Re: database-level lockdown

[Melvin Davidson <melvin6925@xxxxxxxxx>]

Wouldn't it be easier just to prevent connections to the database
 while your transaction is executed?
EG:
<Connect to your_database>
UPDATE pg_database
  SET datallowconn FALSE
 WHERE datname = 'your_database' ;
 START TRANSACTION;
<Do your_transaction>
  COMMIT;
UPDATE pg_database
  SET datallowconn TRUE
 WHERE datname = 'your_database' ;

On Fri, Jul 3, 2015 at 1:25 PM, Filipe Pina <filipe.pina@xxxxxxxxxxxxx> wrote:

So, as database level locks do not exist (as per https://wiki.postgresql.org/wiki/Lock_database), I've thought of having a function that would lock *every* table in the database (far from the same, but would probably work for me).

Something like:

CREATE OR REPLACE FUNCTION lockdown()

    RETURNS void AS $$

DECLARE

  t information_schema.tables.table_name%TYPE;

BEGIN

  FOR t in SELECT table_name

    FROM information_schema.tables

  WHERE table_schema='public'

   AND table_type='BASE TABLE'

  LOOP

    EXECUTE 'LOCK ' || t;

  END LOOP;

END

$$

LANGUAGE plpgsql;

But the problem is that calling the function starts its own transaction and once it returns, locks are removed..

Basically the workflow is (pseudo code coming from Django/python/psycopg2/external, not within pgsql):

function generic_function_restarter(developer_function) {

  # try 4 times to execute developer function and if all of them fail

  # (due to SQLSTATE 40001 serialization failures),

  # lock database and execute one last time

  for 1 in [1..4] {

    try {

      call developer_function()

      return 'success'

    }

    except SQLSTATE_40001 {

      continue

    }

    except other_error {

      return other_error

    }

      

  # only reaches here if all tries failed with SQLSTATE_40001

  try {

    START TRANSACTION

    call lockdown()

    call developer_function()

    COMMIT TRANSACTION

    return 'success'

  }

  except any_error {

    # implicit ROLLBACK

    return any_error

  }

}

So, my problem here is that "call lockdown()" will place the locks and remove them upon returning... Is it possible to execute a function without creating a subtransaction?

I could place the locks from the adapter directly at the outter transaction level but I have the feeling that performance would be worse...

Thanks,

Filipe

On Sex, Jun 12, 2015 at 5:25 , Filipe Pina <filipe.pina@xxxxxxxxxxxxx> wrote:

Exactly, that’s why there’s a limit on the retry number. On the last try I wanted something like full lockdown to make sure the transaction will not fail due to serialiazation failure (if no other processes are touching the database, it can’t happen). So if two transactions were retrying over and over, the first one to reach max_retries would activate that “global lock” making the other one wait and then the second one would also be able to successfully commit...

On 11/06/2015, at 20:27, Tom Lane <tgl@xxxxxxxxxxxxx> wrote: Filipe Pina <filipe.pina@xxxxxxxxxxxxx> writes:

It will try 5 times to execute each instruction (in case of OperationError) and in the last one it will raise the last error it received, aborting.

Now my problem is that aborting for the last try (on a restartable error - OperationalError code 40001) is not an option... It simply needs to get through, locking whatever other processes and queries it needs.

I think you need to reconsider your objectives. What if two or more transactions are repeatedly failing and retrying, perhaps because they conflict? They can't all forcibly win. regards, tom lane

--

Melvin Davidson
I reserve the right to fantasize.  Whether or not you
wish to share my fantasy is entirely up to you.