Dimuthu’s Blog

WSO2 announced an another round of release of their famous SOA products.

• WSO2 Governance Registry 3.0.1
• WSO2 ESB 2.1.1
• WOS2 Web Services Application Server (WSAS) 3.1.1
• WSO2 Identity Server (IS) 2.0.1
• WSO2 Mashup Server 2.0.0

Although the version numbers say this is minor patch release (Other than the Mashup Server which is shipping as a major release), in fact there are new features and improvements. Some basic new features shares among all of these products are

1. Improved registry level transaction Support.
2. Improved Support for deploying on top of Application Servers other than tomcat like WebSphere, WebLogic, and
   JBoss.
3. Support for Eclipse P2 based provisioning. (Yes, you can add/remove features from these WSO2 products , see https://wso2.org/wiki/display/carbon/p2-based-provisioning-support for more details)
4. Improved Remote Registry model

WSO2 is preparing for the first major release of their enterprise java product series after adapting the OSGI technology. You can already try out the betas from the wso2.org site.

1. WSO2 Web Services Application Server (WSAS)
   
2. WSO2 Enterprise Service Bus (ESB)
3. WSO2 Registry
4. WSO2 Business Process Server (BPS)

With the power of OSGI you will be able to customize these products for your need just by mixing and matching the components within these products. If you like to learn more about this, just have a loot at the following ebook released by WSO2.

Making Good SOA Great

Now you can view the article I wrote titling “Introduction to PHP Data Services“. There I explain how you can design and implement Data Services in PHP using WSF/PHP Data Services Library.

This article covers,

1. Designing your Data Service API.
2. Writing the Data Service.
3. Deploying and Testing Data Service.
4. Make the Data Service available in both SOAP and RESTful form.
5. Use of WS-* features in your Data Service.

If you are thinking of adapting SOA in to your database backed PHP applications, this article will be a good starting point.

In simple term, data services are exposing data as web services. Anyway it is not a complete definition. Actually there are situations where we use data services not only to read data, but also to create, update or delete data. So it is better say data services are doing CRUD (Create, Read, Update, Delete) operations for data through web services. Simply it is like providing a web service interface for the database.

Anyway exposing  a database directly as a web service is like violating the first principles of software engineering. It will tightly couple the database structure with the interface, so whenever you do a simple change to the database schema, you will have to change the web service interface which will no doubt break all the clients depending on it.

So first of all, you have to design the service interface independent of the database schema you have. Most of the time you will able to find some query that would map the service interface to the database schema.

For an example think of publishing data in database table (say for table name “Games”) like this.

You will directly able to map these data to data service. So the response payload for a “getGames” operation would be something like,

<getGamesResponse>
   <Game>
      <Venue>
         xxx stadium
      </Venue>
      <Date>
         2008-12-18
      </date>
      <Team1>
          Italy
      </Team1>
      <Team2>
          Sweden
      </Team2>
      <Team1Score>
          34
      </Team1Score>
      <Team2Score>
          33
      </Team2Score>
   </Game>
   <Game>
      <Venue>
         yyy stadium
      </Venue>
      <Date>
         2008-12-19
      </date>
      <Team1>
          France
      </Team1>
      <Team2>
          Spain
      </Team2>
      <Team1Score>
          51
      </Team1Score>
      <Team2Score>
          50
      </Team2Score>
   </Game>
</getGamesResponse>

You can get this done with a SQL query simply as this,

SELECT * FROM `Games`

Say later if you decided to restructure the database table so the new database schema would be like this,

Games


GameId
Venue
Date


1
xxx stadium
2008-12-18


1
yyy stadium
2008-12-19

GamesTeams


GameId
TeamId
score


1
1
70


1
2
33


2
1
51


2
3
50

(Note here the Games and Teams are associated in the GamesTeams table.)

You can still use a query like the one in following to provide the same service interface, because it returns the same result set as the earlier one.

   SELECT Games.Venue,
              Games.Date,
              Team1.Name AS Team1,
              Team2.Name AS Team2,
              GameTeam1.Score AS Score1,
              GameTeam2.Score AS Score2
         FROM Teams Team1,
              Teams Team2,
              GamesTeams GameTeam1,
              GamesTeams GameTeam2,
              Games
        WHERE GameTeam1.gameId = Games.gameId  AND
              GameTeam2.gameId = Games.gameId AND
              GameTeam1.teamId = Team1.teamId AND
              GameTeam2.teamId = Team2.teamId AND
              Team1.teamId <> Team2.teamId AND
              Team1.name=?

This allows you to keep the service interface unchanged, regardless of the changes you done to the database schema.

In addition to that, we can uses the features of the data service libraries to give meaningful names for the response elements. If we take above example itself, say you want to rename ‘Score1′ to ‘Team1-Score’ and ‘Score2′ to ‘Team2-Score’. But the dash character (‘-’) cannot be used as a variable in a database query. But you can provide that in the map of sql name to element name when you are writing the data service.

If you are using WSF/PHP php data services Here is how you provide that mapping,

$outputFormat = array("resultElement" => "getGamesResponse",
                      "rowElement" => "game",  // this is the repeating wrapper element for each game
                      "elements" => array( "Venue" => "Venue", // this is the mapping of xml name => sql name
                                           "Date" => "Date",
                                           "Team1" => "Team1",
                                           "Team2" => "Team2",
                                           "Team1-score" => "Score1", // we are using different names for sql and xml
                                           "Team2-score" => "Score2"));

If you write it using Java Data Services, you can use do this with the following configuration xml,

        <result element="getGamesResponse" rowName="game">
            <element name="Venue" column="Venu" />
            <element name="Date" column="Date" />
            <element name="Team1" column="Team1" />
            <element name="Team2" column="Team2" />
            <element name="Team1-Score" column="Score1" />
            <element name="Team2-Score" column="Score2" />
        </result>

Another consideration, when talking about the best practices of designing web services or data services is the granularity of the service interface. We say a web service is fine grained if the service contains tons of small operations. The opposite of that is the coarse grained services, which contains large but few operations. And it is recommended to use the later approach, you can see why from the following example.

Lets say you are developing a web service to upload some information. In order to upload the information first the user have to be authenticated and then the content should be validated. Then only he can submit the actual information. Say you design a fine grained web service for that. So it has three operations.

1. authenticateUser
2. validateContent
3. submitInformation

If you design a coarse grained service interface it will be just the ‘uploadInformation’ operation. And all the three operation defined earlier will be called within the service logic and the clients will not be aware of that.

So here are some disadvantages that I see in the fine grained interface design compared with coarse grained design.

1. The coupling is too high. Since the client is linked with the service in three adapters. Say you decided to change the service so that you first validate the content and depending on the content it sometime bypasses the authentication. You can’t do this with the former approach (fine grained approach) without changing the clients. But if it were designed as a coarse grained service (just one uploadInformation operation), the client need not to be changed.
2. Two much time consumed for the transmission. Since we used three web service calls to do a single task the latency of the operation will largely depend on the network latency which will no doubt is comparatively very low. So the performance of the operation is degraded.
3. The clients can bypass some steps!, Say in your fine grained service, some client bypass the authentication and validateContent steps and jump directly to the sumbitContent operation. In fact you have to write special code to make sure the clients call the service in the correct sequence, otherwise it will be a big security hole.

So I think these three points will be enough to explain why you should try to design a coarse grained interface for your web service.

You can apply these principle when you write data services as well. Lets take the same example explained above. Say there is another table that keep the scoring shots of each game like the one in following.

Here also you can keep the operation like ‘getGames’ which we defined in the above section. That operation only provide the basic information like the winner and the scores. So if the clients want to know about scoring shots as well, he have to call another operation, say getScoringShots(game) that will return the scoring shots results for a given game. If in practice the clients only need to know about scoring shots of few selected games, then this approach is ok.
But say normally clients need to know about the scoring shots of each and every games. Then they have to call the operation ‘getScoringShots’ multiple times. That’s when the lesson we just learn about the granularity can be applied. We can actually provide another operation, say ‘getGamesDetailed’ that actually bundle the details of scoring shots for all games with other information about the game. Here is snip of the response XML, I’m talking about.

<getGamesDetailedResponse>
   <Game>
      <Venue>
         xxx stadium
      </Venue>
      <Date>
         2008-12-18
      </date>
      <Team1>
          Italy
      </Team1>
      <Team2>
          Sweden
      </Team2>
      <Team1Score>
          34
      </Team1Score>
      <Team2Score>
          33
      </Team2Score>
      <!-- additionally we have ScoringShots element -->
      <ScoringShots>
          <Shot>
              <Time> xxx </Time>
              <ScoringPlayer> xxx </ScoringPlayer>
              <ScoringTeam> xxx </ScoringTeam>
              <Score> xxx </Score>
          </Shot>
          <Shot>
              <Time> yyy </Time>
              <ScoringPlayer> yyy </ScoringPlayer>
              <ScoringTeam> yyy </ScoringTeam>
              <Score> yyy </Score>
          </Shot>
      </ScoringShots>
   </Game>

   <Game>
    <!-- Another game details are mentioned here -->
   </Game>

   <!-- More Games -->
</getGamesDetailedResponse>

You can generate this kind of response using the nested query support of the data services libraries.
You can checkout more details about nested queries in php data services from my old post about php data service API.

LAMP (Linux + Apache + Mysql + PHP) stack powers many servers in the Internet today. For a LAMP  server, PostgreSQL could be the first alternative to Mysql. Similar to PHP + MySQL,  PHP + PostgreSQL too can be easily used in to host data services. Here are the steps to do it.

1. If you already don’t have Apache + PHP + PostgreSQL download them from the following locations. Apache – http://httpd.apache.org/download.cgi, PHP – http://php.net and PostgreSQL – http://www.postgresql.org/download/
2. You have to enable the PHP pdo_pgsql, pdo and pgsql plugins. Read here for the instructions to setup these libraries. (For an example: if you are windows you have to set the system ‘PATH’ variable to the <postgresql_installed_dir>/bin directory.
3. If you already don’t have WSF/PHP, download and install it according to the guidelines provided in wsf/php installation guide.NOTE: You can check pdo_pgsql and wsf/php has properly installed with the help of phpinfo() function.
4. Now lets start with creating a sample Database table. For this example I created a database called ‘workshop’, schema called ‘workshop’ and inside there the table ‘Employee’ with the following schema.
   

   Column Name	Column Type
   employId	integer
   name	character varying
   email	character varying
   jobTitle	character varying
   project	character varying

   Note: You can use phpPgAdmin (web based) or pgAdmin III to create tables from GUI

5. Then you can write a small php script to expose the data in the above table as a web service.

   <?php
   
   //Including the Data Services library
   require_once("wso2/DataServices/DataService.php");
   
   // Including the connection information (i.e. PGSQUL USERNAME
   // and PGSQL_PASSWORD) for my PGSQL Connection
   require_once("constants.php");
   
   // database configurations
   $config = array(
   		"db" => "pgsql",
   		"username" => PGSQL_USERNAME,
   		"password" => PGSQL_PASSWORD,
   		"dbname" => "workshop",
   		"dbhost" => "localhost"
   		);
   
   $output_format = array(
                       "resultElement" => "employees",
                       "rowElement" => "employee",
                       "elements" => array(
   			    "id" => "employeeId",
                               "name" => "name",
                               "email" => "email",
                               "jobTitle" => "jobTitle",
                               "project" => "project"));
   
   $sql = "SELECT * FROM workshop.Employees";
   
   $get_employees_op = array("outputFormat" => $output_format, "sql" => $sql);
   
   $get_employees_url = array("HTTPMethod" => "GET", "RESTLocation" => "employees");
   
   // list of operations
   $operations = array(
                   "getEmployees" => $get_employees_op,
                   );
   
   // list of rest url mappping (operation => url)
   $restmap = array(
                   "getEmployees" => $get_employees_url,
                   );
   
   // creating DSService and reply
   $service = new DataService(array(
            "config" => $config,
            "operations" => $operations, "RESTMapping"=>$restmap));
   $service->reply();

   ?>

6. We just wrote a PostgreSQL Data Services that provides its service as both REST and SOAP form. To deploy this service, We just need to copy this in to the web root directory. And the web URL for the script will be the endpoint to the web service.
7. We can test the service either by calling its SOAP interface, which we may need to write a small SOAP client or by calling its REST interface, which only need a GET request from the browser. Say my script name is “my_dataservice.php” and I’ve put it in the web root directory, then the URL to call the REST interface of the service is

   http://localhost/my_dataservice.php/employees

WSDL Generation for PostgreSQL Data Service
You can get the WSDL for the service from the URL formed adding the suffix “?wsdl” (or “?wsdl2″ to wsdl v2.0) to the service URL,

http://localhost/my_dataservice.php?wsdl

Here all the schema data types are shown as xsd:anyType, which may not be the behavior that you want. In fact you can provide the schema data types to the fields from the code itself. Lets change the $outputFormat variable to provide the schema information as well using the following code snip.

$output_format = array(
                    "resultElement" => "employees",
                    "rowElement" => "employee",
                    "elements" => array(
			    "id" => array("column" => "employeeId",
		    			  "xsdType" => "xsd:int"),
			    "name" => array("column" => "name",
		    			  "xsdType" => "xsd:string"),
			    "email" => array("column" => "email",
		    			  "xsdType" => "xsd:string"),
			    "jobTitle" => array("column" => "jobTitle",
		    			  "xsdType" => "xsd:string"),
			    "project" => array("column" => "project",
		    			  "xsdType" => "xsd:string")));

Note that you provide the xsd type for each field explicitly. In fact this change is not needed for mysql pdo extension since it allows identifying field types programatically. Since this feature is not available in all the other pdo drivers, we have to explicitly give xsd type information for them.

If you wan to provide data services with SQLite or MSSQL, You can check my other posts on MSSQL(Microsoft SQL) Data Services In PHP and Data Services with SQLite in PHP.

With WSF/PHP Data Service library, you can convert a SQL query to a Data Service very easily in few steps.

1. Decide your SQL query first, For the query you may require some input parameters, and you have to decide what should be returned by the query, Say your query is

   $sql_query = "SELECT name, age, email FROM users where country = ?";

   Then the “country” is our input parameter and the name, age, email are our return values.

2. Define the input Format. For the above query it will be something like,

   $inputFormat = array("country" => "INT");

3. Define the output format. We are giving the name “Users” for the out most wrapper element and the name “user” for the wrapper element of each user. Here is how we define it,

   $outputFormat = array("resultElement" => "users",
                   "rowElement" => "user",
                   "elements" => array(
                               "name" => "name",
                               "age" => "age",
                               "email" => "email"));

   For this output format, our expected result payload would be like,

   <users>
     <user>
        <name>xxx</name>
        <age>23</age>
        <email>xxx@xxx.xx</email>
     </user>
     <user>
        <name>yyy</name>
        <age>23</age>
        <email>yyy@yyy.yy</email>
     </user>
     ....
   </users>

4. Define your operation using the sql query, input and output formats.

   $operations = array("getUsersByCountry" => array(
                                 "inputFormat" => $inputFormat,
                                 "sql" => $sql_query,
                                 "outputFormat" => $outputFormat));

5. Define your database configuration in an array like this,

   $config = array(
       "db" => "mysql", // your db engine
       "username" => "myname", // name & password for the db server
       "password" => "mypasswd",
       "dbname" => "db", // the db
       "dbhost" => "localhost");

6. Create a DataService instance using the database configuration and the operations we just created. And call DataServices reply method.

   $ds_service = new DataService(array(
                  "config" => $config,
                  "operations" => $operations));
   
   $ds_service->reply();

That is it. You just exposed your query as a web service. The PHP script URL will be the endpoint URL for the web service.

In a RESTful design, choose of URLs for resources are really important. The URL uniquely represents a resource. Service consumers can change some parts in the URL to access different other resources. So it is clear that the URL consists of some constant parts which describe the resource group or catalog in general and some variable parts which have different and unique values for different resources.

As an example look at the following URL patterns

• students/{name} – The constant ’students’ represent the students group in general and the variable ‘name’ is used to identify each student individually.
• students/{name}/marks/{subject} – The constants ’students’ and ‘marks’ shows that this resource is a marks of some students, The two variables ‘name’ and ’subjects’ addresses which student and marks of which subject is presented in the URL.

You can have a look at some of the uses of such mappings from RESTful School demo.

WSF/PHP allows you to create RESTful Web Services and further more RESTful Data Services in PHP.

In a RESTful Data Service you expose a database query as a web service. There you can write a prepared statement and feed arguments for the statement through the variable parameters of the URL. For an example take the RESTfulSchool Demo Code.

To retrieve a particular student, we can use the following prepared statement and the URL pattern ( This URL Pattern+  HTTP ‘GET’ method is matched to execute this query. )

$sql = "SELECT * FROM Students where StudentName = ?"

$get_students_with_name_url = array("HTTPMethod" => "GET",
                 "RESTLocation" => "students/{name}");

So you can execute this prepared statement with the subject name ‘John’ using the following URL.

http://labs.wso2.org/wsf/php/solutions/RESTFulSchool/school_service.php/students/john

If your service is not exposing the database directly, then you have to choose the general web service API rather than the data service specific API. In there you will be able to write your business logic for publishing student information in a PHP function and expose it as a web service.

In such a cas,e your function is taking an argument which of type WSMessage. This structure hold an XML that contains values for all the variable parameters as in the users request URL. For an example for above REST Mapping, we can expect the following XML.

<getSubject>
    <name>Chemistry</name>
</getSubject>

And your function to expose as a service, would be look like this,

function getSubject($input) {
    /* retrieve the subject name from the
       $input xml using simple xml */

    $input_xml = new SimpleXMLElement($input->str);

    $subject_name = $input->name;

    /* write the logic to retrieve subject information
      for the $subject_name */

    ....
}

We use ‘GROUP BY’ SQL construct to query the data with aggregating some rows according to a field. For an example say if your blog database store your blogs in a table call ‘Blog’ and it has ‘Date’ as a field. If so

SELECT count(*) FROM Blog GROUP BY Date

will give you a set of numbers that represent the number of blog you posted each day.

SELECT Date, count(*) FROM Blog GROUP BY Date

will give you a map of ‘date’ to ‘number of blog posted for that date’ without much trouble. 

Anyway the problem is most of the databases of blogs don’t just keep the ‘date’ for a blog, rather it keep both ‘date and time’ (say in a field called ‘Time’). But you still want to group by date. You may use the MySQL ‘DATE’ function to convert the ‘Date and Time value’ to just ‘Date’ and use it in GROUP BY statement.

SELECT Date, count(*) FROM Blog GROUP BY DATE(Time)

If you take Drupal for a blog database, it save the time of the blog entry as a unix timestamp. So you have to derive the Date from the timestamp using the infamous FROM_UNIXTIME mysql function,

In Drupal the database table name to store blog is ‘node’ and the field name to store the create time is ‘created’ . So your query to get statistics of Drupal would be something like this.

SELECT DATE( FROM_UNIXTIME(`created`)), count(*)
FROM `node`
GROUP BY DATE( FROM_UNIXTIME(`created`))

Rather than converting the timestamp to SQL Date format, You can convert Date to timestamp and your sql statement may look little mathematical.

SELECT ROUND( (
UNIX_TIMESTAMP( NOW( ) ) - `created` ) / ( 24 *60 *60 )
), count( * )
FROM `node`
GROUP BY ROUND( (
UNIX_TIMESTAMP( NOW( ) ) - `created` ) / ( 24 *60 *60 )
)

In fact the expression “UNIX_TIMESTAMP( NOW( ) ) – `created` ) / ( 24 *60 *60 )” derives a number that represent the age of the post in days.

So this way you can derive statistics of your data with the use of ‘GROUP BY” construct. The ability to write complex queries in SQL syntax like this is really useful, specially when you access a remote database through a web services (i.e. Data Services) or using database drivers, you have to minimize the number of sql queries to execute as minimum as possible.

Here are some of the other aggregate function that you may use with GROUP BY, http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html