Fixed rules: More on Parameter processing

In the famous “lets do it right this time” release of Pentaho Reporting 3.5,
we introduced the ability to have parameter on a report. Well, it wasn’t quite
right, with parameters you need to pre and post process the data to make it
sound. That was release 3.6. At that point, so our theory, you, dear user,
should be happy. But apparently, the beast we created wasn’t all pretty.

Well, its PRD-3.7 now and guess what we are improving: Parameters.

So far, the date parameter processing was not quite right. I still wonder
why after 5 years of XAction no one complained about that. Sure, XActions
only have strings, and any processing or parsing is up to you – and so is the
blame if it does not work.

The various system level options on the parameter UIs was .. sub-optimal.
(I’m getting better at phrasing it more positively, don’t you think?)
The list of supported parameter (aka “the list of parameters you shall not
use in your report”) grew with every release. The Swing UI and the server UI
never quite agreed on what setting to accept and how to behave in border
cases. Thus creating formulas that worked in both settings was a chore.

And last but not least: Validating parameters and getting them run in a
consistent way was difficult. Give a Integer where a Long was expected
and you are screwed. Without a error message. Thus even for me working with
the parameters was more a easter-egg search than sane designing.

And last but not least: Even the parameter processing order was a bit
funny. It works for simple cases, but behaves rather funny for the
not so simple ones.

How Pentaho Reporting Processes Parameters

Each parameter in Pentaho Reporting carries at least two formulas that
eventually need to be evaluated.

The default-value-formula is used to produce a valid value if the user
provided no value.

The post-processing-formula is used to transform the user’s input into
something more usable or simply to validate arbitrary business rules
(a deposit cannot be negative, for instance).

And last but not least, if you reference an other parameter, you expect
it to contain the proper post-processed value.

In PRD-3.6, the order of the validation was largely out of sync with
those expectations. In fact, post processing was done in blocks, so that
parameter were not able to use other post-processed parameter values in
their queries. Now that’s bad, and I guess Gretchen will be able to share
a few unhappy tales in Lisbon about that.

In PRD-3.7, each parameter is now fully processed on its own before
the next defined parameter in the chain is processed.

Lets be more formal for a while:

For each parameter defined:

If the value for the parameter is <null>, we compute the parameter’s
default value and use that as untrusted parameter value. The default-value
formula only sees the previously validated parameters.

In a second step, we post process the parameter to get a trusted value.
The post processing formula sees the previously validated parameters
and the untrusted value. So be careful how you use the untrusted value
here, as you cannot trust users and SQL-injections or cross-site-scripting
troubles are never to far away.

If the post-processing formula fails with an error the
trusted value of the parameter will be <null>, a warning message will be
issued and last but not least we refuse to execute the report. The
parameter processing continues with this value set to <null>.

And finally we check the type of the parameter and compare the parameter
value against the list of valid key-values. If the value passes this test
it becomes a trusted value and will be used in the further parameter
processing and ultimately it will be used in the report.

If the parameter fails the test, we report an error, prevent any report
processing and continue to validate the remaining parameters using the
parameter’s default value.

Beginning with this version, the parameter validation also creates the
set of validated values after the validation is complete. For a report without
any parameter values set, this will yield the default values for all parameters.

So what does this mean for you?

The new schema brings a couple of changes to the way the system behaves.
Default values are now context sensitive and can change when the
selection for the previously declared parameters changes. Our parameter
UIs do not directly use that feature for usability reasons.Automatically changing
the user’s input is not very nice and confuses and/or upsets people. A lot.

The post processing formulas are now executed in a timely manor and before
the default-value or selection for a parameter is computed. This way, you
are now able to compute the mondrian-role array in a hidden parameter’s
post-processing formula and be sure that your datasource sees it.

And last but not least, your formulas wont be able to use values that have
not been validated, nor would the report ever include them. Especially with
the SINGLEVALUEQUERY and MULTIVALUEQUERY formula functions, this is mandatory.
Your database is yours and we all want to keep it that way.

This entry was posted in Advanced Topic, BI-Server, Parameter, Tech-Tips on by .
Thomas

About Thomas

After working as all-hands guy and lead developer on Pentaho Reporting for over an decade, I have learned a thing or two about report generation, layouting and general BI practices. I have witnessed the remarkable growth of Pentaho Reporting from a small niche product to a enterprise class Business Intelligence product. This blog documents my own perspective on Pentaho Reporting's development process and our our steps towards upcoming releases.

One thought on “Fixed rules: More on Parameter processing

  1. Yanuar

    Dear Mr.Thomas Morgner

    How about if i want to implemented in the query mdx. now i have more than one parameter, and the value will my input into the MDX query. thank’s

    regards

    Yanuar

Comments are closed.