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.
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
- The thread will wait indefinitely until the lock is acquired.
- While inside the block, you will exclusively own the advisory lock.
- 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
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 option defaults to
false which means an exclusive lock will be
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.
PostgreSQL supports transaction-level locks which remain held until the
transaction completes. You can enable this by setting the
Note: transaction-level locks will not be reflected by
when the block has returned.
The return value of
with_advisory_lock_result is a
instance, which has a
lock_was_acquired? method and a
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
you provided a timeout_seconds value and the lock was not able to be acquired in
Testing for the current lock status
If you needed to check if the advisory lock is currently being held, you can
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
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,
Add this line to your application's Gemfile:
And then execute:
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.
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.
If you're building a CRUD application, this will be your most commonly used lock.
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.
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
with_advisory_lock block, you will be releasing the parent lock (!!!). A
NestedAdvisoryLockErrorwill be raised in this case. If you ask for the same
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: false to the
Is clustered MySQL supported?
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.
minitest_helper.rb, add a
before do ENV['FLOCK_DIR'] = Dir.mktmpdir end after do FileUtils.remove_entry_secure ENV['FLOCK_DIR'] end