[Engine-devel] Global transaction in ovirt-engine

[Michael Kublin]

The issue is a following (I think I had raised it once),
by default when any command is invoked in ovirt-engine a transaction is opened.
Now, lets take a look on complicated command which is performing a lot of XML-RPC/HTTP calls and just simple update , 
such command is a main cause for the following problems:
1. Long transactions, each call increase a time of transaction
2. When transaction is opened a connection which is associated with it also in used, so a number of available DB connections reduced from
connection pool.
3. A errors : transaction was cancelled and operation is failed on engine side, but successes at host side.
4. Performance - "try to keep transactions as short as possible".
5. Performance - to many unneeded transactions
6. Bug debugging - it is very difficult to understand why something was committed and something not why transaction was reverted or not, 
   when we have a lot of nested transactions.

Now, what should be done by my opinion:
1. Change default behaviour - by default not to open a transaction (Today, transaction is opened and if we don't want a command should be marked
   by NonTransactiveCommandAttribute) - it is simple
2. The change described at 1 will require to rewrite and redesign some flows - more difficult, but also can be done pretty fast. 
   By the way this can be or even should be done at any case. 
3. All new commands should not be based on global transaction or keep in mind that global transaction can be disabled

Benefits,
1. Better performance
2. Cleaner code
3. Less bugs
4. Increased scale

Regards Michael