Center for the Handheld Web

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)

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.

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.

Figure 3: A cross domain request PHP technique

Links to other blogs in this project

• Resource pooling and scheduling systems during a disaster and/or crisis situation using a handheld device and the World
  Wide Web (WWW) technologies
• Web-based resource tracking system implemented on a mobile device
• Location Tracking Techniques
• Creating REST Web Service
• REST Client simple Implementation and Test
• REST Service Implementation using HTTP – Cross Domain Request