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DO NOT READ THIS FILE ON GITHUB, GUIDES ARE PUBLISHED ON http://guides.rubyonrails.org.

Configuring Rails Applications

This guide covers the configuration and initialization features available to Rails applications.

After reading this guide, you will know:

How to adjust the behavior of your Rails applications.
How to add additional code to be run at application start time.

Locations for Initialization Code

Rails offers four standard spots to place initialization code:

config/application.rb
Environment-specific configuration files
Initializers
After-initializers

Running Code Before Rails

In the rare event that your application needs to run some code before Rails itself is loaded, put it above the call to require 'rails/all' in config/application.rb.

Configuring Rails Components

In general, the work of configuring Rails means configuring the components of Rails, as well as configuring Rails itself. The configuration file config/application.rb and environment-specific configuration files (such as config/environments/production.rb) allow you to specify the various settings that you want to pass down to all of the components.

For example, you could add this setting to config/application.rb file:

config.time_zone = 'Central Time (US & Canada)'

This is a setting for Rails itself. If you want to pass settings to individual Rails components, you can do so via the same config object in config/application.rb:

config.active_record.schema_format = :ruby

Rails will use that particular setting to configure Active Record.

Rails General Configuration

These configuration methods are to be called on a Rails::Railtie object, such as a subclass of Rails::Engine or Rails::Application.

config.after_initialize takes a block which will be run after Rails has finished initializing the application. That includes the initialization of the framework itself, engines, and all the application's initializers in config/initializers. Note that this block will be run for rake tasks. Useful for configuring values set up by other initializers:
```
config.after_initialize do
  ActionView::Base.sanitized_allowed_tags.delete 'div'
end
```
config.asset_host sets the host for the assets. Useful when CDNs are used for hosting assets, or when you want to work around the concurrency constraints built-in in browsers using different domain aliases. Shorter version of config.action_controller.asset_host.
config.autoload_once_paths accepts an array of paths from which Rails will autoload constants that won't be wiped per request. Relevant if config.cache_classes is false, which is the case in development mode by default. Otherwise, all autoloading happens only once. All elements of this array must also be in autoload_paths. Default is an empty array.
config.autoload_paths accepts an array of paths from which Rails will autoload constants. Default is all directories under app. It is no longer recommended to adjust this. See Autoloading and Reloading Constants
config.cache_classes controls whether or not application classes and modules should be reloaded on each request. Defaults to false in development mode, and true in test and production modes.
config.beginning_of_week sets the default beginning of week for the application. Accepts a valid week day symbol (e.g. :monday).
config.cache_store configures which cache store to use for Rails caching. Options include one of the symbols :memory_store, :file_store, :mem_cache_store, :null_store, or an object that implements the cache API. Defaults to :file_store.
config.colorize_logging specifies whether or not to use ANSI color codes when logging information. Defaults to true.
config.consider_all_requests_local is a flag. If true then any error will cause detailed debugging information to be dumped in the HTTP response, and the Rails::Info controller will show the application runtime context in /rails/info/properties. true by default in development and test environments, and false in production mode. For finer-grained control, set this to false and implement local_request? in controllers to specify which requests should provide debugging information on errors.

config.console allows you to set class that will be used as console you run rails console. It's best to run it in console block:

console do
  # this block is called only when running console,
  # so we can safely require pry here
  require "pry"
  config.console = Pry
end

config.eager_load when true, eager loads all registered config.eager_load_namespaces. This includes your application, engines, Rails frameworks and any other registered namespace.
config.eager_load_namespaces registers namespaces that are eager loaded when config.eager_load is true. All namespaces in the list must respond to the eager_load! method.
config.eager_load_paths accepts an array of paths from which Rails will eager load on boot if cache classes is enabled. Defaults to every folder in the app directory of the application.
config.enable_dependency_loading: when true, enables autoloading, even if the application is eager loaded and config.cache_classes is set as true. Defaults to false.
config.encoding sets up the application-wide encoding. Defaults to UTF-8.
config.exceptions_app sets the exceptions application invoked by the ShowException middleware when an exception happens. Defaults to ActionDispatch::PublicExceptions.new(Rails.public_path).
config.debug_exception_response_format sets the format used in responses when errors occur in development mode. Defaults to :api for API only apps and :default for normal apps.
config.file_watcher is the class used to detect file updates in the file system when config.reload_classes_only_on_change is true. Rails ships with ActiveSupport::FileUpdateChecker, the default, and ActiveSupport::EventedFileUpdateChecker (this one depends on the listen gem). Custom classes must conform to the ActiveSupport::FileUpdateChecker API.
config.filter_parameters used for filtering out the parameters that you don't want shown in the logs, such as passwords or credit card numbers. By default, Rails filters out passwords by adding Rails.application.config.filter_parameters += [:password] in config/initializers/filter_parameter_logging.rb. Parameters filter works by partial matching regular expression.
config.force_ssl forces all requests to be served over HTTPS by using the ActionDispatch::SSL middleware, and sets config.action_mailer.default_url_options to be { protocol: 'https' }. This can be configured by setting config.ssl_options - see the ActionDispatch::SSL documentation for details.
config.log_formatter defines the formatter of the Rails logger. This option defaults to an instance of ActiveSupport::Logger::SimpleFormatter for all modes. If you are setting a value for config.logger you must manually pass the value of your formatter to your logger before it is wrapped in an ActiveSupport::TaggedLogging instance, Rails will not do it for you.
config.log_level defines the verbosity of the Rails logger. This option defaults to :debug for all environments. The available log levels are: :debug, :info, :warn, :error, :fatal, and :unknown.
config.log_tags accepts a list of: methods that the request object responds to, a Proc that accepts the request object, or something that responds to to_s. This makes it easy to tag log lines with debug information like subdomain and request id - both very helpful in debugging multi-user production applications.
config.logger is the logger that will be used for Rails.logger and any related Rails logging such as ActiveRecord::Base.logger. It defaults to an instance of ActiveSupport::TaggedLogging that wraps an instance of ActiveSupport::Logger which outputs a log to the log/ directory. You can supply a custom logger, to get full compatibility you must follow these guidelines:
- To support a formatter, you must manually assign a formatter from the config.log_formatter value to the logger.
- To support tagged logs, the log instance must be wrapped with ActiveSupport::TaggedLogging.
- To support silencing, the logger must include LoggerSilence and ActiveSupport::LoggerThreadSafeLevel modules. The ActiveSupport::Logger class already includes these modules.
```
class MyLogger < ::Logger
  include ActiveSupport::LoggerThreadSafeLevel
  include LoggerSilence
end

mylogger           = MyLogger.new(STDOUT)
mylogger.formatter = config.log_formatter
config.logger      = ActiveSupport::TaggedLogging.new(mylogger)
```
config.middleware allows you to configure the application's middleware. This is covered in depth in the Configuring Middleware section below.
config.reload_classes_only_on_change enables or disables reloading of classes only when tracked files change. By default tracks everything on autoload paths and is set to true. If config.cache_classes is true, this option is ignored.
secret_key_base is used for specifying a key which allows sessions for the application to be verified against a known secure key to prevent tampering. Applications get a random generated key in test and development environments, other environments should set one in config/credentials.yml.enc.
config.public_file_server.enabled configures Rails to serve static files from the public directory. This option defaults to true, but in the production environment it is set to false because the server software (e.g. NGINX or Apache) used to run the application should serve static files instead. If you are running or testing your app in production mode using WEBrick (it is not recommended to use WEBrick in production) set the option to true. Otherwise, you won't be able to use page caching and request for files that exist under the public directory.
config.session_store specifies what class to use to store the session. Possible values are :cookie_store which is the default, :mem_cache_store, and :disabled. The last one tells Rails not to deal with sessions. Defaults to a cookie store with application name as the session key. Custom session stores can also be specified:
```
config.session_store :my_custom_store
```
This custom store must be defined as ActionDispatch::Session::MyCustomStore.
config.time_zone sets the default time zone for the application and enables time zone awareness for Active Record.

Configuring Assets

config.assets.enabled a flag that controls whether the asset pipeline is enabled. It is set to true by default.
config.assets.css_compressor defines the CSS compressor to use. It is set by default by sass-rails. The unique alternative value at the moment is :yui, which uses the yui-compressor gem.
config.assets.js_compressor defines the JavaScript compressor to use. Possible values are :closure, :uglifier and :yui which require the use of the closure-compiler, uglifier or yui-compressor gems respectively.
config.assets.gzip a flag that enables the creation of gzipped version of compiled assets, along with non-gzipped assets. Set to true by default.
config.assets.paths contains the paths which are used to look for assets. Appending paths to this configuration option will cause those paths to be used in the search for assets.
config.assets.precompile allows you to specify additional assets (other than application.css and application.js) which are to be precompiled when rake assets:precompile is run.
config.assets.unknown_asset_fallback allows you to modify the behavior of the asset pipeline when an asset is not in the pipeline, if you use sprockets-rails 3.2.0 or newer. Defaults to true.
config.assets.prefix defines the prefix where assets are served from. Defaults to /assets.
config.assets.manifest defines the full path to be used for the asset precompiler's manifest file. Defaults to a file named manifest-<random>.json in the config.assets.prefix directory within the public folder.
config.assets.digest enables the use of SHA256 fingerprints in asset names. Set to true by default.
config.assets.debug disables the concatenation and compression of assets. Set to true by default in development.rb.
config.assets.version is an option string that is used in SHA256 hash generation. This can be changed to force all files to be recompiled.
config.assets.compile is a boolean that can be used to turn on live Sprockets compilation in production.
config.assets.logger accepts a logger conforming to the interface of Log4r or the default Ruby Logger class. Defaults to the same configured at config.logger. Setting config.assets.logger to false will turn off served assets logging.
config.assets.quiet disables logging of assets requests. Set to true by default in development.rb.

Configuring Generators

Rails allows you to alter what generators are used with the config.generators method. This method takes a block:

config.generators do |g|
  g.orm :active_record
  g.test_framework :test_unit
end

The full set of methods that can be used in this block are as follows:

assets allows to create assets on generating a scaffold. Defaults to true.
force_plural allows pluralized model names. Defaults to false.
helper defines whether or not to generate helpers. Defaults to true.
integration_tool defines which integration tool to use to generate integration tests. Defaults to :test_unit.
system_tests defines which integration tool to use to generate system tests. Defaults to :test_unit.
javascripts turns on the hook for JavaScript files in generators. Used in Rails for when the scaffold generator is run. Defaults to true.
javascript_engine configures the engine to be used (for eg. coffee) when generating assets. Defaults to :js.
orm defines which orm to use. Defaults to false and will use Active Record by default.
resource_controller defines which generator to use for generating a controller when using rails generate resource. Defaults to :controller.
resource_route defines whether a resource route definition should be generated or not. Defaults to true.
scaffold_controller different from resource_controller, defines which generator to use for generating a scaffolded controller when using rails generate scaffold. Defaults to :scaffold_controller.
stylesheets turns on the hook for stylesheets in generators. Used in Rails for when the scaffold generator is run, but this hook can be used in other generates as well. Defaults to true.
stylesheet_engine configures the stylesheet engine (for eg. sass) to be used when generating assets. Defaults to :css.
scaffold_stylesheet creates scaffold.css when generating a scaffolded resource. Defaults to true.
test_framework defines which test framework to use. Defaults to false and will use Minitest by default.
template_engine defines which template engine to use, such as ERB or Haml. Defaults to :erb.

Configuring Middleware

Every Rails application comes with a standard set of middleware which it uses in this order in the development environment:

ActionDispatch::SSL forces every request to be served using HTTPS. Enabled if config.force_ssl is set to true. Options passed to this can be configured by setting config.ssl_options.
ActionDispatch::Static is used to serve static assets. Disabled if config.public_file_server.enabled is false. Set config.public_file_server.index_name if you need to serve a static directory index file that is not named index. For example, to serve main.html instead of index.html for directory requests, set config.public_file_server.index_name to "main".
ActionDispatch::Executor allows thread safe code reloading. Disabled if config.allow_concurrency is false, which causes Rack::Lock to be loaded. Rack::Lock wraps the app in mutex so it can only be called by a single thread at a time.
ActiveSupport::Cache::Strategy::LocalCache serves as a basic memory backed cache. This cache is not thread safe and is intended only for serving as a temporary memory cache for a single thread.
Rack::Runtime sets an X-Runtime header, containing the time (in seconds) taken to execute the request.
Rails::Rack::Logger notifies the logs that the request has begun. After request is complete, flushes all the logs.
ActionDispatch::ShowExceptions rescues any exception returned by the application and renders nice exception pages if the request is local or if config.consider_all_requests_local is set to true. If config.action_dispatch.show_exceptions is set to false, exceptions will be raised regardless.
ActionDispatch::RequestId makes a unique X-Request-Id header available to the response and enables the ActionDispatch::Request#uuid method.
ActionDispatch::RemoteIp checks for IP spoofing attacks and gets valid client_ip from request headers. Configurable with the config.action_dispatch.ip_spoofing_check, and config.action_dispatch.trusted_proxies options.
Rack::Sendfile intercepts responses whose body is being served from a file and replaces it with a server specific X-Sendfile header. Configurable with config.action_dispatch.x_sendfile_header.
ActionDispatch::Callbacks runs the prepare callbacks before serving the request.
ActionDispatch::Cookies sets cookies for the request.
ActionDispatch::Session::CookieStore is responsible for storing the session in cookies. An alternate middleware can be used for this by changing the config.action_controller.session_store to an alternate value. Additionally, options passed to this can be configured by using config.action_controller.session_options.
ActionDispatch::Flash sets up the flash keys. Only available if config.action_controller.session_store is set to a value.
Rack::MethodOverride allows the method to be overridden if params[:_method] is set. This is the middleware which supports the PATCH, PUT, and DELETE HTTP method types.
Rack::Head converts HEAD requests to GET requests and serves them as so.

Besides these usual middleware, you can add your own by using the config.middleware.use method:

config.middleware.use Magical::Unicorns

This will put the Magical::Unicorns middleware on the end of the stack. You can use insert_before if you wish to add a middleware before another.

config.middleware.insert_before Rack::Head, Magical::Unicorns

Or you can insert a middleware to exact position by using indexes. For example, if you want to insert Magical::Unicorns middleware on top of the stack, you can do it, like so:

config.middleware.insert_before 0, Magical::Unicorns

There's also insert_after which will insert a middleware after another:

config.middleware.insert_after Rack::Head, Magical::Unicorns

Middlewares can also be completely swapped out and replaced with others:

config.middleware.swap ActionController::Failsafe, Lifo::Failsafe

They can also be removed from the stack completely:

config.middleware.delete Rack::MethodOverride

Configuring i18n

All these configuration options are delegated to the I18n library.

config.i18n.available_locales whitelists the available locales for the app. Defaults to all locale keys found in locale files, usually only :en on a new application.
config.i18n.default_locale sets the default locale of an application used for i18n. Defaults to :en.
config.i18n.enforce_available_locales ensures that all locales passed through i18n must be declared in the available_locales list, raising an I18n::InvalidLocale exception when setting an unavailable locale. Defaults to true. It is recommended not to disable this option unless strongly required, since this works as a security measure against setting any invalid locale from user input.
config.i18n.load_path sets the path Rails uses to look for locale files. Defaults to config/locales/*.{yml,rb}.
config.i18n.fallbacks sets fallback behavior for missing translations. Here are 3 usage examples for this option:
- You can set the option to true for using default locale as fallback, like so:
```
config.i18n.fallbacks = true
```
- Or you can set an array of locales as fallback, like so:
```
config.i18n.fallbacks = [:tr, :en]
```
- Or you can set different fallbacks for locales individually. For example, if you want to use :tr for :az and :de, :en for :da as fallbacks, you can do it, like so:
```
config.i18n.fallbacks = { az: :tr, da: [:de, :en] }
#or
config.i18n.fallbacks.map = { az: :tr, da: [:de, :en] }
```

Configuring Active Record

config.active_record includes a variety of configuration options:

config.active_record.logger accepts a logger conforming to the interface of Log4r or the default Ruby Logger class, which is then passed on to any new database connections made. You can retrieve this logger by calling logger on either an Active Record model class or an Active Record model instance. Set to nil to disable logging.
config.active_record.primary_key_prefix_type lets you adjust the naming for primary key columns. By default, Rails assumes that primary key columns are named id (and this configuration option doesn't need to be set.) There are two other choices:
- :table_name would make the primary key for the Customer class customerid.
- :table_name_with_underscore would make the primary key for the Customer class customer_id.
config.active_record.table_name_prefix lets you set a global string to be prepended to table names. If you set this to northwest_, then the Customer class will look for northwest_customers as its table. The default is an empty string.
config.active_record.table_name_suffix lets you set a global string to be appended to table names. If you set this to _northwest, then the Customer class will look for customers_northwest as its table. The default is an empty string.
config.active_record.schema_migrations_table_name lets you set a string to be used as the name of the schema migrations table.
config.active_record.internal_metadata_table_name lets you set a string to be used as the name of the internal metadata table.
config.active_record.protected_environments lets you set an array of names of environments where destructive actions should be prohibited.
config.active_record.pluralize_table_names specifies whether Rails will look for singular or plural table names in the database. If set to true (the default), then the Customer class will use the customers table. If set to false, then the Customer class will use the customer table.
config.active_record.default_timezone determines whether to use Time.local (if set to :local) or Time.utc (if set to :utc) when pulling dates and times from the database. The default is :utc.
config.active_record.schema_format controls the format for dumping the database schema to a file. The options are :ruby (the default) for a database-independent version that depends on migrations, or :sql for a set of (potentially database-dependent) SQL statements.
config.active_record.error_on_ignored_order specifies if an error should be raised if the order of a query is ignored during a batch query. The options are true (raise error) or false (warn). Default is false.
config.active_record.timestamped_migrations controls whether migrations are numbered with serial integers or with timestamps. The default is true, to use timestamps, which are preferred if there are multiple developers working on the same application.
config.active_record.lock_optimistically controls whether Active Record will use optimistic locking and is true by default.
config.active_record.cache_timestamp_format controls the format of the timestamp value in the cache key. Default is :nsec.
config.active_record.record_timestamps is a boolean value which controls whether or not timestamping of create and update operations on a model occur. The default value is true.
config.active_record.partial_writes is a boolean value and controls whether or not partial writes are used (i.e. whether updates only set attributes that are dirty). Note that when using partial writes, you should also use optimistic locking config.active_record.lock_optimistically since concurrent updates may write attributes based on a possibly stale read state. The default value is true.
config.active_record.maintain_test_schema is a boolean value which controls whether Active Record should try to keep your test database schema up-to-date with db/schema.rb (or db/structure.sql) when you run your tests. The default is true.
config.active_record.dump_schema_after_migration is a flag which controls whether or not schema dump should happen (db/schema.rb or db/structure.sql) when you run migrations. This is set to false in config/environments/production.rb which is generated by Rails. The default value is true if this configuration is not set.
config.active_record.dump_schemas controls which database schemas will be dumped when calling db:structure:dump. The options are :schema_search_path (the default) which dumps any schemas listed in schema_search_path, :all which always dumps all schemas regardless of the schema_search_path, or a string of comma separated schemas.
config.active_record.belongs_to_required_by_default is a boolean value and controls whether a record fails validation if belongs_to association is not present.
config.active_record.warn_on_records_fetched_greater_than allows setting a warning threshold for query result size. If the number of records returned by a query exceeds the threshold, a warning is logged. This can be used to identify queries which might be causing a memory bloat.
config.active_record.index_nested_attribute_errors allows errors for nested has_many relationships to be displayed with an index as well as the error. Defaults to false.
config.active_record.use_schema_cache_dump enables users to get schema cache information from db/schema_cache.yml (generated by bin/rails db:schema:cache:dump), instead of having to send a query to the database to get this information. Defaults to true.

The MySQL adapter adds one additional configuration option:

ActiveRecord::ConnectionAdapters::Mysql2Adapter.emulate_booleans controls whether Active Record will consider all tinyint(1) columns as booleans. Defaults to true.

The SQLite3Adapter adapter adds one additional configuration option:

ActiveRecord::ConnectionAdapters::SQLite3Adapter.represent_boolean_as_integer indicates whether boolean values are stored in sqlite3 databases as 1 and 0 or 't' and 'f'. Leaving ActiveRecord::ConnectionAdapters::SQLite3Adapter.represent_boolean_as_integer set to false is deprecated. SQLite databases have used 't' and 'f' to serialize boolean values and must have old data converted to 1 and 0 (its native boolean serialization) before setting this flag to true. Conversion can be accomplished by setting up a Rake task which runs
```
ExampleModel.where("boolean_column = 't'").update_all(boolean_column: 1)
ExampleModel.where("boolean_column = 'f'").update_all(boolean_column: 0)
```
for all models and all boolean columns, after which the flag must be set to true by adding the following to your application.rb file:
```
Rails.application.config.active_record.sqlite3.represent_boolean_as_integer = true
```

The schema dumper adds one additional configuration option:

ActiveRecord::SchemaDumper.ignore_tables accepts an array of tables that should not be included in any generated schema file.

Configuring Action Controller

config.action_controller includes a number of configuration settings:

config.action_controller.asset_host sets the host for the assets. Useful when CDNs are used for hosting assets rather than the application server itself.
config.action_controller.perform_caching configures whether the application should perform the caching features provided by the Action Controller component or not. Set to false in development mode, true in production.
config.action_controller.default_static_extension configures the extension used for cached pages. Defaults to .html.
config.action_controller.include_all_helpers configures whether all view helpers are available everywhere or are scoped to the corresponding controller. If set to false, UsersHelper methods are only available for views rendered as part of UsersController. If true, UsersHelper methods are available everywhere. The default configuration behavior (when this option is not explicitly set to true or false) is that all view helpers are available to each controller.
config.action_controller.logger accepts a logger conforming to the interface of Log4r or the default Ruby Logger class, which is then used to log information from Action Controller. Set to nil to disable logging.
config.action_controller.request_forgery_protection_token sets the token parameter name for RequestForgery. Calling protect_from_forgery sets it to :authenticity_token by default.
config.action_controller.allow_forgery_protection enables or disables CSRF protection. By default this is false in test mode and true in all other modes.
config.action_controller.forgery_protection_origin_check configures whether the HTTP Origin header should be checked against the site's origin as an additional CSRF defense.
config.action_controller.per_form_csrf_tokens configures whether CSRF tokens are only valid for the method/action they were generated for.
config.action_controller.default_protect_from_forgery determines whether forgery protection is added on ActionController:Base. This is false by default, but enabled when loading defaults for Rails 5.2.
config.action_controller.relative_url_root can be used to tell Rails that you are deploying to a subdirectory. The default is ENV['RAILS_RELATIVE_URL_ROOT'].
config.action_controller.permit_all_parameters sets all the parameters for mass assignment to be permitted by default. The default value is false.
config.action_controller.action_on_unpermitted_parameters enables logging or raising an exception if parameters that are not explicitly permitted are found. Set to :log or :raise to enable. The default value is :log in development and test environments, and false in all other environments.
config.action_controller.always_permitted_parameters sets a list of whitelisted parameters that are permitted by default. The default values are ['controller', 'action'].

config.action_controller.enable_fragment_cache_logging determines whether to log fragment cache reads and writes in verbose format as follows:

Read fragment views/v1/2914079/v1/2914079/recordings/70182313-20160225015037000000/d0bdf2974e1ef6d31685c3b392ad0b74 (0.6ms)
Rendered messages/_message.html.erb in 1.2 ms [cache hit]
Write fragment views/v1/2914079/v1/2914079/recordings/70182313-20160225015037000000/3b4e249ac9d168c617e32e84b99218b5 (1.1ms)
Rendered recordings/threads/_thread.html.erb in 1.5 ms [cache miss]

By default it is set to false which results in following output:

Rendered messages/_message.html.erb in 1.2 ms [cache hit]
Rendered recordings/threads/_thread.html.erb in 1.5 ms [cache miss]

Configuring Action Dispatch

config.action_dispatch.session_store sets the name of the store for session data. The default is :cookie_store; other valid options include :active_record_store, :mem_cache_store or the name of your own custom class.

config.action_dispatch.default_headers is a hash with HTTP headers that are set by default in each response. By default, this is defined as:

config.action_dispatch.default_headers = {
  'X-Frame-Options' => 'SAMEORIGIN',
  'X-XSS-Protection' => '1; mode=block',
  'X-Content-Type-Options' => 'nosniff',
  'X-Download-Options' => 'noopen',
  'X-Permitted-Cross-Domain-Policies' => 'none',
  'Referrer-Policy' => 'strict-origin-when-cross-origin'
}

config.action_dispatch.default_charset specifies the default character set for all renders. Defaults to nil.
config.action_dispatch.tld_length sets the TLD (top-level domain) length for the application. Defaults to 1.
config.action_dispatch.ignore_accept_header is used to determine whether to ignore accept headers from a request. Defaults to false.
config.action_dispatch.x_sendfile_header specifies server specific X-Sendfile header. This is useful for accelerated file sending from server. For example it can be set to 'X-Sendfile' for Apache.
config.action_dispatch.http_auth_salt sets the HTTP Auth salt value. Defaults to 'http authentication'.
config.action_dispatch.signed_cookie_salt sets the signed cookies salt value. Defaults to 'signed cookie'.
config.action_dispatch.encrypted_cookie_salt sets the encrypted cookies salt value. Defaults to 'encrypted cookie'.
config.action_dispatch.encrypted_signed_cookie_salt sets the signed encrypted cookies salt value. Defaults to 'signed encrypted cookie'.
config.action_dispatch.authenticated_encrypted_cookie_salt sets the authenticated encrypted cookie salt. Defaults to 'authenticated encrypted cookie'.
config.action_dispatch.encrypted_cookie_cipher sets the cipher to be used for encrypted cookies. This defaults to "aes-256-gcm".
config.action_dispatch.signed_cookie_digest sets the digest to be used for signed cookies. This defaults to "SHA1".
config.action_dispatch.cookies_rotations allows rotating secrets, ciphers, and digests for encrypted and signed cookies.
config.action_dispatch.perform_deep_munge configures whether deep_munge method should be performed on the parameters. See Security Guide for more information. It defaults to true.

config.action_dispatch.rescue_responses configures what exceptions are assigned to an HTTP status. It accepts a hash and you can specify pairs of exception/status. By default, this is defined as:

config.action_dispatch.rescue_responses = {
  'ActionController::RoutingError'               => :not_found,
  'AbstractController::ActionNotFound'           => :not_found,
  'ActionController::MethodNotAllowed'           => :method_not_allowed,
  'ActionController::UnknownHttpMethod'          => :method_not_allowed,
  'ActionController::NotImplemented'             => :not_implemented,
  'ActionController::UnknownFormat'              => :not_acceptable,
  'ActionController::InvalidAuthenticityToken'   => :unprocessable_entity,
  'ActionController::InvalidCrossOriginRequest'  => :unprocessable_entity,
  'ActionDispatch::Http::Parameters::ParseError' => :bad_request,
  'ActionController::BadRequest'                 => :bad_request,
  'ActionController::ParameterMissing'           => :bad_request,
  'Rack::QueryParser::ParameterTypeError'        => :bad_request,
  'Rack::QueryParser::InvalidParameterError'     => :bad_request,
  'ActiveRecord::RecordNotFound'                 => :not_found,
  'ActiveRecord::StaleObjectError'               => :conflict,
  'ActiveRecord::RecordInvalid'                  => :unprocessable_entity,
  'ActiveRecord::RecordNotSaved'                 => :unprocessable_entity
}

Any exceptions that are not configured will be mapped to 500 Internal Server Error.

ActionDispatch::Callbacks.before takes a block of code to run before the request.
ActionDispatch::Callbacks.after takes a block of code to run after the request.

Configuring Action View

config.action_view includes a small number of configuration settings:

config.action_view.cache_template_loading controls whether or not templates should be reloaded on each request. Defaults to whatever is set for config.cache_classes.
config.action_view.field_error_proc provides an HTML generator for displaying errors that come from Active Model. The default is
```
Proc.new do |html_tag, instance|
  %Q(<div class="field_with_errors">#{html_tag}</div>).html_safe
end
```
config.action_view.default_form_builder tells Rails which form builder to use by default. The default is ActionView::Helpers::FormBuilder. If you want your form builder class to be loaded after initialization (so it's reloaded on each request in development), you can pass it as a String.
config.action_view.logger accepts a logger conforming to the interface of Log4r or the default Ruby Logger class, which is then used to log information from Action View. Set to nil to disable logging.
config.action_view.erb_trim_mode gives the trim mode to be used by ERB. It defaults to '-', which turns on trimming of tail spaces and newline when using <%= -%> or <%= =%>. See the Erubis documentation for more information.
config.action_view.embed_authenticity_token_in_remote_forms allows you to set the default behavior for authenticity_token in forms with remote: true. By default it's set to false, which means that remote forms will not include authenticity_token, which is helpful when you're fragment-caching the form. Remote forms get the authenticity from the meta tag, so embedding is unnecessary unless you support browsers without JavaScript. In such case you can either pass authenticity_token: true as a form option or set this config setting to true.
config.action_view.prefix_partial_path_with_controller_namespace determines whether or not partials are looked up from a subdirectory in templates rendered from namespaced controllers. For example, consider a controller named Admin::ArticlesController which renders this template:
```
<%= render @article %>
```
The default setting is true, which uses the partial at /admin/articles/_article.erb. Setting the value to false would render /articles/_article.erb, which is the same behavior as rendering from a non-namespaced controller such as ArticlesController.
config.action_view.raise_on_missing_translations determines whether an error should be raised for missing translations.
config.action_view.automatically_disable_submit_tag determines whether submit_tag should automatically disable on click, this defaults to true.
config.action_view.debug_missing_translation determines whether to wrap the missing translations key in a <span> tag or not. This defaults to true.
config.action_view.form_with_generates_remote_forms determines whether form_with generates remote forms or not. This defaults to true.
config.action_view.form_with_generates_ids determines whether form_with generates ids on inputs. This defaults to true.

Configuring Action Mailer

There are a number of settings available on config.action_mailer:

config.action_mailer.logger accepts a logger conforming to the interface of Log4r or the default Ruby Logger class, which is then used to log information from Action Mailer. Set to nil to disable logging.
config.action_mailer.smtp_settings allows detailed configuration for the :smtp delivery method. It accepts a hash of options, which can include any of these options:
- :address - Allows you to use a remote mail server. Just change it from its default "localhost" setting.
- :port - On the off chance that your mail server doesn't run on port 25, you can change it.
- :domain - If you need to specify a HELO domain, you can do it here.
- :user_name - If your mail server requires authentication, set the username in this setting.
- :password - If your mail server requires authentication, set the password in this setting.
- :authentication - If your mail server requires authentication, you need to specify the authentication type here. This is a symbol and one of :plain, :login, :cram_md5.
- :enable_starttls_auto - Detects if STARTTLS is enabled in your SMTP server and starts to use it. It defaults to true.
- :openssl_verify_mode - When using TLS, you can set how OpenSSL checks the certificate. This is useful if you need to validate a self-signed and/or a wildcard certificate. This can be one of the OpenSSL verify constants, :none or :peer -- or the constant directly OpenSSL::SSL::VERIFY_NONE or OpenSSL::SSL::VERIFY_PEER, respectively.
- :ssl/:tls - Enables the SMTP connection to use SMTP/TLS (SMTPS: SMTP over direct TLS connection).
config.action_mailer.sendmail_settings allows detailed configuration for the sendmail delivery method. It accepts a hash of options, which can include any of these options:
- :location - The location of the sendmail executable. Defaults to /usr/sbin/sendmail.
- :arguments - The command line arguments. Defaults to -i.
config.action_mailer.raise_delivery_errors specifies whether to raise an error if email delivery cannot be completed. It defaults to true.
config.action_mailer.delivery_method defines the delivery method and defaults to :smtp. See the configuration section in the Action Mailer guide for more info.
config.action_mailer.perform_deliveries specifies whether mail will actually be delivered and is true by default. It can be convenient to set it to false for testing.

config.action_mailer.default_options configures Action Mailer defaults. Use to set options like from or reply_to for every mailer. These default to:

mime_version:  "1.0",
charset:       "UTF-8",
content_type: "text/plain",
parts_order:  ["text/plain", "text/enriched", "text/html"]

Assign a hash to set additional options:

config.action_mailer.default_options = {
  from: "noreply@example.com"
}

config.action_mailer.observers registers observers which will be notified when mail is delivered.
```
config.action_mailer.observers = ["MailObserver"]
```
config.action_mailer.interceptors registers interceptors which will be called before mail is sent.
```
config.action_mailer.interceptors = ["MailInterceptor"]
```

config.action_mailer.preview_path specifies the location of mailer previews.

config.action_mailer.preview_path = "#{Rails.root}/lib/mailer_previews"

config.action_mailer.show_previews enable or disable mailer previews. By default this is true in development.
```
config.action_mailer.show_previews = false
```
config.action_mailer.deliver_later_queue_name specifies the queue name for mailers. By default this is mailers.
config.action_mailer.perform_caching specifies whether the mailer templates should perform fragment caching or not. By default this is false in all environments.

Configuring Active Support

There are a few configuration options available in Active Support:

config.active_support.bare enables or disables the loading of active_support/all when booting Rails. Defaults to nil, which means active_support/all is loaded.
config.active_support.test_order sets the order in which the test cases are executed. Possible values are :random and :sorted. Defaults to :random.
config.active_support.escape_html_entities_in_json enables or disables the escaping of HTML entities in JSON serialization. Defaults to true.
config.active_support.use_standard_json_time_format enables or disables serializing dates to ISO 8601 format. Defaults to true.
config.active_support.time_precision sets the precision of JSON encoded time values. Defaults to 3.
config.active_support.use_sha1_digests specifies whether to use SHA-1 instead of MD5 to generate non-sensitive digests, such as the ETag header. Defaults to false.
ActiveSupport::Logger.silencer is set to false to disable the ability to silence logging in a block. The default is true.
ActiveSupport::Cache::Store.logger specifies the logger to use within cache store operations.
ActiveSupport::Deprecation.behavior alternative setter to config.active_support.deprecation which configures the behavior of deprecation warnings for Rails.
ActiveSupport::Deprecation.silence takes a block in which all deprecation warnings are silenced.
ActiveSupport::Deprecation.silenced sets whether or not to display deprecation warnings.

Configuring Active Job

config.active_job provides the following configuration options:

config.active_job.queue_adapter sets the adapter for the queueing backend. The default adapter is :async. For an up-to-date list of built-in adapters see the ActiveJob::QueueAdapters API documentation.
```
# Be sure to have the adapter's gem in your Gemfile
# and follow the adapter's specific installation
# and deployment instructions.
config.active_job.queue_adapter = :sidekiq
```
config.active_job.default_queue_name can be used to change the default queue name. By default this is "default".
```
config.active_job.default_queue_name = :medium_priority
```
config.active_job.queue_name_prefix allows you to set an optional, non-blank, queue name prefix for all jobs. By default it is blank and not used.

The following configuration would queue the given job on the production_high_priority queue when run in production:
```
config.active_job.queue_name_prefix = Rails.env
```
```
class GuestsCleanupJob < ActiveJob::Base
  queue_as :high_priority
  #....
end
```
config.active_job.queue_name_delimiter has a default value of '_'. If queue_name_prefix is set, then queue_name_delimiter joins the prefix and the non-prefixed queue name.

The following configuration would queue the provided job on the video_server.low_priority queue:
```
# prefix must be set for delimiter to be used
config.active_job.queue_name_prefix = 'video_server'
config.active_job.queue_name_delimiter = '.'
```
```
class EncoderJob < ActiveJob::Base
  queue_as :low_priority
  #....
end
```
config.active_job.logger accepts a logger conforming to the interface of Log4r or the default Ruby Logger class, which is then used to log information from Active Job. You can retrieve this logger by calling logger on either an Active Job class or an Active Job instance. Set to nil to disable logging.

Configuring Action Cable

config.action_cable.url accepts a string for the URL for where you are hosting your Action Cable server. You would use this option if you are running Action Cable servers that are separated from your main application.
config.action_cable.mount_path accepts a string for where to mount Action Cable, as part of the main server process. Defaults to /cable. You can set this as nil to not mount Action Cable as part of your normal Rails server.

Configuring a Database

Just about every Rails application will interact with a database. You can connect to the database by setting an environment variable ENV['DATABASE_URL'] or by using a configuration file called config/database.yml.

Using the config/database.yml file you can specify all the information needed to access your database:

development:
  adapter: postgresql
  database: blog_development
  pool: 5

This will connect to the database named blog_development using the postgresql adapter. This same information can be stored in a URL and provided via an environment variable like this:

> puts ENV['DATABASE_URL']
postgresql://localhost/blog_development?pool=5

The config/database.yml file contains sections for three different environments in which Rails can run by default:

The development environment is used on your development/local computer as you interact manually with the application.
The test environment is used when running automated tests.
The production environment is used when you deploy your application for the world to use.

If you wish, you can manually specify a URL inside of your config/database.yml

development:
  url: postgresql://localhost/blog_development?pool=5

The config/database.yml file can contain ERB tags <%= %>. Anything in the tags will be evaluated as Ruby code. You can use this to pull out data from an environment variable or to perform calculations to generate the needed connection information.

TIP: You don't have to update the database configurations manually. If you look at the options of the application generator, you will see that one of the options is named --database. This option allows you to choose an adapter from a list of the most used relational databases. You can even run the generator repeatedly: cd .. && rails new blog --database=mysql. When you confirm the overwriting of the config/database.yml file, your application will be configured for MySQL instead of SQLite. Detailed examples of the common database connections are below.

Connection Preference

Since there are two ways to configure your connection (using config/database.yml or using an environment variable) it is important to understand how they can interact.

If you have an empty config/database.yml file but your ENV['DATABASE_URL'] is present, then Rails will connect to the database via your environment variable:

$ cat config/database.yml

$ echo $DATABASE_URL
postgresql://localhost/my_database

If you have a config/database.yml but no ENV['DATABASE_URL'] then this file will be used to connect to your database:

$ cat config/database.yml
development:
  adapter: postgresql
  database: my_database
  host: localhost

$ echo $DATABASE_URL

If you have both config/database.yml and ENV['DATABASE_URL'] set then Rails will merge the configuration together. To better understand this we must see some examples.

When duplicate connection information is provided the environment variable will take precedence:

$ cat config/database.yml
development:
  adapter: sqlite3
  database: NOT_my_database
  host: localhost

$ echo $DATABASE_URL
postgresql://localhost/my_database

$ bin/rails runner 'puts ActiveRecord::Base.configurations'
{"development"=>{"adapter"=>"postgresql", "host"=>"localhost", "database"=>"my_database"}}

Here the adapter, host, and database match the information in ENV['DATABASE_URL'].

If non-duplicate information is provided you will get all unique values, environment variable still takes precedence in cases of any conflicts.

$ cat config/database.yml
development:
  adapter: sqlite3
  pool: 5

$ echo $DATABASE_URL
postgresql://localhost/my_database

$ bin/rails runner 'puts ActiveRecord::Base.configurations'
{"development"=>{"adapter"=>"postgresql", "host"=>"localhost", "database"=>"my_database", "pool"=>5}}

Since pool is not in the ENV['DATABASE_URL'] provided connection information its information is merged in. Since adapter is duplicate, the ENV['DATABASE_URL'] connection information wins.

The only way to explicitly not use the connection information in ENV['DATABASE_URL'] is to specify an explicit URL connection using the "url" sub key:

$ cat config/database.yml
development:
  url: sqlite3:NOT_my_database

$ echo $DATABASE_URL
postgresql://localhost/my_database

$ bin/rails runner 'puts ActiveRecord::Base.configurations'
{"development"=>{"adapter"=>"sqlite3", "database"=>"NOT_my_database"}}

Here the connection information in ENV['DATABASE_URL'] is ignored, note the different adapter and database name.

Since it is possible to embed ERB in your config/database.yml it is best practice to explicitly show you are using the ENV['DATABASE_URL'] to connect to your database. This is especially useful in production since you should not commit secrets like your database password into your source control (such as Git).

$ cat config/database.yml
production:
  url: <%= ENV['DATABASE_URL'] %>

Now the behavior is clear, that we are only using the connection information in ENV['DATABASE_URL'].

Configuring an SQLite3 Database

Rails comes with built-in support for SQLite3, which is a lightweight serverless database application. While a busy production environment may overload SQLite, it works well for development and testing. Rails defaults to using an SQLite database when creating a new project, but you can always change it later.

Here's the section of the default configuration file (config/database.yml) with connection information for the development environment:

development:
  adapter: sqlite3
  database: db/development.sqlite3
  pool: 5
  timeout: 5000

NOTE: Rails uses an SQLite3 database for data storage by default because it is a zero configuration database that just works. Rails also supports MySQL (including MariaDB) and PostgreSQL "out of the box", and has plugins for many database systems. If you are using a database in a production environment Rails most likely has an adapter for it.

Configuring a MySQL or MariaDB Database

If you choose to use MySQL or MariaDB instead of the shipped SQLite3 database, your config/database.yml will look a little different. Here's the development section:

development:
  adapter: mysql2
  encoding: utf8
  database: blog_development
  pool: 5
  username: root
  password:
  socket: /tmp/mysql.sock

If your development database has a root user with an empty password, this configuration should work for you. Otherwise, change the username and password in the development section as appropriate.

Configuring a PostgreSQL Database

If you choose to use PostgreSQL, your config/database.yml will be customized to use PostgreSQL databases:

development:
  adapter: postgresql
  encoding: unicode
  database: blog_development
  pool: 5

Prepared Statements are enabled by default on PostgreSQL. You can disable prepared statements by setting prepared_statements to false:

production:
  adapter: postgresql
  prepared_statements: false

If enabled, Active Record will create up to 1000 prepared statements per database connection by default. To modify this behavior you can set statement_limit to a different value:

production:
  adapter: postgresql
  statement_limit: 200

The more prepared statements in use: the more memory your database will require. If your PostgreSQL database is hitting memory limits, try lowering statement_limit or disabling prepared statements.

Configuring an SQLite3 Database for JRuby Platform

If you choose to use SQLite3 and are using JRuby, your config/database.yml will look a little different. Here's the development section:

development:
  adapter: jdbcsqlite3
  database: db/development.sqlite3

Configuring a MySQL or MariaDB Database for JRuby Platform

If you choose to use MySQL or MariaDB and are using JRuby, your config/database.yml will look a little different. Here's the development section:

development:
  adapter: jdbcmysql
  database: blog_development
  username: root
  password:

Configuring a PostgreSQL Database for JRuby Platform

If you choose to use PostgreSQL and are using JRuby, your config/database.yml will look a little different. Here's the development section:

development:
  adapter: jdbcpostgresql
  encoding: unicode
  database: blog_development
  username: blog
  password:

Change the username and password in the development section as appropriate.

Creating Rails Environments

By default Rails ships with three environments: "development", "test", and "production". While these are sufficient for most use cases, there are circumstances when you want more environments.

Imagine you have a server which mirrors the production environment but is only used for testing. Such a server is commonly called a "staging server". To define an environment called "staging" for this server, just create a file called config/environments/staging.rb. Please use the contents of any existing file in config/environments as a starting point and make the necessary changes from there.

That environment is no different than the default ones, start a server with rails server -e staging, a console with rails console -e staging, Rails.env.staging? works, etc.

Deploy to a subdirectory (relative url root)

By default Rails expects that your application is running at the root (eg. /). This section explains how to run your application inside a directory.

Let's assume we want to deploy our application to "/app1". Rails needs to know this directory to generate the appropriate routes:

config.relative_url_root = "/app1"

alternatively you can set the RAILS_RELATIVE_URL_ROOT environment variable.

Rails will now prepend "/app1" when generating links.

Using Passenger

Passenger makes it easy to run your application in a subdirectory. You can find the relevant configuration in the Passenger manual.

Using a Reverse Proxy

Deploying your application using a reverse proxy has definite advantages over traditional deploys. They allow you to have more control over your server by layering the components required by your application.

Many modern web servers can be used as a proxy server to balance third-party elements such as caching servers or application servers.

One such application server you can use is Unicorn to run behind a reverse proxy.

In this case, you would need to configure the proxy server (NGINX, Apache, etc) to accept connections from your application server (Unicorn). By default Unicorn will listen for TCP connections on port 8080, but you can change the port or configure it to use sockets instead.

You can find more information in the Unicorn readme and understand the philosophy behind it.

Once you've configured the application server, you must proxy requests to it by configuring your web server appropriately. For example your NGINX config may include:

upstream application_server {
  server 0.0.0.0:8080;
}

server {
  listen 80;
  server_name localhost;

  root /root/path/to/your_app/public;

  try_files $uri/index.html $uri.html @app;

  location @app {
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header Host $http_host;
    proxy_redirect off;
    proxy_pass http://application_server;
  }

  # some other configuration
}

Be sure to read the NGINX documentation for the most up-to-date information.

Rails Environment Settings

Some parts of Rails can also be configured externally by supplying environment variables. The following environment variables are recognized by various parts of Rails:

ENV["RAILS_ENV"] defines the Rails environment (production, development, test, and so on) that Rails will run under.
ENV["RAILS_RELATIVE_URL_ROOT"] is used by the routing code to recognize URLs when you deploy your application to a subdirectory.
ENV["RAILS_CACHE_ID"] and ENV["RAILS_APP_VERSION"] are used to generate expanded cache keys in Rails' caching code. This allows you to have multiple separate caches from the same application.

Using Initializer Files

After loading the framework and any gems in your application, Rails turns to loading initializers. An initializer is any Ruby file stored under config/initializers in your application. You can use initializers to hold configuration settings that should be made after all of the frameworks and gems are loaded, such as options to configure settings for these parts.

NOTE: You can use subfolders to organize your initializers if you like, because Rails will look into the whole file hierarchy from the initializers folder on down.

TIP: While Rails supports numbering of initializer file names for load ordering purposes, a better technique is to place any code that need to load in a specific order within the same file. This reduces file name churn, makes dependencies more explicit, and can help surface new concepts within your application.

Initialization events

Rails has 5 initialization events which can be hooked into (listed in the order that they are run):

before_configuration: This is run as soon as the application constant inherits from Rails::Application. The config calls are evaluated before this happens.
before_initialize: This is run directly before the initialization process of the application occurs with the :bootstrap_hook initializer near the beginning of the Rails initialization process.
to_prepare: Run after the initializers are run for all Railties (including the application itself), but before eager loading and the middleware stack is built. More importantly, will run upon every request in development, but only once (during boot-up) in production and test.
before_eager_load: This is run directly before eager loading occurs, which is the default behavior for the production environment and not for the development environment.
after_initialize: Run directly after the initialization of the application, after the application initializers in config/initializers are run.

To define an event for these hooks, use the block syntax within a Rails::Application, Rails::Railtie or Rails::Engine subclass:

module YourApp
  class Application < Rails::Application
    config.before_initialize do
      # initialization code goes here
    end
  end
end

Alternatively, you can also do it through the config method on the Rails.application object:

Rails.application.config.before_initialize do
  # initialization code goes here
end

WARNING: Some parts of your application, notably routing, are not yet set up at the point where the after_initialize block is called.

`Rails::Railtie#initializer`

Rails has several initializers that run on startup that are all defined by using the initializer method from Rails::Railtie. Here's an example of the set_helpers_path initializer from Action Controller:

initializer "action_controller.set_helpers_path" do |app|
  ActionController::Helpers.helpers_path = app.helpers_paths
end

The initializer method takes three arguments with the first being the name for the initializer and the second being an options hash (not shown here) and the third being a block. The :before key in the options hash can be specified to specify which initializer this new initializer must run before, and the :after key will specify which initializer to run this initializer after.

Initializers defined using the initializer method will be run in the order they are defined in, with the exception of ones that use the :before or :after methods.

WARNING: You may put your initializer before or after any other initializer in the chain, as long as it is logical. Say you have 4 initializers called "one" through "four" (defined in that order) and you define "four" to go before "four" but after "three", that just isn't logical and Rails will not be able to determine your initializer order.

The block argument of the initializer method is the instance of the application itself, and so we can access the configuration on it by using the config method as done in the example.

Because Rails::Application inherits from Rails::Railtie (indirectly), you can use the initializer method in config/application.rb to define initializers for the application.

Initializers

Below is a comprehensive list of all the initializers found in Rails in the order that they are defined (and therefore run in, unless otherwise stated).

load_environment_hook: Serves as a placeholder so that :load_environment_config can be defined to run before it.
load_active_support: Requires active_support/dependencies which sets up the basis for Active Support. Optionally requires active_support/all if config.active_support.bare is un-truthful, which is the default.
initialize_logger: Initializes the logger (an ActiveSupport::Logger object) for the application and makes it accessible at Rails.logger, provided that no initializer inserted before this point has defined Rails.logger.
initialize_cache: If Rails.cache isn't set yet, initializes the cache by referencing the value in config.cache_store and stores the outcome as Rails.cache. If this object responds to the middleware method, its middleware is inserted before Rack::Runtime in the middleware stack.
set_clear_dependencies_hook: This initializer - which runs only if cache_classes is set to false - uses ActionDispatch::Callbacks.after to remove the constants which have been referenced during the request from the object space so that they will be reloaded during the following request.
initialize_dependency_mechanism: If config.cache_classes is true, configures ActiveSupport::Dependencies.mechanism to require dependencies rather than load them.
bootstrap_hook: Runs all configured before_initialize blocks.
i18n.callbacks: In the development environment, sets up a to_prepare callback which will call I18n.reload! if any of the locales have changed since the last request. In production mode this callback will only run on the first request.
active_support.deprecation_behavior: Sets up deprecation reporting for environments, defaulting to :log for development, :notify for production and :stderr for test. If a value isn't set for config.active_support.deprecation then this initializer will prompt the user to configure this line in the current environment's config/environments file. Can be set to an array of values.
active_support.initialize_time_zone: Sets the default time zone for the application based on the config.time_zone setting, which defaults to "UTC".
active_support.initialize_beginning_of_week: Sets the default beginning of week for the application based on config.beginning_of_week setting, which defaults to :monday.
active_support.set_configs: Sets up Active Support by using the settings in config.active_support by send'ing the method names as setters to ActiveSupport and passing the values through.
action_dispatch.configure: Configures the ActionDispatch::Http::URL.tld_length to be set to the value of config.action_dispatch.tld_length.
action_view.set_configs: Sets up Action View by using the settings in config.action_view by send'ing the method names as setters to ActionView::Base and passing the values through.
action_controller.assets_config: Initializes the config.actions_controller.assets_dir to the app's public directory if not explicitly configured.
action_controller.set_helpers_path: Sets Action Controller's helpers_path to the application's helpers_path.
action_controller.parameters_config: Configures strong parameters options for ActionController::Parameters.
action_controller.set_configs: Sets up Action Controller by using the settings in config.action_controller by send'ing the method names as setters to ActionController::Base and passing the values through.
action_controller.compile_config_methods: Initializes methods for the config settings specified so that they are quicker to access.
active_record.initialize_timezone: Sets ActiveRecord::Base.time_zone_aware_attributes to true, as well as setting ActiveRecord::Base.default_timezone to UTC. When attributes are read from the database, they will be converted into the time zone specified by Time.zone.
active_record.logger: Sets ActiveRecord::Base.logger - if it's not already set - to Rails.logger.
active_record.migration_error: Configures middleware to check for pending migrations.
active_record.check_schema_cache_dump: Loads the schema cache dump if configured and available.
active_record.warn_on_records_fetched_greater_than: Enables warnings when queries return large numbers of records.
active_record.set_configs: Sets up Active Record by using the settings in config.active_record by send'ing the method names as setters to ActiveRecord::Base and passing the values through.
active_record.initialize_database: Loads the database configuration (by default) from config/database.yml and establishes a connection for the current environment.
active_record.log_runtime: Includes ActiveRecord::Railties::ControllerRuntime which is responsible for reporting the time taken by Active Record calls for the request back to the logger.
active_record.set_reloader_hooks: Resets all reloadable connections to the database if config.cache_classes is set to false.
active_record.add_watchable_files: Adds schema.rb and structure.sql files to watchable files.
active_job.logger: Sets ActiveJob::Base.logger - if it's not already set - to Rails.logger.
active_job.set_configs: Sets up Active Job by using the settings in config.active_job by send'ing the method names as setters to ActiveJob::Base and passing the values through.
action_mailer.logger: Sets ActionMailer::Base.logger - if it's not already set - to Rails.logger.
action_mailer.set_configs: Sets up Action Mailer by using the settings in config.action_mailer by send'ing the method names as setters to ActionMailer::Base and passing the values through.
action_mailer.compile_config_methods: Initializes methods for the config settings specified so that they are quicker to access.
set_load_path: This initializer runs before bootstrap_hook. Adds paths specified by config.load_paths and all autoload paths to $LOAD_PATH.
set_autoload_paths: This initializer runs before bootstrap_hook. Adds all sub-directories of app and paths specified by config.autoload_paths, config.eager_load_paths and config.autoload_once_paths to ActiveSupport::Dependencies.autoload_paths.
add_routing_paths: Loads (by default) all config/routes.rb files (in the application and railties, including engines) and sets up the routes for the application.
add_locales: Adds the files in config/locales (from the application, railties and engines) to I18n.load_path, making available the translations in these files.
add_view_paths: Adds the directory app/views from the application, railties and engines to the lookup path for view files for the application.
load_environment_config: Loads the config/environments file for the current environment.
prepend_helpers_path: Adds the directory app/helpers from the application, railties and engines to the lookup path for helpers for the application.
load_config_initializers: Loads all Ruby files from config/initializers in the application, railties and engines. The files in this directory can be used to hold configuration settings that should be made after all of the frameworks are loaded.
engines_blank_point: Provides a point-in-initialization to hook into if you wish to do anything before engines are loaded. After this point, all railtie and engine initializers are run.
add_generator_templates: Finds templates for generators at lib/templates for the application, railties and engines and adds these to the config.generators.templates setting, which will make the templates available for all generators to reference.
ensure_autoload_once_paths_as_subset: Ensures that the config.autoload_once_paths only contains paths from config.autoload_paths. If it contains extra paths, then an exception will be raised.
add_to_prepare_blocks: The block for every config.to_prepare call in the application, a railtie or engine is added to the to_prepare callbacks for Action Dispatch which will be run per request in development, or before the first request in production.
add_builtin_route: If the application is running under the development environment then this will append the route for rails/info/properties to the application routes. This route provides the detailed information such as Rails and Ruby version for public/index.html in a default Rails application.
build_middleware_stack: Builds the middleware stack for the application, returning an object which has a call method which takes a Rack environment object for the request.
eager_load!: If config.eager_load is true, runs the config.before_eager_load hooks and then calls eager_load! which will load all config.eager_load_namespaces.
finisher_hook: Provides a hook for after the initialization of process of the application is complete, as well as running all the config.after_initialize blocks for the application, railties and engines.
set_routes_reloader_hook: Configures Action Dispatch to reload the routes file using ActiveSupport::Callbacks.to_run.
disable_dependency_loading: Disables the automatic dependency loading if the config.eager_load is set to true.

Database pooling

Active Record database connections are managed by ActiveRecord::ConnectionAdapters::ConnectionPool which ensures that a connection pool synchronizes the amount of thread access to a limited number of database connections. This limit defaults to 5 and can be configured in database.yml.

development:
  adapter: sqlite3
  database: db/development.sqlite3
  pool: 5
  timeout: 5000

Since the connection pooling is handled inside of Active Record by default, all application servers (Thin, Puma, Unicorn etc.) should behave the same. The database connection pool is initially empty. As demand for connections increases it will create them until it reaches the connection pool limit.

Any one request will check out a connection the first time it requires access to the database. At the end of the request it will check the connection back in. This means that the additional connection slot will be available again for the next request in the queue.

If you try to use more connections than are available, Active Record will block you and wait for a connection from the pool. If it cannot get a connection, a timeout error similar to that given below will be thrown.

ActiveRecord::ConnectionTimeoutError - could not obtain a database connection within 5.000 seconds (waited 5.000 seconds)

If you get the above error, you might want to increase the size of the connection pool by incrementing the pool option in database.yml

NOTE. If you are running in a multi-threaded environment, there could be a chance that several threads may be accessing multiple connections simultaneously. So depending on your current request load, you could very well have multiple threads contending for a limited number of connections.

Custom configuration

You can configure your own code through the Rails configuration object with custom configuration under either the config.x namespace, or config directly. The key difference between these two is that you should be using config.x if you are defining nested configuration (ex: config.x.nested.nested.hi), and just config for single level configuration (ex: config.hello).

config.x.payment_processing.schedule = :daily
config.x.payment_processing.retries  = 3
config.super_debugger = true

These configuration points are then available through the configuration object:

Rails.configuration.x.payment_processing.schedule # => :daily
Rails.configuration.x.payment_processing.retries  # => 3
Rails.configuration.x.payment_processing.not_set  # => nil
Rails.configuration.super_debugger                # => true

You can also use Rails::Application.config_for to load whole configuration files:

# config/payment.yml:
production:
  environment: production
  merchant_id: production_merchant_id
  public_key:  production_public_key
  private_key: production_private_key
development:
  environment: sandbox
  merchant_id: development_merchant_id
  public_key:  development_public_key
  private_key: development_private_key

# config/application.rb
module MyApp
  class Application < Rails::Application
    config.payment = config_for(:payment)
  end
end

Rails.configuration.payment['merchant_id'] # => production_merchant_id or development_merchant_id

Search Engines Indexing

Sometimes, you may want to prevent some pages of your application to be visible on search sites like Google, Bing, Yahoo or Duck Duck Go. The robots that index these sites will first analyze the http://your-site.com/robots.txt file to know which pages it is allowed to index.

Rails creates this file for you inside the /public folder. By default, it allows search engines to index all pages of your application. If you want to block indexing on all pages of your application, use this:

User-agent: *
Disallow: /

To block just specific pages, it's necessary to use a more complex syntax. Learn it on the official documentation.

Evented File System Monitor

If the listen gem is loaded Rails uses an evented file system monitor to detect changes when config.cache_classes is false:

group :development do
  gem 'listen', '>= 3.0.5', '< 3.2'
end

Otherwise, in every request Rails walks the application tree to check if anything has changed.

On Linux and macOS no additional gems are needed, but some are required for *BSD and for Windows.

Note that some setups are unsupported.

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