Exposing an Excel PowerPivot model as a Web Service

In my last post I demonstrated a simple way to interrogate an Excel based PowerPivot model using either MDX,DMV or the new DAX Query Table commands. This is a great way to dig deep into a PowerPivot model, for example, this new Denali DMV discover_calc_dependency can be use to  describe the dependencies between a model’s DAX measures.

Being able to directly access the PowerPivot model opens up all sorts of interesting options, one such is a web service.

In the past I’ve written about exposing Excel as a simple DIY web service, see here for a JavaScript example and here for a Python version. Both were capable of serving PowerPivot data but they both had to trigger events in Excel to do so.

With this newly discovered ability to query the model directly, I figured a third attempt at a PowerPivot Web Service was overdue.  This time I’m using the new kid on the block, HAMMER, and in particular using its in-built IronPython interpreter.

I could have built a simple server using VB.NET or C# and added it as a HAMMER command, but the nature of a web service makes it hard to frame as a parametrised command. It was for this type of requirement that I added the ability to script using Python, i.e. situations where a generic command would always come up  short, for example:

  • Should the service allow only local connections?
  • Should it use authentication, and if so, what type?
  • Should it allow free-format commands or offer only packaged commands?
  • Should it use HTTPListener (which is in-built and very powerful, but requires Admin privileges) or use a more basic socket based utility that can run without Admin privileges.
  • Should it block or handle asynchronous requests. (For services that access the Excel Object Model, it has to be blocking, but perhaps if the service simply accessed the AS engine, it night support multi-threaded queries? Must try it sometime.)

For this example I decided to use HTTPListener (in blocking mode and without authentication) and to expose services that accept free-format MDX,DMV or (if Denali) DAX Table Queries.

See the PPWebService workbook example in the latest (V0.5.0) version of HAMMER. To activate, make sure latest version of HAMMER is installed (or use activate button on Hammer sheet) and click the Expose PowerPivot Model as Web Service button (again on Hammer sheet). This will start a server on port 8070.

The service exposes 6 endpoints:
  • /exit – this will shutdown the service.
  • /html – this will return data as a HTML table (ideal for importing into Excel using a Web Query).
  • /xml – this returns a ‘canonical’ XML table (ideal for importing to Excel as an XML Map).
  • /json – returns a JSON table (as a list of lists).
  • /odata – returns a ATOM-base ODATA feed suitable for direct import into PowerPivot using its Data Feed import facility.
  • /csv – returns in CSV format.

Each service (except /exit) expects to be followed by a MDX, DMV or DAX command. e.g. /html/evaluate(InvoiceHeaders). In the case of /csv an optional file name may be specified e.g. /csv/invhead.csv/evaluate(InvoiceHeaders).

It’s also possible to issue a POST request with the command in the message body.

As with all URL requests, the commands may need to be URL encoded (use the HAMMER command “_URLENCODE” if need be).

For the latest versions and articles on HAMMER follow the HAMMER tag on my blog …


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