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Re: debugging a db migration script


On 07/16/2012 11:59 PM, Jim Fehlig wrote:
I'm working on a patch that adds a column to the compute_nodes table in
the nova db, but it seems my db migration script fails when calling 'db
sync' in stack.sh.  I tried running the command manually, same failure:

stack@virt71:~> /opt/stack/nova/bin/nova-manage --debug -v db
sync2012-07-16 21:42:52 DEBUG nova.utils [-] backend <module
'nova.db.sqlalchemy.migration' from
'/opt/stack/nova/nova/db/sqlalchemy/migration.pyc'> from (pid=19230)
__get_backend /opt/stack/nova/nova/utils.py:484
SADeprecationWarning: The 'listeners' argument to Pool (and
create_engine()) is deprecated.  Use event.listen().
   Pool.__init__(self, creator, **kw)
SADeprecationWarning: Pool.add_listener is deprecated.  Use event.listen()
Command failed, please check log for more info

I can't find anything useful in any log (/var/log/*, /opt/stack/log/*).
I ran the above in strace and saw my migration script was opened and
then shortly thereafter writing of "Command failed, please check log for
more info" and exit(1) :).

The patch also adds a 'hypervisor_type' column to the instances table,
and that migration script succeeds!

Any hints for debugging a db migration script?


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I just went through this with a Keystone change.  What I would suggest is:

1.  Get a blank Database.
2.  Run the DB migration without  your scripts.
3. Get the SQL you want to run to work correctly from that step.
4. Add in a Database appropriate SQL script that has exactly your SQL from above.
5. Run the whole migration.

Yes, it is a labor intensive and painful as it sounds. You want to make sure that you have exactly the preconditions that your script expects. What I had to do was actually go back and modify earlier DB init code due to the SQL alchemy column definition changing.

Note this change:
I now have to explicitly create the token table to make sure it is the state it would be today. Since my code had modified the token table, had I not done this, by the end of "stage 1" SQL processing, the database would have had this table in "stage 2" state.

Then I Went and added a SQL script for modifying the table. Since I was altering a a table without dumping the data in it, it was a non-trivial change that SQL Alchemy couldn't handl (AFAICT). Instead, I added a SQL script:

Make sure you have a comparable Downgrade script, too.


For Keystone, we run the upgrade using a stand alone executable keystone/bin/keystone-manage. In nove, it looks like there is bin/nova-manage to do the same thing. I am running using Eclipse and PyDev as my development environment, and using the integrated debugger to step through the code. Makes it a lot less painful to debug.

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