[erlang-questions] mnesia race condition in add_table_copy

[Dan Gudmundsson]

Can you send me (privately) a small test program that shows the problem.


/Dan

On Fri, Jul 8, 2011 at 10:30 AM, karol skocik <karol.skocik@REDACTED>wrote:

> In addition to this, the obvious question:
> what should I do to ensure that schema change is propagated after
> mnesia:add_table_copy?
>
> Or alternatively, what should I call **before**
> mnesia:table_info(Table, disc_copies) to get all nodes where the copy
> resides?
>
> Thanks for any suggestions,
> Karol
>
> On Thu, Jul 7, 2011 at 7:01 PM, karol skocik <karol.skocik@REDACTED>
> wrote:
> > Hi,
> >  I think I have found a race condition in mnesia:add_table_copy.
> > I am trying to add table copy, when new node appears in cluster (or
> > add table copy to another node, when the one having a copy fails), and
> > the number of copies is less than some required count.
> >
> > The idea is simple, I spawn a new process on every node in cluster
> > first, and in these processes I want to create a global transaction
> > using global:trans with ID = {add_table_trans, table_name}.
> > The first process which grabbed the transaction lock, checks if more
> > table copies are required, and creates new copy on some node not
> > having one, when needed.
> > When the copy is created, this process exits, and another process on
> > different node gets the transaction lock and tries to do the same.
> >
> > The problem here is, that the second process checks where are the
> > copies using mnesia:table_info(table_name, disc_copies), and this list
> > is sometimes incomplete, missing the very last node which got a table
> > copy in the first process.
> > It can be verified easily - in the second process:
> > Copies1 = mnesia:table_info(table_name, disc_copies),
> > timer:sleep(2000),
> > Copies2 = mnesia:table_info(table_name, disc_copies).
> >
> > Then, mnesia:add_table_copy fails with
> > {aborted,{already_exists,table_name,LastAddedNode}}
> >
> > Since the transaction lock ensures that no other process can add
> > another table copy, I guess this is a race condition where new table
> > copy node is not propagated to the schema on all nodes before the
> > function mnesia:add_table_copy returns.
> >
> > Karol
> >
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