1ee4a8812f
This PR moves advisory lock to it's own connection instead of `ActiveRecord::Base` to fix #37748. As a note the issue is present on both mysql and postgres. We don't see it on sqlite3 because sqlite3 doesn't support advisory locks. The underlying problem only appears if: 1) the app is using multiple databases, and therefore establishing a new connetion in the abstract models 2) the app has a migration that loads a model (ex `Post.update_all`) which causes that new connection to get established. This is because when Rails runs migrations the default connections are established, the lock is taken out on the `ActiveRecord::Base` connection. When the migration that calls a model is loaded, a new connection will be established and the lock will automatically be released. When Rails goes to release the lock in the ensure block it will find that the connection has been closed. Even if the connection wasn't closed the lock would no longer exist on that connection. We originally considered checking if the connection was active, but ultimately that would hide that the advisory locks weren't working correctly because there'd be no lock to release. We also considered making the lock more granular - that it only blocked on each migration individually instead of all the migrations for that connection. This might be the right move going forward, but right now multi-db migrations that load models are very broken in Rails 6.0 and master. John and I don't love this fix, it requires a bit too much knowledge of internals and how Rails picks up connections. However, it does fix the issue, makes the lock more global, and makes the lock more resilient to changing connections. Co-authored-by: John Crepezzi <john.crepezzi@gmail.com> |
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Welcome to Rails
What's Rails?
Rails is a web-application framework that includes everything needed to create database-backed web applications according to the Model-View-Controller (MVC) pattern.
Understanding the MVC pattern is key to understanding Rails. MVC divides your application into three layers: Model, View, and Controller, each with a specific responsibility.
Model layer
The Model layer represents the domain model (such as Account, Product,
Person, Post, etc.) and encapsulates the business logic specific to
your application. In Rails, database-backed model classes are derived from
ActiveRecord::Base. Active Record allows you to present the data from
database rows as objects and embellish these data objects with business logic
methods.
Although most Rails models are backed by a database, models can also be ordinary
Ruby classes, or Ruby classes that implement a set of interfaces as provided by
the Active Model module.
Controller layer
The Controller layer is responsible for handling incoming HTTP requests and
providing a suitable response. Usually, this means returning HTML, but Rails controllers
can also generate XML, JSON, PDFs, mobile-specific views, and more. Controllers load and
manipulate models, and render view templates in order to generate the appropriate HTTP response.
In Rails, incoming requests are routed by Action Dispatch to an appropriate controller, and
controller classes are derived from ActionController::Base. Action Dispatch and Action Controller
are bundled together in Action Pack.
View layer
The View layer is composed of "templates" that are responsible for providing appropriate representations of your application's resources. Templates can come in a variety of formats, but most view templates are HTML with embedded Ruby code (ERB files). Views are typically rendered to generate a controller response or to generate the body of an email. In Rails, View generation is handled by Action View.
Frameworks and libraries
Active Record, Active Model, Action Pack, and Action View can each be used independently outside Rails. In addition to that, Rails also comes with Action Mailer, a library to generate and send emails; Action Mailbox, a library to receive emails within a Rails application; Active Job, a framework for declaring jobs and making them run on a variety of queuing backends; Action Cable, a framework to integrate WebSockets with a Rails application; Active Storage, a library to attach cloud and local files to Rails applications; Action Text, a library to handle rich text content; and Active Support, a collection of utility classes and standard library extensions that are useful for Rails, and may also be used independently outside Rails.
Getting Started
-
Install Rails at the command prompt if you haven't yet:
$ gem install rails -
At the command prompt, create a new Rails application:
$ rails new myappwhere "myapp" is the application name.
-
Change directory to
myappand start the web server:$ cd myapp $ bin/rails serverRun with
--helpor-hfor options. -
Go to
http://localhost:3000and you'll see: "Yay! You’re on Rails!" -
Follow the guidelines to start developing your application. You may find the following resources handy:
Contributing
We encourage you to contribute to Ruby on Rails! Please check out the Contributing to Ruby on Rails guide for guidelines about how to proceed. Join us!
Trying to report a possible security vulnerability in Rails? Please check out our security policy for guidelines about how to proceed.
Everyone interacting in Rails and its sub-projects' codebases, issue trackers, chat rooms, and mailing lists is expected to follow the Rails code of conduct.
Code Status
License
Ruby on Rails is released under the MIT License.