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,
(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.
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.
- 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.
- 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.
- 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.