SSRS: Full report not rendered via ReportExecutionService

I've come across an issue when using the ReportExecutionService to render a report through a .Net App using code similar to that demonstrated here.

Essentially, I'm trying to render a report which contains 3 layers of nested subreports all using different datasets. When viewing the report through the SSRS report viewer, you get a question mark on the paging section of the toolbar:



This question mark is a result of the On-demand processing engine introduced in SSRS 2008 and basically means that the report is rendered in stages, giving a potential performance gain on large reports. The question mark indicates that it knows there is more to come, just not sure exactly how much.

So, as I understand it, if you have a report with 2 subreports accessing different datasets - only the first one will be run to minimise the data sent to the client. When user starts navigating through the report and comes to the end of the first report, the rest of the report will be rendered. Rob Bruckner explains more here.

All well and good from a performance perspective and typically doesn't cause any issues apart from the strange question mark appearing (even this can be worked around) and users just navigate through the report. However, when trying to access it through the webservice, the same behaviour is honoured and only the first section of the report is rendered - equivalent to the first page when viewing through the SSRS report viewer. This is incredibly frustrating as I know that the report is over 3 pages long but I only ever get the first page in my deliverable. Worse still, there doesn't seem to be a way to override this behaviour (its the same in R2 I believe) and something i'd like to see addressed by MS (on a report by report basis).

At first, I was stumped as even using the TotalPages workaround referenced above, I couldn't get the full report to render. But, then I took a closer look a the "master" reports and noticed that they all had orphaned data sources defined - a legacy from when the reports held definitions rather than subreports. As a last shot, I removed these from the reports as they weren't needed at all and was delighted to see that the full report now rendered correctly through the webservice.

I'm not clear exactly why these orphaned connection strings were causing such an issue but all I know is that clearing them did the job.