Prevent race conditions on postgres

[samsondav]

Tested with 50 simultaneous connections. This 100% guarantees correctness of the sequential_id in Postgres and makes no change to the previous behavior for other database adapters.

Unfortunately this is not so trivial to implement in MySQL because table locks must be manually released.

[samsondav]

Here is the test code:

Model

class SequentialTest < ActiveRecord::Base
    acts_as_sequenced scope: :sequential_scope_id
end

Migration

class CreateSequentialTests < ActiveRecord::Migration
  def change
    create_table :sequential_tests do |t|
      t.integer :sequential_id, null: false
      t.integer :sequential_scope_id, null: false, default: 1
    end

    add_index :sequential_tests, [:sequential_id, :sequential_scope_id], unique: true
  end
end

Test

THREADS = 50
def test
  SequentialTest.destroy_all
  initial = 0

  threads = (1..THREADS).map do |i|
    Thread.new do
      ActiveRecord::Base.connection_pool.with_connection do
        SequentialTest.create!
      end
    end
  end

  threads.each {|t| t.join}

  final = SequentialTest.pluck('max(sequential_id)')[0]

  fail('not concurrent-safe') unless final == initial + THREADS
end

[samsondav]

@djreimer I enabled postgresql for your test suite and wrote a test case. It fails without the locking, and succeeds with locking enabled.

Travis CI will automatically run all tests on both databases. You can run the test manually using postgres like this: DB=postgresql rake test

[derrickreimer]

Rock on! 🤘 This is great. I want to run some tests on my local machine, but looks like you've covered all the bases. This should be merged soon. Thanks!

[samsondav]

@djreimer Hey man. Let me know if there's anything you need from me or comments you have on the code that I can fix 👍

[derrickreimer]

Cool will do. This is in on my todo list for a lazy post-Thanksgiving Friday

On Friday, November 27, 2015, Sam Philip notifications@github.com wrote:

@djreimer https://github.com/djreimer Hey man. Let me know if there's
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Derrick