I should THINK you are safe.
What v9 and 10 can't see or do can't really effect the file itself, only the data within it.
The problem will be that something you do to the database is unsupported by the earlier versions and it just won't work.
I can provide anecdotal 'evidence' that there don't appear to be any problems.
I have one client who has an FM 10 server. Internally they have a mix of FM 9 and FM 10 clients - I've used FM 11 Advanced to do work on the the DB for them. Another client have an FM 11 server, they have a mix of FM 10 and FM 11 clients, and I again, use FMA 11…
The only issue that we've ever come across has been where I've implemented something for them using functionality from 11, which doesn't work in 10 or 9 and as Lyndsay said nothing harmful happens, stuff just doesn't work…
I now have FM 9, 10 and 11 all installed on my main dev machine so that I can test using the lowest common machine for any given client
I did mention this to an engineer at DevCon, and he sort of rolled his eyes, but did not object
We've been doing it successfully for quite awhile, with about 30 concurrent users
> Is it safe to host files with FileMaker Server 11 to clients who are versions 11,10 & 9.
I've been hosting with FMS 11, with coworkers accessing via FMP9, for about a year now, and have not run into any issues. I upgraded the server to make use of some features that FMS11 offers that I couldn't get with FMS9, and I do my development work with FMPA9. I don't have mixed clients accessing the host, though- they're all either FMP9 or FMPA9.
I too have done this for years and it's fine. FMS11 will host the .pf7 file format regardless of the version of FileMaker that created it without any problems and it really is the best version of FMS for that format so if you still using it (as many people are) then FMS 11 is the way to go regardless of what clients you are using.
Thank you all for the advice, it seems I am fine to upgrade the server, given the usual precautions, of backup, backup and more backup.