Unified Data Access Layer


The following section describes how to demonstrate Unified Data Access Layer which estimates data access cost.

1      Accessing  - Tester Desktop

In order to test the Crossgrid package installation, use a web browser to connect with the appropriate Web Server node.

Web Server Node is available for CrossGrid users at the address:  http://zeus04.cyf-kr.edu.pl/.

Open page http://zeus04.cyf-kr.edu.pl/. The result should be similar to the picture presented below.


2      Getting data access cost

Once you have properly configured access to the web site you should select one site address for the list e.g. instance Cracow Zeus Test (zeus04.cyf-kr.edu.pl) by clicking on the respective link. As the result you should get a page showing a graphical representation of the situation which you are now in. Simply, your browser connects to our Demo Client Web Server, which using SOAP connects to the selected storage element host (Tested Storage Site). On that host should be active three main components: Data Access Estimator (DAES), Component-Expert Subsystem (CEXS) and Storage Element (STEL). Those components realize unified data access estimation part from functionality covered by the UDAL package. By clicking on respective component from the graphically represented structure you will be able checking how it works and trying simulating of a real usage of those components by external clients – EDG Replica Manager. Please keep in mind that usually users do not have access to those components directly and this demonstration is supposed to reveal their internal behavior.



Please click on Data Access Estimator component icon. As the result you get the page as in the picture below. This page allows you getting estimation of data access costs. However, the most interesting one is that one used by EDG Replica Manager – SE Cost provided by the first form displayed in the page. All of these forms require to provide one srm (please keep in mind that host address in that srm must be exactly the same as the server name which you are currently testing). Please notice that below the form you can find propositions of values which you can put into the forms.


In order to get some very first estimations please put into the first field value     
You should get result like that one presented in the figure below.



If you get a negative access cost value then it means that the file physically is not located in the tested storage element.


During session above, estimation of the latency of data arrival by DAES localized on zeus machine has been tested. However, in similar way other interfaces of other components can be tested.

3      Component selection for access cost estimation

Please back the component selection page.


Please click the Component-Expert Subsystem icon. As the result you should get the following page.


Using the obtained web page you can exploits low level module (CEXS) used by DAES for selection of the best data access cost estimators.

As the first thing you should try to get a list of all currently register components used for estimation. To do that, please type Estimate in the first form “Get All Components for Type”. As the result you should get the page similar to that one presented in the picture below. The page contains list of all components being estimators. The components must differ by specialization. In the example component Estimate6/ceec is specialized for data access cost estimation for HSM DiskXtender (former UniTree) and the second one Estimate9/ceec is specialized for hard drives.



Then please go back and scroll down to the form “Get Best Component for Call Environment”. You should get the following page:


Using the “Get Best Component for Call Environment” form you can simulate process of selection best estimator for specific context in the same way as DAES does. Here you need to provide two parameters: Component Type and SRM of file which you are processing. In the first filed please type Estimate and in the second please select one from the list of propositions stated below that field e.g. srm://zeus04.cyf-kr.edu.pl/cgdata/f100MB.dat. Submitting that form you can should get similar page to the one stated below.



The obtained page presents what is the best component estimation component used for that specific file and what are values returned in the call-environment passed back by CEXS. The returned call-environment contains all attributes found out during the deduction process (component selection). For instance a type of storage keeping that particular file is returned, physical location, connection way between the storage element and the device keeping file etc. All these data can be used by CEXS’s client which in this case is DAES.