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Advisory locking for ActiveRecord
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README.md

with_advisory_lock

Adds advisory locking (mutexes) to ActiveRecord 4.2, 5.x and 6.0, with ruby 2.4, 2.5 and 2.6, when used with MySQL or PostgreSQL. SQLite resorts to file locking.

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What's an "Advisory Lock"?

An advisory lock is a mutex used to ensure no two processes run some process at the same time. When the advisory lock is powered by your database server, as long as it isn't SQLite, your mutex spans hosts.

Usage

This gem automatically includes the WithAdvisoryLock module in all of your ActiveRecord models. Here's an example of how to use it where User is an ActiveRecord model, and lock_name is some string:

User.with_advisory_lock(lock_name) do
  do_something_that_needs_locking
end

What happens

  1. The thread will wait indefinitely until the lock is acquired.
  2. While inside the block, you will exclusively own the advisory lock.
  3. The lock will be released after your block ends, even if an exception is raised in the block.

Lock wait timeouts

with_advisory_lock takes an options hash as the second parameter. The timeout_seconds option defaults to nil, which means wait indefinitely for the lock.

A value of zero will try the lock only once. If the lock is acquired, the block will be yielded to. If the lock is currently being held, the block will not be called.

Note that if a non-nil value is provided for timeout_seconds, the block will not be invoked if the lock cannot be acquired within that time-frame.

For backwards compatability, the timeout value can be specified directly as the second parameter.

Shared locks

The shared option defaults to false which means an exclusive lock will be obtained. Setting shared to true will allow locks to be obtained by multiple actors as long as they are all shared locks.

Note: MySQL does not support shared locks.

Transaction-level locks

PostgreSQL supports transaction-level locks which remain held until the transaction completes. You can enable this by setting the transaction option to true.

Note: transaction-level locks will not be reflected by .current_advisory_lock when the block has returned.

Return values

The return value of with_advisory_lock_result is a WithAdvisoryLock::Result instance, which has a lock_was_acquired? method and a result accessor method, which is the returned value of the given block. If your block may validly return false, you should use this method.

The return value of with_advisory_lock will be the result of the yielded block, if the lock was able to be acquired and the block yielded, or false, if you provided a timeout_seconds value and the lock was not able to be acquired in time.

Testing for the current lock status

If you needed to check if the advisory lock is currently being held, you can call Tag.advisory_lock_exists?("foo"), but realize the lock can be acquired between the time you test for the lock, and the time you try to acquire the lock.

If you want to see if the current Thread is holding a lock, you can call Tag.current_advisory_lock which will return the name of the current lock. If no lock is currently held, .current_advisory_lock returns nil.

Installation

Add this line to your application's Gemfile:

gem 'with_advisory_lock'

And then execute:

$ bundle

Lock Types

First off, know that there are lots of different kinds of locks available to you. Pick the finest-grain lock that ensures correctness. If you choose a lock that is too coarse, you are unnecessarily blocking other processes.

Advisory locks

These are named mutexes that are inherently "application level"—it is up to the application to acquire, run a critical code section, and release the advisory lock.

Row-level locks

Whether optimistic or pessimistic, row-level locks prevent concurrent modification to a given model.

If you're building a CRUD application, this will be your most commonly used lock.

Table-level locks

Provided through something like the monogamy gem, these prevent concurrent access to any instance of a model. Their coarseness means they aren't going to be commonly applicable, and they can be a source of deadlocks.

FAQ

Transactions and Advisory Locks

Advisory locks with MySQL and PostgreSQL ignore database transaction boundaries.

You will want to wrap your block within a transaction to ensure consistency.

MySQL < 5.7.5 doesn't support nesting

With MySQL < 5.7.5, if you ask for a different advisory lock within a with_advisory_lock block, you will be releasing the parent lock (!!!). A NestedAdvisoryLockErrorwill be raised in this case. If you ask for the same lock name, with_advisory_lock won't ask for the lock again, and the block given will be yielded to.

This is not an issue in MySQL >= 5.7.5, and no error will be raised for nested lock usage. You can override this by passing force_nested_lock_support: true or force_nested_lock_support: false to the with_advisory_lock options.

Is clustered MySQL supported?

No.

There are many lock-* files in my project directory after test runs

This is expected if you aren't using MySQL or Postgresql for your tests. See issue 3.

SQLite doesn't have advisory locks, so we resort to file locking, which will only work if the FLOCK_DIR is set consistently for all ruby processes.

In your spec_helper.rb or minitest_helper.rb, add a before and after block:

before do
  ENV['FLOCK_DIR'] = Dir.mktmpdir
end

after do
  FileUtils.remove_entry_secure ENV['FLOCK_DIR']
end
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