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Call web service with Arduino using SOAP message over HTTP

There are many ways to communicate between two devices. One of that is the efficient, simple and cheap way: Web Service. Web Service is a software system designed for support communication over a network. To communicate between Arduino and host computer, we can apply Web Service which uses Arduino Ethernet Shield. The example from http://arduino.cc/en/Main/ArduinoEthernetShield describe how to call web server over HTTP protocol. We are able to apply to the call to a web service provider by sending a SOAP message.

This article shows how to call the web service provider with Arduino and ethernet shield.
Server-side install web service provider, developed with Java language and deploy on apache tomcat and client-side is an Arduino with Ethernet Shield installed.


Arduino and Ethernet Shield

From WebClient in Arduino Example, demonstrate how to call web server by using an ethernet shield. Like use telnet command call to server with puting some instruction command within the request.
client.println("GET /search?q=arduino%20web%20service HTTP/1.1");
client.println("Host: www.google.com");
client.println("Connection: close");
client.println();
“GET” is a method of request “/search?q=arduino” is URI which is a path on server to call,
“HTTP/1.1” is protocol/version.
“Host” tell server what host is request to and
“Connection:” tell the server state of the connection.
And the last “client.println();” is telling server for the end of the message.

The “GET” method tells the web server this is a request for some information. There are several methods which you can find an information here:  http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html
Then, if the server and the request path are available, we will receive some message with starting by “HTTP/1.1 200 OK”.

So, if we need to send web server some data, we can use “POST” method look like:
client.println("POST /search HTTP/1.1");
client.println("Host: www.google.com");
client.println("Connection: close");
client.println("Content-length: 20");
client.println("Content-type: text/html");
client.println();
client.println("q=arduino%20web%20service");
client.println();

“Content-length” tell the server how many characters that client send to. In this case is length of word “q=arduino%20web%20service” which is 20. The server reads request data and count to “20” character and terminate the request. Then, if “Content-length” is not equal to the data character length the server will lose data or connection is timed out because server waiting until data is full of length.

“client.println();” after “client.println("Content-type: text/html");” is telling the server to start to receive an input data. And the last “client.println();” is telling server for the end of the message.

SOAP Message Over HTTP

An information about SOAP are here: http://www.w3schools.com/soap/ . SOAP is a protocol for accessing a Web Service. SOAP Message is an ordinary XML document containing the elements Envelope, Header, Body and Fault.

<?xml version="1.0"?>
<soap:Envelope xmlns:soap="http://www.w3.org/2001/12/soap-envelope"
soap:encodingStyle="http://www.w3.org/2001/12/soap-encoding">
<soap:Header>
...
</soap:Header>
<soap:Body>
...
  <soap:Fault>
  ...
  </soap:Fault>
</soap:Body>
</soap:Envelope>
An Envelope element that identifies the XML document as a SOAP message. A Header element that contains header information. A Body element that contains call and response information. And a Fault element containing errors and status information.

Why SOAP Message?

SOAP Message without the hassle of communicating across platform or programming language. We have just sent the xml message in the correct SOAP format to the Web Service endpoint which disregard about what OS of a server or development language of provider.

How to call Web Service with SOAP Message?

Sending soap message to the web service server is the same as request data from web server. However, we use "POST" method to send the soap message to the server. The following is a send message:


POST /SimpleService/services/DataLoggerService HTTP/1.1
Host:192.168.1.176
Content-Type: text/xml; charset=utf-8
Content-Length: 253
Connection: close
SOAPAction: ""
<soapenv:envelope xmlns:ns1="http://test" xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/"><soapenv:header><soapenv:body><ns1:log><arg0>167</arg0><arg1> 3</arg1></ns1:log></soapenv:body></soapenv:header>


The "SOAPAction" is from the WSDL file of the web service provider under path "/wsdl:definitions/ wsdl:binding/wsdlsoap:operation/soapaction". Below is an example of WSDLfile.

<wsdl:definitions xmlns:apachesoap="http://xml.apache.org/xml-soap" xmlns:impl="http://test" xmlns:intf="http://test" xmlns:soapenc="http://schemas.xmlsoap.org/soap/encoding/" xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/"xmlns:wsdlsoap="http://schemas.xmlsoap.org/wsdl/soap/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" targetNamespace="http://test">
<!--
WSDL created by Apache Axis version: 1.4
Built on Apr 22, 2006 (06:55:48 PDT)
-->
<wsdl:message name="logRequest">
<wsdl:part name="temp" type="xsd:string"></wsdl:part>
<wsdl:part name="time" type="xsd:string"></wsdl:part>
</wsdl:message>
<wsdl:message name="logResponse">
<wsdl:part name="logReturn" type="xsd:string"></wsdl:part>
</wsdl:message>
<wsdl:portType name="DataLoggerService">
<wsdl:operation name="log" parameterOrder="temp time">
<wsdl:input message="impl:logRequest" name="logRequest"></wsdl:input>
<wsdl:output message="impl:logResponse" name="logResponse"></wsdl:output>
</wsdl:operation>
</wsdl:portType>
<wsdl:binding name="DataLoggerServiceSoapBinding" type="impl:DataLoggerService">
<wsdlsoap:binding style="rpc" transport="http://schemas.xmlsoap.org/soap/http"/>
<wsdl:operation name="log">
<wsdlsoap:operation soapAction=""/>
<wsdl:input name="logRequest">
<wsdlsoap:body encodingStyle="http://schemas.xmlsoap.org/soap/encoding/" namespace="http://test" use="encoded"/>
</wsdl:input>
<wsdl:output name="logResponse">
<wsdlsoap:body encodingStyle="http://schemas.xmlsoap.org/soap/encoding/" namespace="http://test" use="encoded"/>
</wsdl:output>
</wsdl:operation>
</wsdl:binding>
<wsdl:service name="DataLoggerServiceService">
<wsdl:port binding="impl:DataLoggerServiceSoapBinding" name="DataLoggerService">
<wsdlsoap:address location="http://localhost:8080//SimpleService/services/DataLoggerService"/>
</wsdl:port>
</wsdl:service>
</wsdl:definitions>

Arduino Web Service Client



An example below creates an XML in the SOAP format with converting the analog value of pin A0 to String parameters of soap message.
Then use EthernetClient write command to call to the web service provider and wait for a response.

#include <Ethernet.h>
#include <EthernetClient.h>
#include <SPI.h>

#define ETH_SS_PIN 10

byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };
IPAddress ip(192, 168, 1, 177);
IPAddress _dns(192, 168, 1, 176);
char host[] = "192.168.1.176";    
uint16_t port = 8080;
String path = "/SimpleService/services/DataLoggerService";
int count = 0;
EthernetClient client;

void setup() {
 pinMode(ETH_SS_PIN, OUTPUT);
 digitalWrite(ETH_SS_PIN, LOW);
 Serial.begin(9600);
 while (!Serial) {
  ; 
 }
 Serial.println("Start ethernet");

 Ethernet.begin(mac, ip, _dns);
 delay(1000);

}

String callService(String arg1, String arg2) {
 String soapMsg1 =
   "<?xml version=\"1.0\" encoding=\"UTF-8\"?><soapenv:Envelope xmlns:soapenv=\"http://schemas.xmlsoap.org/soap/envelope/\"  xmlns:ns1=\"http://test\"><soapenv:Header/><soapenv:Body><ns1:log>";
 String a = "<arg0>" + arg1 + "</arg0><arg1>" + arg2 + "</arg1>";
 String soapMsg2 = "</ns1:log></soapenv:Body></soapenv:Envelope>";

 Serial.print("connect to :");
 Serial.print(host);
 Serial.print(":");
 Serial.println(port);
 if (client.connect(host, port)) {
  Serial.println("connected.");
  Serial.println("POST " + path + " HTTP/1.1");
  Serial.print("Host:");
  Serial.println(host);
  Serial.println("Content-Type: text/xml; charset=utf-8");
  Serial.print("Content-Length: ");
  Serial.println(soapMsg1.length() + a.length() + soapMsg2.length());
  Serial.println("Connection: keep-alive");
  Serial.print("SOAPAction: \"http://");
  Serial.print(host);
  Serial.print(":");
  Serial.print(port);
  Serial.print(path);
  Serial.println("\"");
  Serial.println("");
  Serial.print(soapMsg1);
  Serial.print(a);
  Serial.println(soapMsg2);
  Serial.println("");

  client.println("POST " + path + " HTTP/1.1");
  client.print("Host:");
  client.println(host);
  client.println("Content-Type: text/xml; charset=utf-8");
  client.print("Content-Length: ");
  client.println(soapMsg1.length() + a.length() + soapMsg2.length());
  client.println("Connection: keep-alive");
  client.print("SOAPAction: \"http://");
  client.print(host);
  client.print(":");
  client.print(port);
  client.print(path);
  client.println("\"");
  client.println("");
  client.print(soapMsg1);
  client.print(a);
  client.println(soapMsg2);
  client.println("");

 } else {
  Serial.println();
  Serial.println("Fail to connect.");
 }

 String ret;
 while (true) {

  if (client.available()) {
   ret = client.readStringUntil('{');
   ret = client.readStringUntil('}');
   Serial.println(ret);
   Serial.println();
   Serial.println("disconnecting.");
   client.stop();
   break;
  }

  if (!client.connected()) {
   Serial.println();
   Serial.println("disconnecting.");
   client.stop();
   break;
  }

 }

 return ret;

}

void loop() {
 int value = analogRead(0);
 count += 1;

 Serial.println("calling service ......");

 String result = callService(String(value), String(count));

 Serial.println("result:" + result);

 Serial.println("waiting for 2 sec .....");
 delay(2000);

}

In the loop, starting to read an analog value from pin 0 and count. Then call the callService() function by passing String of analog value and count and waiting for 2 second.

Web Service Provider

The advantages of using a soap message is not depends on platform or technology.
So I created a simple web service project with Java to receive a request and responds some information back to client. The web service provider receives two parameters from Arduino and print out to the console, and then combine into a JSON string before return it back.

public class DataLoggerService {
 public String log(String temp,String time){
  String result = "{Temp:"+temp+",Time:"+time+"}";
  System.out.println(result);
  return result;
 }
}

Download




Note

Before you installing and testing, incase direct connect arduino ethernet shield to computer, we need to set computer ip address to : 192.168.1.176 ( this is depend on the arduino dns)

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