Re: [MonetDB-users] Serious concurrency issues via python api

2 Sep 2009

      Are you saying that if someone reads data from a table while another
transaction is bulk-loading data that that will cause a concurrency conflict
and a forced rollback?

I'm looking at the behavior of one of my long running batch loads and that
looks to be the case... if so, that seems pretty extreme.   I could
understand protecting from simultaneous modification but it seems wrong to
cause a write to fail for any read on a table.

73,
Matthew W. Jones (KI4ZIB)
http://matburt.net

On Wed, Sep 2, 2009 at 3:41 PM, Martin Kersten wrote:
...
Matthew Jones wrote:
...
There's no relational data in these tables, I guess I just need
something internal to manage concurrency... like blocking table locks so
I don't have to wait for a failure in order to manage concurrency on my
own.
you either have to serialize the updates/access at your client interface,
or hook into the MAL kernel to get access to the general lock/semaphore
module.
...
I noticed in one of my operations  which was a single update to a table
that no other processes were using that it gave me that error, informed
Optimistic concurrency control should also ensure that non of the
concurrent
transactions reads stale data.
This becomes tricky if you have multiple user sessions with both autocommit
and compound transactions, because then the timespans to trigger
a transaction failure are larger.
...
me of the rollback, but the changes still took effect.... which is
alarming since I will now need to:
1) Try the operation
2) If it fails, check to see if the change actually took effect
3) if the changes didn't take effect re-try the operation
This is disconcerting.
73,
Matthew W. Jones (KI4ZIB)
http://matburt.net
On Wed, Sep 2, 2009 at 3:02 PM, Martin Kersten mailto:Martin.Kersten@cwi.nl> wrote:
Dear Matthew,
A quick initial reaction. MonetDB uses an optimistic concurrency
control
    scheme, which is geared at high-read, low-write applications. If you
hit
    the system with multiple concurrent writes on the same table, then
    indeed
    to secure acid properties, transactions are bound to fail. MonetDB
    indeed
    conforms to the semantics of a transaction safe system.
Blocking queries is equivalent to relation level locking.
    This is not the preferred way to go for the kernel, because someone
    will immediately ask for more granularities and als relaxation of
    the ACID principles.
There are several application scenarios to deal with this issue,
    but all depend on the semantics required.
regards, Martin
Matthew Jones wrote:
     > I have multiple processes reading and writing to the same table
    and I've
     > noticed really persistent concurrency problems:
     >
     >   File "/usr/lib/python2.5/site-packages/monetdb/sql/cursors.py",
    line
     > 208, in execute
     >     self.__store_result(block)
     >   File "/usr/lib/python2.5/site-packages/monetdb/sql/cursors.py",
    line
     > 520, in __store_result
     >     self.__exception_handler(InterfaceError, "Unknown state, %s"
    % block)
     >   File "/usr/lib/python2.5/site-packages/monetdb/sql/cursors.py",
    line
     > 574, in __exception_handler
     >     raise exception_class(message)
     > InterfaceError: Unknown state, &2 65541 -1
     > !ERROR: COMMIT: transaction is aborted because of concurency
    conflicts,
     > will ROLLBACK instead
     >
     > I'm not sure what the problem is, but it exists in such a way
    that all
     > operations that load or update information in MonetDB have to be
put
     > into a loop to continuously retry.
     >
     > I'm using autocommit, but I would expect MonetDB to handle
    concurrency
     > issues in a decent way, like maybe blocking a query until the
other
     > query finished.  I'm not clobbering data, most of these are just
bulk
     > load operations that insert new rows.
     >
     > Is there any way to address this issue without having to putting
    retry
     > loops around all of the code I'm writing to write to the database?
     >
     > 73,
     > Matthew W. Jones (KI4ZIB)
     > http://matburt.net
     >
     >
     >
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