Fatalistic is a Ruby gem that adds table-level locking to Active Record.
Table-level locks can be used to restrict read and write access to a table. Neither Postgres nor MySQL currently support truly serializabile transactions, so table locks are sometimes necessary to reliably avoid the "phantom record" problem. See this Wikipedia article for more details.
The MySQL docs show a classic usage scenario for table locking:
LOCK TABLES trans READ, customer WRITE; SELECT SUM(value) FROM trans WHERE customer_id=some_id; UPDATE customer SET total_value=sum_from_previous_statement WHERE customer_id=some_id; UNLOCK TABLES;
Table-level locks are generally best avoided when possible because of their potential impact on performance. MySQL/Innodb's locking implementation is also clunky and fraught with bizarre behaviors, particularly when used with transactions. Before relying on table locks, see if there's some other way to accomplish your goal. However, they can be useful when used sparingly.
Doesn't Active Record already support locking?
Active Record supports row-level locking, but not table locking.
If you do something like
Person.lock with Active Record, it will emit the
SELECT * FROM people FOR UPDATE. This is bad for performance because if
you have a lot of rows, it's going to be very slow. It's also nearly useless,
because it still doesn't prevent new records from being inserted. Finally, it's
foolish because if you want to lock every row in a table, it makes much more
sense to lock the table itself.
Active Record comes with 2 locking modules: optimistic and pessimistic. Since this locking mode is the most "extreme" of the three, I've named it "fatalistic."
What Fatalistic does
MySQL and Postgres use the same row locking syntax, but quite different table locking syntax. This library provides the abstraction needed to use them both with Active Record. SQLite does not support table level locking, so the methods provided here are just no-ops with SQLite.
Fatalistic changes the behavior of the top-level
lock method so that
Person.lock will lock the entire table, but
continue to lock just the selected records.
require "active_record/locking/fatalistic" class Person < ActiveRecord::Base end Person.lock do # your code here end
Just install with RubyGems:
gem install fatalistic
The source code is on Github.
Copyright (c) 2011-2012 Norman Clarke and Business Vision SA
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