Awesome

Fatalistic

Fatalistic is a Ruby gem that adds table-level locking to Active Record.

Table locks

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 query 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 Person.where(...).lock will continue to lock just the selected records.

Example

require "active_record/locking/fatalistic"

class Person < ActiveRecord::Base
end

Person.lock do
  # your code here
end

Getting it

Just install with RubyGems:

gem install fatalistic

The source code is on Github.

License

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.