Data Virtualization and user-concurrency

Question

Does anyone have any pointers / examples on how to resolve many issues that are possible in multi-user scenarios when using data-virtualization. Lets say that we are talking about WPF and DataGrid. Implementing a virtualized collection which loads on demand is not too difficult. However, without a staging area where temporary results of the original query are stored, we get into concurrency issues like:

1. Loading new page could fetch incorrect data (concurrent user adds and removes some records, leading to same total count of records, but which results in page fetching duplicate entries that are already displayed somewhere above in the grid)
2. Preserving user-selection in the grid when scrolling and loading new pages, in which there could be a possibility that once selected items have expired from cache, and once reloaded, we find out that someone deleted them. We can deselect everything and show the message to the user, but :/ Also, if selecting with Shift-click (multiselect) somewhere close to the end of the list, what should be done when some items "appear" in the middle of list upon loading some of the middle pages (concurrent user added items).

Answer 1

It is useful to keep in mind that nothing on the screen of your user is technically up to date. The moment you show it you are lagging on the master dataset.

1) Yes, of course. But you can always keep track of the first record you are showing on your grid and get your next page sized set from there. Those which are deleted will drop out of view, of course. Optionally, you might try and use a library such as ZeroMQ or RabbitMQ and broadcast dataset changes and update your datagrid live if they are currently shown. It still won't be perfectly in sync, obviously but you will reduce the window in which they won't be in sync.

2) When you have selected items you keep track of their primary keys. I don't know what you want to do with those which are already deleted from the master set. But you can always act on all the others, right? Even if they aren't shown anymore you can track the PK's. And reselect them when loading a page.