I want to be able to generate serial numbers for use in (for example) uniquely identifying list items. The serial numbers all need to be unique, and preferably monotonically increasing (0001, 0002, 0004 etc).
In SharePoint Standard I could create a database which I poll for a serial number, update it and then return the result. Another (better?) option is to create a Windows Service on the server which is polled as a Web Service and keeps track of and returns the required number. However, none of these solutions are possible in SharePoint Online.
I could set up a web service on a separate server, but I'd ideally want something that's self-contained. It is important that the serial numbers are unique even if there are concurrent requests to the number service.