We’ve managed to meet the requirements of our
demonstration scenario and conveniently in the process have touched upon
all the major features of Excel Services. Although our demonstration
site works as required, in a real-world environment, we’d need to
consider a few other aspects, particularly those with regard to how data connection information is stored and used.
Our sample workbook
includes an embedded data connection that has been configured to use the
credentials of the currently logged-in user. This approach has a few
drawbacks, however. First, all users of the web site must also be
granted permissions to access the data source referred to by the
workbook. Second, anybody with permissions to edit the workbook can make
changes to the data connection, possibly creating a connection to a
server that’s not generally accessible. In such a case, the generated
workbook would display just fine for users with appropriate permissions
on the data source but would display an error message for other users.
As well as creating connections to restricted servers, a user might also
be able to create a connection that returned an unnecessarily large
volume of data. For example, a user could read every single sales
transaction record from an ERP system into a pivot table and then
summarize the total sales data by quarter. While this would deliver the
required end result, the performance implications of using such a
Workbook in Excel Services are considerable.
To resolve these issues,
you can restrict data connection availability to specific data
connection libraries. Permissions can be set on the libraries so that
only authorized users can create connections. This provides a much
higher degree of control over what data sources can be used, how
authentication is handled, and how queries are written. Furthermore, it
allows users who are not familiar with the nuances of connecting to
database servers and retrieving data to create useful Excel workbooks
simply by selecting the appropriate data source from a list.
Restricting Data Connection Types
Let’s start by denying our embedded connection the rights to run under Excel Services:
Using SharePoint 2010 Central Administration, select Manage Service Applications from the Application Management section.
Select the appropriate Excel Web Service Application instance from the list of available services.
In
the Trusted File Locations section, add a new location specifically for
our sample site. This will allow us to override the default settings
for our sample site without affecting the settings for other sites that
use our Excel Services instance. Click the Add Trusted File Location
link.
In the Location section, type the Address as http://<your server name>/12
and then check the Children Trusted checkbox. Notice that we’re using
the physical server name rather than localhost because Excel Services
configuration uses the URL that was assigned to an application when it
was first created. Although we’ve accessed our sample site using the URL
http://localhost/12, this URL isn’t configured within SharePoint
and therefore can’t be used as a SharePoint trusted file location.
In the External Data section, select the Trusted Data Connection Libraries Only option for Allow External Data.
Currently
our workbook is set up to refresh external data content manually. As it
happens, the first time we select a currency from the drop-down list,
the workbook is
refreshed, causing the underlying external data to be reloaded. The
caching settings within the External Data section determine how often
external data is reloaded, and the default values mean that external
data is cached for a period of 5 minutes. When we reconfigure our data
connection to use a Data Connection library, we’ll set it up to refresh
automatically when the workbook is loaded. To prevent the user from
having to confirm this refresh every time the workbook is opened,
uncheck the Refresh Warning Enabled checkbox.
In the User-Defined Functions section. check the User-Defined Functions Allowed checkbox.
Click OK to apply the settings.
We can now revisit our sample
site home page to see the damage that we’ve done to our application.
Bearing in mind that external data is refreshed only whenever the
currency code is changed, select an alternative currency from the
drop-down list to trigger a refresh. If our configuration changes have
been properly applied, we should see an error, as shown:
Excel services makes extensive
use of caching, both in terms of the workbooks and the external data
that’s used within them. If the expected error is not shown, it’s most
likely because the workbook has been cached on the server.
Adding Connections to Data Connection Libraries
Now that we’ve broken our
sample application, we need to fix it again. We can do this by adding a
new data connection library and then creating an Office Data Connection
(.odc) file containing our connection settings. We’ll then tweak our
workbook to use our new connection file instead of an embedded
connection.
Create
a new Data Connection library. From our sample site home page, click
the Documents link to view the list of document libraries. Click the
Create link to show the Create dialog.
From the Content & Data category, select Data Connection Library. Type the name of the new library as Sample Data Connections, and then click the Create button.
With
our new library available, we need to let Excel Services know that all
data connections stored there can be trusted. Switch back to the Manage
Excel Services page in Central Administration and click the Trusted Data
Connection Libraries link.
Click the Add Trusted Data Connection Library to add a new library. Type the address http://<your server name>/12/Sample%20Data%20Connections, and then click OK to save the changes.
Tip
In the real world, where
trusted connection libraries are used, it makes sense to have a single
central connection library at a well-known location. Given that the
purpose of the connection library is to allow all users to access
trusted business data freely, making connections to the data as easy to
find as possible is a worthwhile aim.
Now
we’ll create a data connection in the library and reconfigure our Excel
workbook to use that instead. Navigate to the Excel Workbooks document
library in our sample site and then edit our Last30DaysSales workbook
using Excel client.
From the Data menu, select the Connection button.
In
the Workbook Connection dialog, make sure the
AdventureWorksList30DaysSales connection is selected and then click the
Properties button.
While
we’re changing stuff, we’ll configure our data connection to reload
external data when the file is opened. In the Usage tab, check the
Refresh Data When Opening The File checkbox.
Switch
to the Definition tab. Click the Export Connection File button and then
in the File Save dialog, save the connection to http://<your server
name>/12/Sample Data Connections.
Since
we’re uploading the file to SharePoint, we’ll be prompted for some
additional metadata. Make sure that the Content Type is set to Office
Data Connection File, and then click OK to complete the upload.
We
can see in the Connection Properties dialog that the Connection file
path has changed to our data connection library. Even though we’ve saved
the connection details to our SharePoint server, Excel still uses an
embedded copy of the connection details. To force a reload every time
the connection is used, check the Always Use Connection File checkbox.
Click
OK to close the Connection Properties dialog, and then click Close to
return to Excel. We can now save our revised workbook back to SharePoint
by clicking the Save icon in the upper-left corner.
When we return to the home
page of our sample site, we’ll find that our chart now functions
properly. If an error is still being displayed, try recycling the
application pool to clear out any cached copies of the workbook.
