Report processing engine of Reporting Services 2008 works significantly different compared to previous releases. One of the important architectural changes in SSRS 2008 is on demand processing behaviour of the service engine.

What is on demand processing?

Earlier versions of reporting services, i.e. 2000 and 2005 processing engine processed the report as a snapshot, evaluating all expressions exactly once.

But RS 2008 on-demand processing engine pre-computes and persists only certain invariants, such as grouping, sorting, filtering, aggregates, and report/group variables. All other expressions (style expressions, textbox values) are evaluated on-demand based on the invariants, at the time a rendering extension requests a specific page containing those values.

How does on-demand processing enhance reporting service?

• All datasets are not necessarily processed

If user opens only initial 2 pages of a report where data region refers to only Dataset1 and Dataset2, then SSRS engine only runs those two datasets. It delays the processing of other datasets untill the user navigates further to other pages.

• Time for interactivity with the report is reduced

Toggles, drill-through operations are performed on the fly in SSRS 2008. These are done as and when the user interacts with the page and hence the initial time to load the page is reduced.

• Faster page by page navigation

The new architecture allows us to cache a few bytes per page that enable quick restart of pagination of a partially/fully paginated report. As a result, navigating to a page of a report which was already previously accessed during the same session is now faster, particularly the larger the report in size.

• Reduced memory footprnt

Memory footprints will be smaller, specially for large reports.

What is the draw back of having on demand processing?

• Re-evaluating same expressions

Most expressions in a report, particularly textbox values and style expressions, are evaluated on-demand in Reporting Services 2008 as you navigate to a particular page. If you go there multiple times, it might be re-evaluated again.

As a report author /designer or end user it will be useful if you are aware of this change in reporting services behavious. That will help you to leverage new features of SSRS 2008