Thanks for the surprisingly prompt and helpful advice everyone. :-) The primary question I had in my mind was whether persistent objects were necessarily manifest in a folder. If at least hundreds of users might create umpteen new objects/records each day, I was wondering if there might be two kinds of persistent object, i.e. visible and invisible (and whether there would be performance issues if not). I think this is pretty well answered now. BTreeFolder2 sounds like a reasonable solution to the folder containment issue. However, given that creation of new objects/records (bids) is likely to be quite frequent and simultaneous by a wide variety of users, I'm beginning to think that perhaps I'd be better off using an external database. (NB we're talking around 100K bids per item) An external database is also likely to help separate out reliability and scalability issues of the eCommerce data separately from taht of the web site. Now down to the external database. I've gathered so far that probably the best database to start off with would be MySQL for Python (with ZMySQLDA). I expect this can be swapped out with another SQL database if MySQL starts creaking under the load.