Writing an Application in Six Steps

There are six steps that any Xgrid client application must take:

1. Locate a controller—You need to locate a controller, which typically involves bringing up a service browser and letting the user choose a controller.

2. Connect and authenticate—You need to connect to the controller, which is typically password protected.

3. Submit the job—The tasks must be assembled and submitted to the controller with the proper syntax, specifying a grid.

4. Identify the job—When you submit a job, an action monitor object is returned. Use this object to determine whether your job submission was successful and to obtain the job ID for future reference.

5. Receive notifications—Once you have a job ID, you can register to be notified when the job completes, when tasks complete, or when errors occur.

6. Collect the data—When the job completes, you need to collect the returned data and deal with it appropriately.

Of course, there are housekeeping tasks as well, such as deleting the job when you are done.

This chapter highlights the functions you need to use to accomplish each step, and points you to code samples that illustrate their use.

All code samples are included as part of the GridSample project, which is installed in the Developer/Examples/Xcode directory on your hard disk when you install the Mac OS X Developer Tools (Tiger) or XCode Tools (Leopard).

Step One: Locate a Controller and Connect

To browse for Xgrid controllers, use the NSNetServiceBrowser function. GridSampleServiceBrowser.m in GridSample provides a working code sample of an Xgrid service browser.

Connect to an Xgrid server using the XGConnection and XGController functions. See GridSampleConnectionController.m in GridSample for working code samples.

Step Two: Authenticate

You may not need to authenticate to access an Xgrid controller, but you typically do.

Authentication by means of a simple password is supported, as is authentication using single sign-on (SSO or Kerberos). The XGConnection function requires an authentication parameter. If the controller is unprotected, pass nil. If it is protected, pass the user name and password, using XGTwoWayRandomAuthenticator. If using Kerberos, use XGGSSAuthenticator.

GridSampleConnectionController.m attempts an initial connection with the connection authenticator set to nil. If the connection fails with an authentication error, the user is prompted for login information and XGAuthenticator is set.

Refer to GridSampleLoginController.m in GridSample for a working code sample.

Step Three: Submit a Job

Submit jobs to Xgrid using the XGController function, using this syntax:

-[XGController performSubmitJobActionWithJobSpecification:gridIdentifier:]

Note that you need to pass a job specification and a grid identifier, of type XGGrid.

GridSampleNewJobWindow.m includes sample code for a pop-up window that allows the user to choose an available grid.

Constructing a properly formatted job submission is non-trivial. Refer to the man page for xgrid for the correct syntax.

Refer to GridSampleNewJobWindowController.m in GridSample for working job submission sample code. You may also find GridCalendarNewJobWindowController.m helpful. If you have not already done so, download the GridCalendar sample code from http://developer.apple.com/samplecode/GridCalendar/.

Step Four: Retrieve Job ID

When you submit a job using XGController, an XGActionMonitor object is returned. Monitor the XGActionMonitor object until the isUpdating attribute is false, then verify that the dictionary contains a “success” entry. If the job submission is successful, the jobIdentifier attribute contains the job ID, an object of type XGJob. Pass this identifier to the appropriate functions to monitor the job status and retrieve the data.

Refer to GridSampleNewJobWindowController.m in GridSample for working sample code.

Step Five: Register for Notifications

Register for notification of job state changes, or query the controller for the current job state, using the XGActionMonitor function. Pass in the job identifier you received in step four.

Refer to GridSampleNewJobWindowController.m in GridSample for working sample code.

Step Six: Data Collection

When the job completes, collect your data using XGFile and XGFileDownload functions. Identify the job using the identifier object (XGJob).

Refer to GridSampleJobResultsWindowController.m in GridSample for working sample code.

Good Housekeeping

When the job is complete, and you have collected the results, the job should be deleted, notification observers should be removed, and unneeded structures and objects should be deallocated.

See the dealloc function in GridSampleJobResultsWindowController.m for an example of good housekeeping practices.