Design of Prevalent System and Commands With Regard to Transaction Security

Hi,

I've been fiddling around with Prevayler and prevalence for some hours 
now and worked around most of the kinks. However, how to design the 
system and commands to get classical transaction features is still 
something I'm not sure about, especially how to get the A in ACID.

Since there is no easy rollback per se and exceptions are ignored and 
even replayed, the commands must do all error checking before modifying 
any data in the prevalent system. This gets harder to do the more 
complex the commands are, e.g. when modifying entities that depend on 
each other. And it still does not solve the problem what happens when an 
exception occurs in a live system where despite error checking parts of 
the transaction are executed and some are not.

How do you work with this, especially, how are the system and commands 
designed to mitigate these problems?

I'm currently using a prevalent system class that just holds the state 
and only exposes modifying methods to the sub package where the commands 
are. The commands then do two things, first perform consistency checks 
and then modify the data in the prevalent system when everything is OK. 
This works to some degree but does not solve the actual problem 
described above (exception after some data has already been modified), 
or when executing several simple commands in a compound command.

For a live system where consistency is a fundamental requirement, there 
are only two options I could come up with (first one was apparently part 
of an older Prevayler version?):
1. "classical" rollback, make copy of system before applying transaction
     (relational DBs can optimize this very good with regard to required
     memory, to implement this efficiently in a prevalent system would
     require some effort)
2. Something along the following lines: when error occurs
   - remove/ignore failed transactions from journal
   - hold command execution and perform a restore of the system from the
     last stored snapshot and journal
   - resume command execution

Thanks & regards,
Elmar

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