编辑“︁Asynchronous Database Access”︁（章节）

== Implementation ==

Marauroa has a DBCommandQueue, which is singleton and has a background thread to execute enqueued DBCommands. DBCommand is an interface that specifies an execute()-method, that processed the database request by invoking the DAO-classes. It is called from the background thread of DBCommandQueue.

The following example for a simple write operation assumes that GameEvent implements/extends DBCommand:

 DBCommandQueue.get().enqueue(
     new GameEvent(attacker.getName(),
     "attack", target.getName());

Note that this example works for case 3), too.

Since we ignore 1) for now, this leaves us with 2): We need a way to receive the data produced by an read operation:

 UUID uuid = DBCommandQueue.get().enqueueAndAwaitResult(
     this, new GetCharacterList(username));

The DBCommandQueue will process the GetCharacterList at some future time in the background thread. It will then put the GetCharacterList which now contains the results from the database in a second queue ("waiting to be fetched by program"). Here comes the tricky part: We need a way to notify the program in the thread that requested the operation about the results. The easiest way is to remember the requesting thread by Thread.getCurrentThread() and provide a List<DBCommand>
getResults() method, which fetches all results of Commands enqueued by the current thread.

We had to decide here whether we wanted an ''interrupt'' or ''polling'' based approach: An interrupt based approach would then distribute the results to various program parts of the current thread using an interface DBCommandResultReceiver<T extends DBCommand>, which is implemented by those classes who requested the data. We chose a a polling based
approach. Here, the program parts look for specific results. The program part that sends the character list back to the client calls: 
 List<GetCharacterList> DBCommandQueue.get().getResults(this).


[[Category:Marauroa]]
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