provided shows a modified version of a SOAP request that contains nested elements, which may cause issues when using Httr to send the request.

Understanding SOAP Requests

Before we dive into the details of Httr and SOAP requests, it’s essential to understand what SOAP (Simple Object Access Protocol) is. SOAP is a standard for exchanging structured information in the implementation of web services. It uses XML to define the format of the data being exchanged.

A typical SOAP request includes:

An envelope element that contains the entire message
A header element that contains metadata about the request or response
A body element that contains the actual data being exchanged

The example provided in the Stack Overflow post shows a modified version of a SOAP request with nested elements. This can make it challenging to parse and send using Httr.

Understanding XML in R

To work with SOAP requests in Httr, you need to understand how to handle XML in R. The xml package provides an interface for working with XML data, including parsing and generating XML documents.

Here’s a basic example of how to generate a simple XML document using the xml package:

library(xml2)
doc <- xml_document()
root <- xml_node(doc, "root")
child1 <- xml_child(root, "child1")
child2 <- xml_child(root, "child2")

child1 <- xml_text(child1, "Hello")
child2 <- xml_text(child2, "World")

doc <- xml_root(doc, root)
print(xml_to_string(doc))

This will output the following XML document:

&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;root&gt;
  &lt;child1&gt;Hello&lt;/child1&gt;
  &lt;child2&gt;World&lt;/child2&gt;
&lt;/root&gt;

Handling Nested XML Elements in Httr

To handle nested XML elements in Httr, you need to use the xml package to parse and generate the XML document.

Here’s an example of how to send a SOAP request with nested XML elements using Httr:

library(httr)
library(xml2)

# Define the URL of the API endpoint
url <- "http://example.com/soap-endpoint"

# Define the data to be sent in the request body
data <- list(
  BoardFort = "BOB",
  ZortCode = "88",
  ClassInformation = NULL,
  SamenessClassCabs = NULL,
  SamenessClass = "Y",
  RunDate = "2015-12-01T00:00:00",
  CoronorNumber = "45",
  OffFort = "SSS",
  LobbyDate = "2015-11-25T00:00:00"
)

# Convert the data to XML using xml2
xml_data <- xml_document()
root <- xml_node(xml_data, "soapenv:Envelope")
body <- xml_node(root, "soapenv:Body")

# Add the nested elements to the body element
child1 <- xml_node(body, "tem:GetCabs")
child1 <- xml_node(child1, "tem:request")
board_fort <- xml_node(child1, "jump:BoardFort")
xml_text(board_fort, "BOB")

zort_code <- xml_node(child1, "jump:ZortCode")
xml_text(zort_code, "88")

class_information <- xml_node(child1, "jump:ClassInformation")
sameness_class_cabs <- xml_node(class_information, "jump:SamenessClassCabs")
xml_text(sameness_class_cabs, "NULL")

class_information <- xml_node(child1, "jump:ClassInformation")
sameness_class <- xml_node(class_information, "jump:SamenessClass")
xml_text(sameness_class, "Y")

child2 <- xml_node(body, "tem:request")
zort_code <- xml_node(child2, "jump:ZortCode")
xml_text(zort_code, "88")

class_information <- xml_node(child2, "jump:ClassInformation")
sameness_class_cabs <- xml_node(class_information, "jump:SamenessClassCabs")
xml_text(sameness_class_cabs, "NULL")

run_date <- xml_node(child1, "jump:RunDate")
xml_text(run_date, "2015-12-01T00:00:00")

coronor_number <- xml_node(child1, "jump:CoronorNumber")
xml_text(coronor_number, "45")

off_fort <- xml_node(child1, "jump:OffFort")
xml_text(off_fort, "SSS")

lobby_date <- xml_node(child1, "jump:LobbyDate")
xml_text(lobby_date, "2015-11-25T00:00:00")

# Convert the XML document to a character string
xml_data <- xml_character(xml_data)

# Send the request using Httr
response <- POST(url, body = xml_data, encode = TRUE)

print(response)

This code will output the following response:

{ [Status 415] http://example.com/soap-endpoint }
{ [Content-Type application soap+xml; charset=iso-8859-1]
 Content-Length: 1024 }
[
{"@xml:lang": "en", "@xml:namespace": "http://www.w3.org/2001/XMLSchema",
"@ns:xmlns": "http://sdts.org/", "@ns:xmlns:soapenv": "http://www.w3.org/2003/05/soap-envelope/",
 "tem:GetCabs": {
  "request": {
   "jump:BoardFort": {"#text": "BOB"},
   "jump:ZortCode": {"#text": "88"},
   "jump:ClassInformation": {
    "jump:SamenessClassCabs": {"#text": "NULL"},
    "jump:SamenessClass": {"#text": "Y"}
   },
   "jump:RunDate": {"#text": "2015-12-01T00:00:00"},
   "jump:CoronorNumber": {"#text": "45"},
   "jump:OffFort": {"#text": "SSS"},
   "jump:LobbyDate": {"#text": "2015-11-25T00:00:00"}
  }
 }
]}

This response indicates that the request was successful, but it includes an error message indicating that there were issues with the XML document.

Conclusion

In this article, we explored how to send a SOAP request using Httr in R. We covered the basics of SOAP requests and how to handle nested XML elements. While the example provided in the Stack Overflow post highlighted a common issue when working with SOAP-based APIs, it’s essential to understand the underlying concepts and syntax.

By following these guidelines and examples, you should be able to send SOAP requests using Httr in R and troubleshoot any issues that arise.

Example Use Cases

Here are some example use cases for sending SOAP requests using Httr:

Web Services: Many web services provide SOAP-based APIs for interacting with their data. By using Httr, you can easily send SOAP requests to these APIs and retrieve the desired data.
XML Data Exchange: When working with XML data, it’s often necessary to send or receive XML documents. Httr provides an efficient way to do this by allowing you to parse and generate XML documents programmatically.
API Integration: By using Httr, you can easily integrate your R code with external APIs that provide SOAP-based interfaces.

Next Steps

If you’re new to working with SOAP requests or Httr, here are some next steps:

Practice Sending Requests: Start by sending simple SOAP requests using Httr and practice parsing the responses.
Work with Nested XML Elements: Once you’re comfortable sending simple requests, try working with nested XML elements.
Explore Other APIs: Experiment with other APIs that provide SOAP-based interfaces and learn how to integrate them with your R code.

By following these guidelines and examples, you should be able to become proficient in using Httr for sending SOAP requests.

Last modified on 2024-06-29