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https://github.com/ClosureTree/with_advisory_lock

Advisory locking for ActiveRecord
https://github.com/ClosureTree/with_advisory_lock

activerecord advisory-locks mutexes mysql postgres postgresql ruby transaction

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Advisory locking for ActiveRecord

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# with_advisory_lock



Adds advisory locking (mutexes) to ActiveRecord  6.0+, with ruby 2.7+, jruby or truffleruby, when used with

[MySQL](https://dev.mysql.com/doc/refman/8.0/en/miscellaneous-functions.html#function_get-lock)

or

[PostgreSQL](https://www.postgresql.org/docs/current/static/functions-admin.html#FUNCTIONS-ADVISORY-LOCKS).

SQLite resorts to file locking.



[![Gem Version](https://badge.fury.io/rb/with_advisory_lock.svg)](https://badge.fury.io/rb/with_advisory_lock)

[![CI](https://github.com/ClosureTree/with_advisory_lock/actions/workflows/ci.yml/badge.svg)](https://github.com/ClosureTree/with_advisory_lock/actions/workflows/ci.yml)



## What's an "Advisory Lock"?



An advisory lock is a [mutex](https://en.wikipedia.org/wiki/Mutual_exclusion)

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:



```ruby

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**

> 

> 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. In this case, `with_advisory_lock` will return `false`, while `with_advisory_lock!` will raise a `WithAdvisoryLock::FailedToAcquireLock` error.



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.



`with_advisory_lock!` is similar to `with_advisory_lock`, but raises a `WithAdvisoryLock::FailedToAcquireLock` error if 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`.



### ActiveRecord Query Cache



You can optionally pass `disable_query_cache: true` to the options hash of

`with_advisory_lock` in order to disable ActiveRecord's query cache. This can

prevent problems when you query the database from within the lock and it returns

stale results. More info on why this can be a problem can be

[found here](https://github.com/ClosureTree/with_advisory_lock/issues/52)



## Installation



Add this line to your application's Gemfile:



```ruby

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](http://api.rubyonrails.org/classes/ActiveRecord/Locking/Optimistic.html)

or [pessimistic](http://api.rubyonrails.org/classes/ActiveRecord/Locking/Pessimistic.html),

row-level locks prevent concurrent modification to a given model.



**If you're building a

[CRUD](http://en.wikipedia.org/wiki/Create,_read,_update_and_delete)

application, this will be 2.4, 2.5 and  your most commonly used lock.**



### Table-level locks



Provided through something like the

[monogamy](https://github.com/ClosureTree/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](http://en.wikipedia.org/wiki/Deadlock).



## 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

`NestedAdvisoryLockError`will 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.](https://github.com/ClosureTree/with_advisory_lock/issues/16)



### 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](https://github.com/ClosureTree/with_advisory_lock/issues/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:



```ruby

before do

  ENV['FLOCK_DIR'] = Dir.mktmpdir

end



after do

  FileUtils.remove_entry_secure ENV['FLOCK_DIR']

end

```