loosing the complete message store because of a hardware or software failure would be ok. Web users won't get status reports for completed jobs (but the final report is saved permanently anyway, they could look it up at any time) or "cancel this"-messages won't be processed and users would have to re-issue the cancel-request. Users might also not get search results but a error message instead - perfectly fine if something is broken (and also current procedure in case of database problems).
I'll take a look at both suggested solutions.