The Problem with SpreadsheetsΒΆ

date:2007-01-23 11:13:49
category:Data Structures and Algorithms

See “When a Column is Not a Column ” for a rant on failed attempts to scale spreadsheets up into relational databases. Bottom Line: A spreadsheet column is rarely an RDBMS column. Often the SS column’s label is a key value, just like the SS row’s label, and the cell’s value is RDBMS data in the RDBMS row. Both models borrow terminology, but the concepts under those terms don’t map very well. It’s almost universally a mistake to transform a spreadsheet page (which is table-like) into a proper RDBMS table.

Here’s the interesting problem du jour . The customer has a collection of massive spreadsheets (20 tabs) that are prepared by 20 or so people. This collection of 20 copies of the same template must then be consolidated into one amalgam.

If the spreadsheets were just data entry vehicles, we’d be replacing them with a simple web application, and that would be straight-forward.

The essential problem (to consolidate the data) is compounded by the spreadsheets having some pretty fancy calculations and feedback. We could replace them with a web application that had enough AJAX to be similar to the Google Docs spreadsheet: it would provide a rich user interface, and the data would be centrally located.

Collaboration and Consolidation.

In this case, the collaboration is more than just consolidation. While the proximate problem is consolidation, we quickly uncovered additional use cases. There are at least three classes of actors: directors who originate the templates, managers who fill in the templates, and analysts who consolidate and report on the data.

The managers filling in the templates, in some cases, actively use them to control their business. They create alternate scenarios, they make decisions, they take action and they compare predicted results with actual results. Other managers, however, don’t do more than fill in the templates and dutifully post them to the analysts.

Our collaboration goes beyond simple consolidation. The superficial collaborative task is only the tip of a larger iceberg of use cases for the spreadsheets. What is a technology choice that supports consolidation, but preserves the other use cases?

Excel Extraction.

Some managers make extensive use of the original spreadsheet functionality; the spreadsheet’s native use cases are essential to the enterprise. In addition to the spreadsheet use cases, consolidation and refreshing history must be added to this. One of the decisions we have to make is where we allocate the additional ETL (Extract Transform Load) processing that produces consolidated information. Since we working with (against?) MS-Office products, we’re talking about some fairly complex use cases with Excel at the center.

Note that we have to strike a fine balance between two opposing forces.

  • The Managers tinker with the spreadsheets as part of their business modeling. The Directors also tinker with the templates as part of the larger planning cycle.
  • The Analysts need a fixed, standard set of spreadsheets they can consolidate without a lot of study and reverse engineering. They need to prevent tinkering.

We have a spectrum of platform alternatives.

  • Excel. We can write gloriously complex macros and VB scripts and embed them in the spreadsheet. This makes them large, difficult to email, and presents a security nightmare. First, some email and firewall programs are grumpy about this kind of embedded functionality. Second, and more important, managers are tinkering with the spreadsheets, but directors need standardization in the spreadsheets. If we “lock things down” for the analysts’ benefit, we prevent the managers from making good use of the tool. If we allow the managers the freedom to use the tool, the analysts struggle to consolidate.
  • Desktop Application. We can write a desktop application; this will exploit the .Net API’s to extract from the local spreadsheet and load to a remote database. This has to use more sophisticated parsing and pattern matching to tolerate tinkering. However, it can also provide immediate feedback if names are dropped or changed, or cells moved in a way that makes the results hard to use.
  • Web Application. We can write a web application to which people upload their spreadsheet, do the ETL on the web server, and then provide reports (or pivot tables) to help them manage their business. Like a desktop application, this must have more sophisticated parsing and pattern-matching. However, this can’t even make good use of the .NET API’s, but must work around a number of tragic limitations.

A Show Stopper.

Here’s a potential show-stopper for the web alternative, “Considerations for service-side Automation of Office ”: “Microsoft does not currently recommend, and does not support, Automation of Microsoft Office applications from any unattended, non-interactive client application or component (including ASP, DCOM, and NT Services), because Office may exhibit unstable behavior and/or deadlock when run in this environment.”


The web application could have had a pretty nifty overview:

  1. The Director creates a new template and populates it with the most recent plan and the actual performance information. It’s posted to the collaboration website.
  2. The Manager gets the template from the web, makes changes, creates models, does the managerial thing. The Manager uploads spreadsheets from time to time. At some point, one is the “current” plan, and is canonized for purposes of overall enterprise consolidated business planning.
  3. The Analysts have all of the Manager’s uploads to consolidate and report on.

The end-user experience would be slightly different than today’s mass-emailing frenzy. There’d be a nice collaboration web site for upload and download. The upload would validate and provide feedback as part of the upload process.

It appears that we can’t make this work very easily.

Desktop and Excel Platforms.

I’m not a big fan of the desktop as a platform. Primarily, I hate the configuration management problem: who has what version of the application.

“But there are tools to help.”

While true, desktop deployment tools merely plaster over the essential problem: the people who “control” the desktop aren’t very disciplined. We can only make desktop software work as part of a total lock-down of the end-user’s computer. Since the client doesn’t do this (and won’t for this one business application), we can’t really make use of the desktop as a platform.

Excel is a viable programming platform. However, it, too is prone to getting out of control. The current 20-plus-tab monstrosity is packed full of macros and VB modules and doesn’t work reliably. In addition to bugs, people can easily add inter-workbook links to documents on their C: drive and in their Windows TEMP directory. The whole thing rapidly spins out of control when we try to make use of the clever features of Excel. We need something simpler and more reliable.

Enter POI and XML.

We can exploit two technologies to make a simple, reliable web-based solution. First, we have Jakarta POI , which allows us to read Excel files directly. This is pleasant, and the HSSF reliably picks apart a spreadsheet. Second, we can use XML versions of the spreadsheets, making them readable by SAX or Xerces .

Here’s the overall Compiler design pattern, and how we would implement it:

  • Lexical scanning is done by POI or SAX. From this, we get a sequence of tokens which are Worksheets, Rows and Cells.
  • Parsing is done by our application. From the sequence of Cells, Rows and Worksheets, we assemble higher-level constructs that are the essential Business Entities described in the spreadsheets. If the user has made the wrong kinds of changes, we can’t interpret the spreadsheet, and must reject the upload with an error. Since we know the Worksheet, Row and Cell where parsing fell apart, we can report an error pretty precisely.

Once we’ve parsed the spreadsheet and have the Business Entities, we can then do the required transform and load operations. These will lead to the consolidated data. We can then cough out the next generation template, or a reporting pivot table, or simply redirect the user to a typical data warehouse reporting portal.

Spreadsheet as Syntax.

This leaves us with the spreadsheet document filling an interesting role in this processing. Rather than being an active platform, the spreadsheet is downgraded to a mostly passive document with a few active elements.

Once we look at a spreadsheet as a kind of syntax – a sequence of tokens – we can parse it using either of a couple of techniques. We can try to create an LR or LL kind of grammar, which may work out, depending on how complex the spreadsheet is. Often, user inputs are preceded by labels which allow us to do very simple LR(0) parsing.

We can, for example, look for the cell which contains the “Weekly Forecast” data. In the next row, a cell will have a product name, and the following cell will have a forecast number of cases sold.

The other technique is to use a more sophisticated regular expression technique where we need to see a longer sequence of cells or rows to determine the pattern. These aren’t as easy to implement because most RE processing software works with individual characters. We would need to write a RE matcher as a non-deterministic finite automaton that worked with Cells and Rows instead of characters.

Solution Outline.

Here’s a fun kind of solution. It works best if the spreadsheets are pared down to just the input sections with just enough calculation and history to facilitate creating high-quality plans. From the current spreadsheets, we would delete the various tabs that are simply reporting and consolidation within the spreadsheet.

  1. Directors build their templates, including ODBC queries which pull historical data into the spreadsheets for use by managers. They save them as XML documents. These are large, but also very easy to cope with. They post them to the web site for use by managers.
  2. Managers download the spreadsheets and work with them. They upload their various planning scenarios so that the plans can be validated, and reports can be generated from plans and actuals.
  3. Analysts use the same reporting tools that managers use. The only practical difference between an analyst and a manager is the breadth of information which is visible. A manager can see their plan, an analyst can see multiple plans.

The upload process uses a SAX application to parse, validate, extract, transform and load the spreadsheet. In the (all-too-common) situation where the spreadsheet doesn’t parse successfully, there are two kinds of feedback:

  • An error page in the web application.
  • A revised spreadsheet with a different style for the erroneous section.

We’ll come back to spreadsheet as syntax in future posts.

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