Posts Tagged ‘web service’

REST Service Implementation using HTTP – Cross Domain Request

Sunday, March 29th, 2009

By Phavanhna Douangboupha, 03/29/09

The three main basic ideas of a REST process are to process a client request, to response to the request, and return a result/data in XML format according to the request.

Two techniques that can be used are getting parameters from client through a URL query string and the use of HTTP methods. The first technique is easier to implement compared to the second technique. The main disadvantages of the first technique include the size of the URL string, the maximum length of the URL string consisting of query parameters, and a possible negative side effect. The first limitation, the size of URL string, can be overcome by using POST method instead of GET method for a client request. However, POST and GET methods should be applied according to a specific task (see the blog on Creating REST Web Service for more detail). Consequently, the HTTP methods are a better solution for REST service.

I have already talked about how to implement a REST client using a URL query string from my previous post, now I will talk about how I implement a REST service for the cross server client request using HTTP methods.

For this exercise, a database server, a web host server and a web client server are assigned in different machines and hosted by different domains. The database server is hosted at STREAMER. The web host server is hosted by CHW domain and finally the demonstrated web client request comes from GIBSON domain (Figure 1 shows system architect for the REST cross domain service). STREAMER is a house to MySQL database which is hosted in a different domain from the web server.

System architect for REST cross domain service
System architect for REST cross domain service

Figure 1: System architect for REST cross domain service

GIBSON is used as a demonstration client to request data from another domain (CHW). The four main files implemented here are a normal HTML file (index.html), a JavaScript file (client_local.js), and two php using cURL libraries files (getclient_chw.php and postclient_chw.php). The Javascript file is used to implement Ajax to send a request and to receive responded data sent back by a server asynchronously. The data is displayed on the HTML file. The JavaScript file uses HTTP request object to send a request to either getclient_chw.php (”How Many Users?” in the database) or postclient_chw.php (”Add New User” to the database). getclient_chw.php and postclient_chw.php use cURL libraries to set up a HTTP request where getclient_chw.php uses the GET method to request to get data from a server and postclient_chw.php uses the POST method to request to post data to a server (please see table 1 for the correspondent HTTP methods to the database query). Both of the files make a request to process data with the web server on CHW. The client side has no relation to the database server on STREAMER and it only sends a HTTP request which will later be checked by the web server.

Use Gibson to act as a client to make REST request​ (Figure 2) - http://people.rit.edu/~pxd8840/restclient/index.html . As you click one of the two buttons – “How Many Users?” (HTTP GET) and “Add New User” (HTTP POST), you will see that the displayed results are updated asynchronously. These are done via Ajax using REST service to perform a cross-domain request instead of the <script> tag hack solution as mentioned from my previous post.

index.html, client requests on GIBSON

index.html, client requests on GIBSON

Figure 2: index.html, client requests on GIBSON

On the web host server (CHW), there are two main files – rest_database.php and ConnectDB.php. ConnectDB.php is an Object Oriented PHP class that contains methods to connect to the database and the required methods to process or retrieve data from STREAMER database server. The rest_database.php file also contains all the logic check method that received from a client. The rest_database.php file is used to check what kind of request being made if it is a HTTP GET request or a HTTP POST request. According to the request, it retrieves data from the database. Then it creates XML responses to the client.

Table 1: HTTP method, REST action, and SQL command for a client request

HTTP Request Method REST Action SQL database command Description
GET GET SELECT Search/Request for
data (getdata)
POST POST INSERT Add/Insert new data
(postdata)

List of files

Client Side

  • index.html
  • client_local.js
  • postclient_chw.php
  • getclient_chw.php

Server Side

  • rest_database_php
  • ConnectDB.php class code on the server side contains database connection and database process methods

References:

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REST Client simple Implementation and Test

Sunday, March 8th, 2009

by Phavanhna Douangboupha

There are many ways to implement a REST web service client request which includes PHP, JavaScript XMLHttpRequest, C# system web HTTP web request, Java HttpClient, the command line:curl, Python, and Ruby. I am going to talk about PHP and JavaScript XMLHttpRequest techniques in this blog.

The purpose of using REST web service in this example is to enable cross domain database access and to take advantage of REST as mentioned in my previous blog about REST. Wikipedia has a good explaination about the technology.

There are three main portions in my small test system. First, it is a database server (data tier). The data tier is the MySQL database created on a server. Second, it is a logic tier that contains logical decision and evaluation functions between a client and the database server. At this logic tier, there are two separate files – a client side file and a server side file. The client side consists of an implementation method for a request made to the server. On the other hand, the server side contains implementation of logical decisions and evaluations. It handles all logic checks, retrieving data from the database and sending back response to a client (Figure 1 shows the flowchart for the server side logic tier). In this case, “getdata” type represents the retrieval request and the “putdata” represents the insert data operation request from a client.

The client side requests are created for both on the same domain with the server and a client request from a cross domain.

Finally, it is a very simple presentation layer to display results of the request.

Logic flow for the server side logic tier (rest_database.php)
Logic flow for the server side logic tier (rest_database.php)

Figure 1: Flowchart for the server side logic tier

A request response in this test is in the form of XML format. Two main responses are the error notification response and the data or status response. While, the error XML response is used to indicate that there is something wrong with the request or the request is not allowed. The data or status response either contains requested data to send back or status message of the request operation performed by the server.

JavaScript:XMLHttpRequest

In this test XMLHttpRequest is used to make a request for data from the database and to insert new data to the database from a client. The client is located on the same domain with the server. First a method is set up to initiate either retrieval data request or insert data request. The rest is simply a standard XMLHttpRequest with the requested parameter passing.

client_js

Figure 2: same domain JavaScript XMLHttpRequest technique

JavaScript XMLHttpRequest is widely used and it has many advantages. One includes the idea of Ajax. However, this method only works if the client and the server are both located on the same domain. How about if a client request is made from a cross domain, then this technique will not work. A well known solution to this problem is to use a cross-domain request called <script> tag hack. In brief, the way this technique works is that the data is encoded as JavaScript objects in the file instead of being located inside a function. The file is seen as a JavaScript file from a domain instead of a data request. This way, the objects will be parsed and immediately executed. I am not going to implement the method in this test; since, there is another easier and cleaner way to do it; that is the use of PHP cURL extension.

PHP cURL extension

One of the PHP methods to make a REST request is using cURL extension. I use this technique to make a data request for cross domain web service for my database; Prof. Jeffrey Sonstein has a more complete picture of RESTful API example and I use his example as a reference for my solution.

It is pretty simple to implement cURL for a cross domain request. To implement this technique, it requires cURL extension library which can be downloaded from http://curl.haxx.se/. In addition, PHP server side is required to have cURL enable configuration. Following are the steps for implementing the PHP cURL cross domain client request. First of all, http_build_query() is used to create a query string from an array which is part of the request URI. To implement a client request using cURL, we first start by creating and initializing a cURL request by curl_init() method. Then, curl_setopt() is used to set the request. The curl_exec() method is called to make the request to a server and it returns the response from the server. Finally, curl_close() is used to close the connection of the transaction after it completes the transaction.

For this test system, to request data or to insert a new data to a database on a server located in another domain, difference query command is used – “getdata” or “putdata”. The command is checked by the server side logic tier to differentiate if the request is to retrieve data or to insert data. The logic tier handles all the operations and sends back a message after it completes executing the request.

REST client

Figure 3: A cross domain request PHP technique

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Location Tracking Techniques

Saturday, January 31st, 2009

by Phavanhna Douangboupha

This blog contains information about some of the current practices in location tracking technologies. It mainly discusses the three common methodologies – GPS, Cell Towers, and Wi-Fi positioning service. The blog is a part of the ongoing investigation for the proposed capstone project on Web-based resource tracking during a disaster or crisis situation.

In general, there are four geo-location methods including triangulate, associate, geo term extraction, and data entry or geo-coding. The Global Positioning System (GPS), Cell Towers, and Wi-Fi positioning service (WPS) are the three well known triangulation techniques to identify a mobile device geo-location. GPS and Cell Towers are based on triangulation to identify an object position by using the location of known objects.

To put a location on a map, the geo-coding position technology relies on getting a location of an object via a meaningful X, Y coordinate or a latitude and longitude coordinate. In fact any available mapping tool such as Microsoft live search maps, Google maps, and Yahoo maps use the coordinate system to identify a requested location.

GPS relies on satellites that send microwave signals information back to the earth. The information is a navigation message of each satellite’s position and time. An object location is calculated by GPS receivers by the use of triangulation and the signal information provided by at least three satellites in order to determine an object’s location and four satellites for greater accuracy. The use of fourth satellite enhances the accuracy in the order of nanoseconds. GPS receivers compare the time difference between the arrival of satellite signals to tell the position. At least three satellites are needed in the calculation for an accuracy result. The first satellite provides a possible location of an object narrowed down to the surface of a sphere. The position is recorded as a radius equal to range 1. On the same token, satellite number two provides confirmation that the object is located within the first sphere (as located by the first satellite). Satellite number two provides an additional position circle of a radius range 2. The first and second satellite indicate that the object is positioned between the intersection between their two spheres – sphere one and sphere two. Finally, the third satellite shows a third sphere of radius range 3 for the positions. The object position is the intersect location of the three spheres. Finally, the fourth satellite is used to confirm the location and hence provides the time reference.

A GPS-enabled device provides geo-code location accuracy about 1 to 5 meters. Despite the fact that GPS system is widely used with many position tracking systems, it does come with some drawbacks. GPS does not work within indoor environment positioning and it requires costly power-consumption on mobile devices. In addition, not all mobile devices are GPS-enabled.

Another solution for location finding is the Cell Tower technique. Similar to GPS system, it requires at least three different cell towers within range of the device to calculate an object location for a high accuracy result. Otherwise, at least two cell towers are required. Each cell tower ,with a unique cell identifier, returns a positioning data to a requested mobile phone. Each mobile phone constantly pings a signal to nearby cell towers to get the cellular radio signal and hence some mobile phones also require costly power-consumption just like in the GPS  positioning techniuqe. Having data from all the cell towers, an algorithm can be used to calculate a final location of the mobile phone which lies in the middle. The advantage of cell tower triangulation technique is that is available for all mobile phones that have registered service providers . In addition, cell towers can be set up to send signal to mobile devices. Unlike GPS, cell towers will work in both indoor and outdoor environment. The accuracy of a position allocation depends on the density of cell towers in the area.

Wi-Fi Positioning Service (WPS) is the least accurate technique for location finding among the three techniques (GPS, Cell Towers, and WPS). IP address from a mobile device Wi-Fi connection is used to get a guessed location back from a service provider database such as Google map using API. Some of the available and well known mobile device location locator technologies are Google gear (Google map version for a mobile device), iPhone Core Location, and Navizon. Google mobile map and Navizon peer-to-peer wireless positioning tools  make the use of their massive data collection to provide a best guessed location for a device without GPS-enabled and for thoser devices that can not communicate with cell towers.

Google gear uses all three triangulation technologies to get the best accurate result. Google gear is compatible with many mobile devices including Windows Mobile, and Android. On the other hand, Core Location is another tool specifically programmed for iPhone. Apart from Google gear and iPhone Core Location, Navizon is another map locator tool. Navizon is free for a cellular enabled device, with some limitations, using cell ID positioning. However,  it is not free for a Wi-Fi or cellular enable device using cellular and Wi-Fi triangulation. Navizon database collects geo-coding data from registered users or devices with GPS-enabled. These data is collected and used as a virtual GPS. The tool uses the bank of data as a reference positioning point to locate a mobile device geo-location.

Another alternative technology is a tool so called PhoneGap that utilizes web application technology and Objective-C core features available on three mobile devices – iPhone, Android, and Blackberry.

References

  1. Bellavista, P., & Corradi, A. (2007). Mobile Middleware for Location-Dependent Services. In The Handbook of Mobile Middleware. USA: Auerbach Publications
  2. Berka, J. (2008, January 22). PhoneGa tool provides JavaScript access to iPhone features. In PhoneGap tool provides JavaScript access to iPhone features - Ars Technica [Internet Article]. Retrieved January 31, 2009, from
    http://arstechnica.com/apple/news/2008/10/
    PhoneGap-tool-provides-javascript-access-to-iphone-features.
  3. B’Far, R. (2005). Mobile Computing Principles: Designing and Developing Mobile Applications with UML and XML. United Kingdom: Cambridge University Press.
  4. Google. (2008). Services – Google Maps API – Google Code. In Google Maps API [Google Maps API Reference]. Retrieved November 9, 2008, from Google Web site: http://code.google.com/apis/maps/documentation/
    services.html#XML_Requests
  5. Google. (2009). Geolocation API. In Geolocation API – Gears API – Google Code [documentation]. Retrieved January 31, 2009, from Google Web site: http://code.google.com/apis/gears/api_geolocation.html#getCurrent
  6. Katsaros, D., Nanopoulos, A., & Manolopoulos (eds), Y. (2005). Location-Based Services. In Wireless Information Highways (section iv – location-based
    services). United States of America: Idea Group Publishing . Retrieved
    November 6, 2008
  7. Mallick, M. (2003). Mobile and Wireless Design Essentials. Indianapolis,
    Indiana, USA: Wiley Publishing. Retrieved November 8, 2008
  8. Mark, D., & LaMarche, J. (2009). Where Am I? Finding Your Way with Core
    Location. In Beginning iPhone Development: Exploring the iPhone SDK (pp. 429-439). USA: Apress
  9. Mexens. (2005-2008). How it works. In Peer-to-peer wireless positionin [product description]. Retrieved January 31, 2009, from http://www.navizon.com
  10. Olla, P. (2008). Global Navigation and Satellite Systems and Services. In
    Commerce in Space: Infrastructures, Technologies, and Applications
    (chapter v). USA: IGI Publishing. Retrieved November 8, 2008
  11. Wu, S.-L., & Tseng, Y.-C. (2007). Wireless Ad Hoc Networking-Personal-Area, Local-Area, and the Sensory-Area Networks (S.-L. Wu & Y.-C. Tseng, Eds.). USA: Auerbach Publications. Retrieved January 30, 2009

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Creating REST Web Service

Tuesday, December 9th, 2008

by Phavanhna Douangboupha

Representational State Transfer (REST) is a web service that offers many advantages compared to Simple Object Access Protocol (SOAP) web service. REST can be a solution to mobile device web limitations. Amazon, EBay, and Yahoo are the examples of REST web services. REST service architect includes XML, HTTP, URI, and MIME type.

The first thing to consider when creating a REST is URI. Unlike URL, URI is suitable for REST since it points to a resource of a web service and hence does not change over time. Richards (2006) suggests a structure of URI for a web service.

A web service returns data in the form of XML format as defined by the service implementer (Richards, 2006). Therefore, different REST web services can have different XML format and there is no particular standard. MIME must be in the type of text/xml for XML (Content-type: text/xml).

HTTP methods that are commonly used for REST are GET, HEAD, POST, PUT, and DELETE (Create, Retrieve, Update, and Delete). Richards (2006) suggests to use GET and POST methods differently according to each functionality. He states that GET should only be used for retrieving a resource representation. In contrast, POST can be used for other operations rather than the resource retrieval including resource creation, medication, addition, and deletion. For security purposes, as a result of performing GET request, according to Gregorio (2004) and Richards (2006), there should not be any side-effects that users unaware of and therefore the implementation of GET method should be safe and idempotent. The idempotent method is the method that provides the same result every time a service is requested.

Apart from URI, data format and methods, we also have to consider the other types of web service status codes (Gregorio, 2004) – 2xx for success, 3xx for redirection, and 4xx for errors.

The next blog is going to address how REST will be used in the resource pooling and prediction project using handheld devices.

References

Gregorio, J. (2004, December 1). How to create a REST protocol. Retrieved December 8, 2008

Richards, R. (2006). Pro PHP XML and Web Services. New York, USA: Apress . Retrieved December 8, 2008, from Books24×7 database: http://library.books24×7.com.ezproxy.rit.edu/

Singh, M. P., & Huhns, M. N. (2005). Service-Oriented Computing: Semantics, Processes, Agents. England: John Wiley & Sons. Retrieved December 8, 2008, from Books24×7 database: http://library.books24×7.com.ezproxy.rit.edu/

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