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rails--rails/activerecord/lib/active_record.rb
Eileen Uchitelle 0abcec416b Adds basic automatic database switching to Rails
The following PR adds behavior to Rails to allow an application to
automatically switch it's connection from the primary to the replica.

A request will be sent to the replica if:

* The request is a read request (`GET` or `HEAD`)
* AND It's been 2 seconds since the last write to the database (because
we don't want to send a user to a replica if the write hasn't made it
to the replica yet)

A request will be sent to the primary if:

* It's not a GET/HEAD request (ie is a POST, PATCH, etc)
* Has been less than 2 seconds since the last write to the database

The implementation that decides when to switch reads (the 2 seconds) is
"safe" to use in production but not recommended without adequate testing
with your infrastructure. At GitHub in addition to the a 5 second delay
we have a curcuit breaker that checks the replication delay
and will send the query to a replica before the 5 seconds has passed.
This is specific to our application and therefore not something Rails
should be doing for you. You'll need to test and implement more robust
handling of when to switch based on your infrastructure. The auto
switcher in Rails is meant to be a basic implementation / API that acts
as a guide for how to implement autoswitching.

The impementation here is meant to be strict enough that you know how to
implement your own resolver and operations classes but flexible enough
that we're not telling you how to do it.

The middleware is not included automatically and can be installed in
your application with the classes you want to use for the resolver and
operations passed in. If you don't pass any classes into the middleware
the Rails default Resolver and Session classes will be used.

The Resolver decides what parameters define when to
switch, Operations sets timestamps for the Resolver to read from. For
example you may want to use cookies instead of a session so you'd
implement a Resolver::Cookies class and pass that into the middleware
via configuration options.

```
config.active_record.database_selector = { delay: 2.seconds }
config.active_record.database_resolver = MyResolver
config.active_record.database_operations = MyResolver::MyCookies
```

Your classes can inherit from the existing classes and reimplment the
methods (or implement more methods) that you need to do the switching.
You only need to implement methods that you want to change. For example
if you wanted to set the session token for the last read from a replica
you would reimplement the `read_from_replica` method in your resolver
class and implement a method that updates a new timestamp in your
operations class.
2019-01-30 13:37:25 -05:00

196 lines
5.3 KiB
Ruby

# frozen_string_literal: true
#--
# Copyright (c) 2004-2019 David Heinemeier Hansson
#
# Permission is hereby granted, free of charge, to any person obtaining
# a copy of this software and associated documentation files (the
# "Software"), to deal in the Software without restriction, including
# without limitation the rights to use, copy, modify, merge, publish,
# distribute, sublicense, and/or sell copies of the Software, and to
# permit persons to whom the Software is furnished to do so, subject to
# the following conditions:
#
# The above copyright notice and this permission notice shall be
# included in all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
# LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
# OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
# WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
#++
require "active_support"
require "active_support/rails"
require "active_model"
require "arel"
require "yaml"
require "active_record/version"
require "active_model/attribute_set"
module ActiveRecord
extend ActiveSupport::Autoload
autoload :Base
autoload :Callbacks
autoload :Core
autoload :ConnectionHandling
autoload :CounterCache
autoload :DynamicMatchers
autoload :Enum
autoload :InternalMetadata
autoload :Explain
autoload :Inheritance
autoload :Integration
autoload :Migration
autoload :Migrator, "active_record/migration"
autoload :ModelSchema
autoload :NestedAttributes
autoload :NoTouching
autoload :TouchLater
autoload :Persistence
autoload :QueryCache
autoload :Querying
autoload :CollectionCacheKey
autoload :ReadonlyAttributes
autoload :RecordInvalid, "active_record/validations"
autoload :Reflection
autoload :RuntimeRegistry
autoload :Sanitization
autoload :Schema
autoload :SchemaDumper
autoload :SchemaMigration
autoload :Scoping
autoload :Serialization
autoload :StatementCache
autoload :Store
autoload :Suppressor
autoload :Timestamp
autoload :Transactions
autoload :Translation
autoload :Validations
autoload :SecureToken
autoload :DatabaseSelector, "active_record/middleware/database_selector"
eager_autoload do
autoload :ActiveRecordError, "active_record/errors"
autoload :ConnectionNotEstablished, "active_record/errors"
autoload :ConnectionAdapters, "active_record/connection_adapters/abstract_adapter"
autoload :Aggregations
autoload :Associations
autoload :AttributeAssignment
autoload :AttributeMethods
autoload :AutosaveAssociation
autoload :LegacyYamlAdapter
autoload :Relation
autoload :AssociationRelation
autoload :NullRelation
autoload_under "relation" do
autoload :QueryMethods
autoload :FinderMethods
autoload :Calculations
autoload :PredicateBuilder
autoload :SpawnMethods
autoload :Batches
autoload :Delegation
end
autoload :Result
autoload :TableMetadata
autoload :Type
end
module Coders
autoload :YAMLColumn, "active_record/coders/yaml_column"
autoload :JSON, "active_record/coders/json"
end
module AttributeMethods
extend ActiveSupport::Autoload
eager_autoload do
autoload :BeforeTypeCast
autoload :Dirty
autoload :PrimaryKey
autoload :Query
autoload :Read
autoload :TimeZoneConversion
autoload :Write
autoload :Serialization
end
end
module Locking
extend ActiveSupport::Autoload
eager_autoload do
autoload :Optimistic
autoload :Pessimistic
end
end
module ConnectionAdapters
extend ActiveSupport::Autoload
eager_autoload do
autoload :AbstractAdapter
end
end
module Scoping
extend ActiveSupport::Autoload
eager_autoload do
autoload :Named
autoload :Default
end
end
module Middleware
extend ActiveSupport::Autoload
autoload :DatabaseSelector, "active_record/middleware/database_selector"
end
module Tasks
extend ActiveSupport::Autoload
autoload :DatabaseTasks
autoload :SQLiteDatabaseTasks, "active_record/tasks/sqlite_database_tasks"
autoload :MySQLDatabaseTasks, "active_record/tasks/mysql_database_tasks"
autoload :PostgreSQLDatabaseTasks,
"active_record/tasks/postgresql_database_tasks"
end
autoload :TestDatabases, "active_record/test_databases"
autoload :TestFixtures, "active_record/fixtures"
def self.eager_load!
super
ActiveRecord::Locking.eager_load!
ActiveRecord::Scoping.eager_load!
ActiveRecord::Associations.eager_load!
ActiveRecord::AttributeMethods.eager_load!
ActiveRecord::ConnectionAdapters.eager_load!
end
end
ActiveSupport.on_load(:active_record) do
Arel::Table.engine = self
end
ActiveSupport.on_load(:i18n) do
I18n.load_path << File.expand_path("active_record/locale/en.yml", __dir__)
end
YAML.load_tags["!ruby/object:ActiveRecord::AttributeSet"] = "ActiveModel::AttributeSet"
YAML.load_tags["!ruby/object:ActiveRecord::Attribute::FromDatabase"] = "ActiveModel::Attribute::FromDatabase"
YAML.load_tags["!ruby/object:ActiveRecord::LazyAttributeHash"] = "ActiveModel::LazyAttributeHash"