May 26, 2009
by Colin Copeland
0 comments
Categories:
Technical

Explicit Table Locking with PostgreSQL and Django

By default, Django doesn't do explicit table locking. This is OK for most read-heavy scenarios, but sometimes you need guaranteed, exclusive access to the data. Caktus uses PostgreSQL in most of our production environments, so we can use the various lock modes it provides to control concurrent access to the data. Once we obtain a lock in PostgreSQL, it is held for the remainder of the current transaction. Django provides transaction management, so all we need to do is execute a SQL LOCK statement within a transaction, and Django and PostgreSQL will handle the rest.

Below is an example decorator we came up with to provide easy table-locking access in Django:

from django.db import transaction

LOCK_MODES = (
    'ACCESS SHARE',
    'ROW SHARE',
    'ROW EXCLUSIVE',
    'SHARE UPDATE EXCLUSIVE',
    'SHARE',
    'SHARE ROW EXCLUSIVE',
    'EXCLUSIVE',
    'ACCESS EXCLUSIVE',
)

def require_lock(model, lock):
    """
    Decorator for PostgreSQL's table-level lock functionality
    
    Example:
        @transaction.commit_on_success
        @require_lock(MyModel, 'ACCESS EXCLUSIVE')
        def myview(request)
            ...
    
    PostgreSQL's LOCK Documentation:
    http://www.postgresql.org/docs/8.3/interactive/sql-lock.html
    """
    def require_lock_decorator(view_func):
        def wrapper(*args, **kwargs):
            if lock not in LOCK_MODES:
                raise ValueError('%s is not a PostgreSQL supported lock mode.')
            from django.db import connection
            cursor = connection.cursor()
            cursor.execute(
                'LOCK TABLE %s IN %s MODE' % (model._meta.db_table, lock)
            )
            return view_func(*args, **kwargs)
        return wrapper
    return require_lock_decorator

This is, by no means, a perfect solution. Feel free to comment below.

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