Communication Between WCF Service and Android Client

Overview of REST in WCF

Windows Communication Foundation (WCF), part of the .NET Framework, provides the first unified programming model for rapidly building service-oriented applications. It enables the development of secure, reliable, transacted services that interoperate with current Microsoft investments and non-Microsoft platforms.

With the .NET Framework 3.5 release, WCF added support for building REST style services. REST, an architectural style for building distributed hypermedia driven applications, involves building resource-oriented services by defining resources that implement uniform interfaces using standard HTTP verbs (GET, POST, PUT, and DELETE), and that can be located/identified by a URI.

REST support within WCF was enhanced with the release of .NET Framework 3.5 SP1 to add make REST development easier and to support the ADO.NET Entity Framework entities in WCF contracts. Improvements were made around UriTemplate flexibility and the Visual Studio tooling to increase developer productivity.

Code example

A good tutorial on how to create RESTful Service using Visual Studio can be found here.

First we need to define the structure (class) which will be used for data exchange between server and client. In this case I created a base class named Response and extended classes named ActivityStatusResponse and ConnectionStatusResponse:

public class Response
{
private bool successful;
private string comment;

public bool Successful
{
get
{
  return successful;
}
set
{
  successful = value;
}
}

public string Comment
{
get
{
  return comment;
}
set
{
  comment = value;
}
}
}

public class ActivityStatusResponse : Response
{
private string activity;

public string Activity
{
get
{
  return activity;
}
set
{
  activity = value;
}
}
}

Next we need to define the methods, message formats and URI templates in the interface:

[OperationContract]
[WebGet(ResponseFormat = WebMessageFormat.Json, UriTemplate = "/{strSessionString}/activityStatus")]
ActivityStatusResponse GetActivityStatus(string strSessionString);

[OperationContract]
[WebGet(ResponseFormat = WebMessageFormat.Json,
UriTemplate = "/{strSessionString}/time={time}&lat={latitude}&long={longitude}")]
Response StoreLocation(string strSessionString, string time, string latitude, string longitude);

WebMessageFormat parameter defines the data exchange format. In this example JSON format is used.

UriTemplate parameter defines the URI template.

The parameters within curly braces represent variable values. Everything else in the URI (not enclosed within curly braces) is considered a static part of the URI.

After the definition we have to implement the methods. The following code shows the implementation of StoreAcceleration method:

public Response StoreAcceleration(string strSessionString, string strMeasurementTime, string strAccelerationX, string strAccelerationY, string strAccelerationZ)
      {
SQLWorks sqlWorks = new SQLWorks();
Response response = new Response();
try
{
  string strTime = strMeasurementTime.Replace("_", " ");
  DateTime measurementTime = DateTime.ParseExact(strTime, "yyyy-MM-dd HH:mm:ss:fff", null);
  double accelerationX = Convert.ToDouble(strAccelerationX.Replace(".", ","));
  double accelerationY = Convert.ToDouble(strAccelerationY.Replace(".", ","));
  double accelerationZ = Convert.ToDouble(strAccelerationZ.Replace(".", ","));

  sqlWorks.StoreAcceleration(strSessionString, measurementTime, accelerationX, accelerationY, accelerationZ);

  response.Successful = true;
  response.Comment = "Stored!";
}
catch(Exception ex)
{
  string sDummy = ex.ToString();
  response.Comment = "an error occured!";
  response.Successful = false;
}

return response;
}

The service Endpoint and other properties are defined in .config file:

<configuration>
<system.web>
<compilation debug="true">
</compilation>
<system.servicemodel>
<bindings>
<webhttpbinding>
<binding name="DefaultBinding">
</binding>
</webhttpbinding>
<behaviors>
<endpointbehaviors>
<behavior name="RESTFriendly">
<webhttp>
</webhttp>
<behavior name="RESTServer.JsonServiceAspNetAjaxBehavior">
<enablewebscript>
</enablewebscript>
</behavior>
<servicebehaviors>
<behavior name="RESTFriendly">
<servicemetadata httpgetenabled="true">
<servicedebug includeexceptiondetailinfaults="false">
</servicedebug>
</servicemetadata>
</behavior>
<services>
<service name="RESTServer.GaitLinkService">
<endpoint address="" behaviorconfiguration="RESTFriendly" binding="webHttpBinding" bindingconfiguration="DefaultBinding" contract="RESTServer.IGaitLinkService">
<identity>
<dns value="localhost">
</dns>
</identity>
</endpoint>
</service>
<servicehostingenvironment aspnetcompatibilityenabled="true">
</servicehostingenvironment>
</services></servicebehaviors></behavior></endpointbehaviors></behaviors></bindings></system.servicemodel></system.web></configuration>

At this point everything is ready to host the service in the windows application:

public void StartRestService()
{
try
{
   WebHttpBinding binding = new WebHttpBinding();
   RESTServiceHost = new ServiceHost(typeof(GaitLinkService), new Uri("http://localhost:8000/GaitLink"));
   RESTServiceHost.AddServiceEndpoint(typeof(IGaitLinkService), binding, "GaitLink");
   RESTServiceHost.Open();
}
catch(Exception ex)
{
   string sDummy = ex.ToString();
   //TODO: notify user about the error!
            }
}

Client application

On client side there is a service called CommunicationService and additional class RestClient. Function arguments are sent to the server by generating the url address which has to be the same format as the template specified in the service interface on the server side.

Class RestClient.java:

package com.client.gaitlink;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import android.util.Log;

public class RestClient {

private static String loginParameters;
private static String accelerationParameters;
private static String activityRequestParameters;
private static String locationParameters;
private static String serviceAddress = "http://testserver:8000/GaitLink/";

private static String convertStreamToString(InputStream is) {
BufferedReader reader = new BufferedReader(new InputStreamReader(is));
StringBuilder sb = new StringBuilder();

String line = null;
try {
while ((line = reader.readLine()) != null) {
  sb.append(line + "\n");
}
} catch (IOException e) {
e.printStackTrace();
} finally {
try {
  is.close();
} catch (IOException e) {
  e.printStackTrace();
}
}
return sb.toString();
}

public static void connect(String url) {

HttpClient httpclient = new DefaultHttpClient();

HttpGet httpget = new HttpGet(url);

//HttpPost httppost = new HttpPost(url);

HttpResponse response;
try {

response = httpclient.execute(httpget);

Log.i("REST:Response Status line", response.getStatusLine().toString());

HttpEntity entity = response.getEntity();

if (entity != null) {

  InputStream instream = entity.getContent();
  String result = convertStreamToString(instream);
  Log.i("REST: result", result);

  JSONObject json = new JSONObject(result);
  Log.i("REST", "<jsonobject>\n" + json.toString()
          + "\n</jsonobject>");

  // Parsing
                JSONArray nameArray = json.names();
  JSONArray valArray = json.toJSONArray(nameArray);
  for (int i = 0; i < valArray.length(); i++) {
      Log
              .i("REST", "<jsonname" + i + ">\n"
                      + nameArray.getString(i) + "\n</jsonname"
                      + i + ">\n" + "<jsonvalue" + i + ">\n"
                      + valArray.getString(i) + "\n</jsonvalue"
                      + i + ">");
  }

  json.put("sample key", "sample value");
  Log.i("REST", "<jsonobject>\n" + json.toString()
          + "\n</jsonobject>");


  instream.close();
}

} catch (ClientProtocolException e) {
// TODO Auto-generated catch block
            e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
            e.printStackTrace();
} catch (JSONException e) {
// TODO Auto-generated catch block
            e.printStackTrace();
}
}

public class Response
{
public boolean isSuccessful() {
return successful;
}

public void setSuccessful(boolean successful) {
this.successful = successful;
}

public String getComment() {
return comment;
}

public void setComment(String comment) {
this.comment = comment;
}

private boolean successful;
private String comment;

public Response()
{
this(false, null);
}

public Response(boolean successful, String comment)
{
this.successful = successful;
this.comment = comment;
}
}

public class LoginResponse extends Response
{
private String strSessionString;

public LoginResponse()
{
this(false, null, "LoginFailed");
}

public LoginResponse(boolean successful, String strSessionString, String comment)
{
super(successful, comment);
this.strSessionString = strSessionString;
}

public String getStrSessionString() {
return strSessionString;
}

public void setStrSessionString(String strSessionString) {
this.strSessionString = strSessionString;
}
}

public class ActivityStatusResponse extends Response
{
private String activity;

public String getActivity() {
return activity;
}

public void setActivity(String activity) {
this.activity = activity;
}

public ActivityStatusResponse(boolean successfull, String activity, String comment)
{
super(successfull, comment);
this.activity = activity;
}

public ActivityStatusResponse()
{
this(false, null, "activity request failed");
}
}

private static void SetLoginParameters(String username, String password)
{
loginParameters = "Login?username=" + username + "&password=" + password;
}

public LoginResponse Login(String username, String password)
{
SetLoginParameters(username, password);

HttpClient httpclient = new DefaultHttpClient();
HttpGet httpget = new HttpGet(serviceAddress + loginParameters);

HttpResponse response;

LoginResponse loginResponse = null;

try
{
response = httpclient.execute(httpget);

HttpEntity entity = response.getEntity();

if(entity != null)
{
  InputStream instream = entity.getContent();
  String result = convertStreamToString(instream);
  JSONObject json = new JSONObject(result);

  // Parsing
                JSONArray nameArray = json.names();
  JSONArray valArray = json.toJSONArray(nameArray);

  loginResponse = new LoginResponse(valArray.getBoolean(1), valArray.getString(0), valArray.getString(2));


  instream.close();
}

}
catch(Exception e)
{
loginResponse = new LoginResponse();
String sDummy = e.toString();
}
return loginResponse;
}

public Response SendAccelerations(String sSession, String measurementTime, double ax, double ay, double az)
{
setAccelerationParameters(sSession, measurementTime, ax, ay, az);

HttpClient httpclient = new DefaultHttpClient();
HttpGet httpget = new HttpGet(serviceAddress + accelerationParameters);

HttpResponse response;

Response accelerationResponse = null;

try
{
response = httpclient.execute(httpget);

HttpEntity entity = response.getEntity();

if(entity != null)
{
  InputStream instream = entity.getContent();
  String result = convertStreamToString(instream);
  JSONObject json = new JSONObject(result);

  // Parsing
                JSONArray nameArray = json.names();
  JSONArray valArray = json.toJSONArray(nameArray);

  accelerationResponse = new Response(valArray.getBoolean(0), valArray.getString(1));


  instream.close();
}

}
catch(Exception e)
{
accelerationResponse = new Response();
String sDummy = e.toString();
}
return accelerationResponse;
}

private static void setAccelerationParameters(String sSession, String measurementTime, double ax, double ay, double az)
{
accelerationParameters = sSession + "/measurementTime=" + measurementTime + "&accelerationX=" + ax + "&accelerationY=" + ay + "&accelerationZ=" + az;
accelerationParameters = accelerationParameters.replaceAll(" ", "_");
}

public Response SendLocation(String sSession, String locationTime, double latitude, double longitude)
{
setLocationParameters(sSession, locationTime, latitude, longitude);

HttpClient httpclient = new DefaultHttpClient();
HttpGet httpget = new HttpGet(serviceAddress + locationParameters);

HttpResponse response;

Response locationResponse = null;

try
{
response = httpclient.execute(httpget);

HttpEntity entity = response.getEntity();

if(entity != null)
{
  InputStream instream = entity.getContent();
  String result = convertStreamToString(instream);
  JSONObject json = new JSONObject(result);

  // Parsing
                JSONArray nameArray = json.names();
  JSONArray valArray = json.toJSONArray(nameArray);

  locationResponse = new Response(valArray.getBoolean(0), valArray.getString(1));


  instream.close();
}

}
catch(Exception e)
{
locationResponse = new Response();
String sDummy = e.toString();
}
return locationResponse;
}

private static void setLocationParameters(String sSession, String locationTime, double latitude, double longitude)
{
///{strSessionString}/time={time}&lat={latitude}&long={longitude}
        locationParameters = sSession + "/time=" + locationTime + "&lat=" + latitude + "&long=" + longitude;
locationParameters = locationParameters.replaceAll(" ", "_");
}

public ActivityStatusResponse GetActivity(String sSession)
{
setActivityRequestParameters(sSession);

HttpClient httpclient = new DefaultHttpClient();
HttpGet httpget = new HttpGet(serviceAddress + activityRequestParameters);

HttpResponse response;

ActivityStatusResponse activityStatusResponse = null;

try
{
response = httpclient.execute(httpget);

HttpEntity entity = response.getEntity();

if(entity != null)
{
  InputStream instream = entity.getContent();
  String result = convertStreamToString(instream);
  JSONObject json = new JSONObject(result);

  // Parsing
                JSONArray nameArray = json.names();
  JSONArray valArray = json.toJSONArray(nameArray);

  activityStatusResponse = new ActivityStatusResponse(valArray.getBoolean(0), valArray.getString(1), valArray.getString(2));


  instream.close();
}

}
catch(Exception e)
{
activityStatusResponse = new ActivityStatusResponse();
String sDummy = e.toString();
}
return activityStatusResponse;
}

private static void setActivityRequestParameters(String sSession)
{
activityRequestParameters = "/" + sSession + "/activityStatus";
}
}

RestClient methods are called from CommunicationService class. The following example shows the method which retrieves the activity status:

private void requestActivity()
{
try
{
if(strSessionString != null)
{
  ActivityStatusResponse response = restClient.GetActivity(strSessionString);
  connectionAvailable = response.isSuccessful();
  strActivity = response.getActivity();
  if(strActivity != null && strActivity != "")
  {
      announceActivityStatus();
  }

}
}
catch(Exception e)
{
connectionAvailable = false;
}
}

The example above is just a brief overview of practical RESTful service realization using WCF and data exchange with Android client. More detailed explanations of each segment could be found at the given addresses.

References

A Guide to Designing and Building RESTful Web Services with WCF 3.5
Windows Communication Foundation Architecture
What Is Windows Communication Foundation?

Client Architecture: Conceptual Overview

The client application is the part of solution which runs on mobile devices. In general Android applications consit of the following elements:

• Foreground Activity (Class with processes wihich run in the foreground and enable user interaction - User Interface)
• Background Service (Class with processes which run in the background and exchange data with foreground activity and other services)
• Intermittent Activity (The processes which can be modified directly by user and run in the background after user's modifications)

In case of solution for motion determination the main client's task is to retrieve the values from acceleration sensor. Optionally, in case we want to enable automatic emergency call with location report, the client has to determine the location by simply reading GPS values or getting the information from available web services which are able to determine the approximate location by base station the phone currently uses. In order to enable storing informations for later motion analysis in case the internet connection is unavailable, we can add the database and the logic for data storing and later retrieval before sending.

The picture represents the client application concept. The red rectangles represent activities and blue are the services. The application consits of three dialogs: main dialog, login and settings. The main dialog displays current activity status, acceleration values and location informations. On application start the user has to sign-in to the server with username and password so at the beginning the login dialog appears. Some options modifications such as enable/disable location report are possible by using Settings dialog.

There are four services running in the background . The Location Service retrieves the location informations and broadcasts the location data which are received at main (GaitLink) activity, Communication Service and optionally at Database Service. The Acceleration Service retrieves the data from accelerometers and also broadcasts the data which are received by the same components. The Database Service listens for the broadcasted messages which can represent the data that have to be stored or the commands for data reading and sending to Communication Service.

Communication Service implements multiple threads. Broadcast Receivers for acceleration and location messages run in separate thread. Every received message is sent to queue. Another thread is periodically retrieving the messages from queues and send the messages to the server. In order to prevent queue congestion the reading frequency has to be higher than queue writing frequency. The main reason for using queues are possible transitory network delays or drops. Without intermediate measurements storing to the queue every value would have to be successfully sent before reading next values from the sensors which is certainly fair too unreliable.

Determining Body Movement with Mobile Phone

Currently I'm trying to develop a solution which would determine the type of movement by measuring accelerations with mobile phone.

By reading acceleration data many things can be discovered. The persons's walk pattern is almost as unique as fingerprint. By reading the walk (movement) patterns early stages of a possible disease could be recognized as well. Another thing that could be implemented is fall/collision detection and automatic emergency call. These are just a few of the possible uses of the solution.

So far the mobile devices haven't been powerful enough to perform all the required processes independently, but luckily the data transmission is cheap and fast enough so the main analysis can be performed on the server.

The accelerometers are cheap so almost every new phone have built-in sensors. Currently I'm using HTC Dream G1 (Android) phone and developing client application in Java. The server-side application has been built in C# .NET.

So far I have established the connection between the phone and server. The phone sends the location data and acceleration measurements to the server which runs the analysis.
I've managed to determine some simple user behaviour patterns such as walk, run and rest by measuring the frequency components of the pattern.

Below there are the charts for collision and walk in time and frequency domain.

These are the acceleration values of simulated collision:



and these are the accelerations of walk:
In both cases The DFT has been performed on 32 sample points at sampling frequency 2.5 Hz.
This is the phone screen (any suggestion about the design is more than welcome). The phone gets the location address by performing reverse geocoding (part of the existing framework) and the activity status by questioning the server.