Guides.rubyonrails.org

Chapters

1. Setting up your application
2. Migrations
3. Activating automatic connection switching
4. Using manual connection switching
5. Caveats
   • Sharding
   • Load Balancing Replicas
   • Joining Across Databases
   • Schema Cache

As an application grows in popularity and usage you'll need to scale the application to support your new users and their data. One way in which your application may need to scale is on the database level. Rails now has support for multiple databases so you don't have to store your data all in one place.

At this time the following features are supported:

• Multiple primary databases and a replica for each
• Automatic connection switching for the model you're working with
• Automatic swapping between the primary and replica depending on the HTTP verb and recent writes
• Rails tasks for creating, dropping, migrating, and interacting with the multiple databases

The following features are not (yet) supported:

• Sharding
• Joining across clusters
• Load balancing replicas
• Dumping schema caches for multiple databases

1 Setting up your application

While Rails tries to do most of the work for you there are still some steps you'll need to do to get your application ready for multiple databases.

Let's say we have an application with a single primary database and we need to add a new database for some new tables we're adding. The name of the new database will be "animals".

The database.yml looks like this:

production:
  database: my_primary_database
  user: root
  adapter: mysql

Let's add a replica for the primary, a new writer called animals and a replica for that as well. To do this we need to change our database.yml from a 2-tier to a 3-tier config.

production:
  primary:
    database: my_primary_database
    user: root
    adapter: mysql
  primary_replica:
    database: my_primary_database
    user: root_readonly
    adapter: mysql
    replica: true
  animals:
    database: my_animals_database
    user: animals_root
    adapter: mysql
    migrations_paths: db/animals_migrate
  animals_replica:
    database: my_animals_database
    user: animals_readonly
    adapter: mysql
    replica: true

When using multiple databases there are a few important settings.

First, the database name for the primary and replica should be the same because they contain the same data. Second, the username for the primary and replica should be different, and the replica user's permissions should be to read and not write.

When using a replica database you need to add a replica: true entry to the replica in the database.yml. This is because Rails otherwise has no way of knowing which one is a replica and which one is the primary.

Lastly, for new primary databases you need to set the migrations_paths to the directory where you will store migrations for that database. We'll look more at migrations_paths later on in this guide.

Now that we have a new database, let's set up the model. In order to use the new database we need to create a new abstract class and connect to the animals databases.

class AnimalsBase < ApplicationRecord
  self.abstract_class = true

  connects_to database: { writing: :animals, reading: :animals_replica }
end

Then we need to update ApplicationRecord to be aware of our new replica.

class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true

  connects_to database: { writing: :primary, reading: :primary_replica }
end

By default Rails expects the database roles to be writing and reading for the primary and replica respectively. If you have a legacy system you may already have roles set up that you don't want to change. In that case you can set a new role name in your application config.

config.active_record.writing_role = :default
config.active_record.reading_role = :readonly

It's important to connect to your database in a single model and then inherit from that model for the tables rather than connect multiple individual models to the same database. Database clients have a limit to the number of open connections there can be and if you do this it will multiply the number of connections you have since Rails uses the model class name for the connection specification name.

Now that we have the database.yml and the new model set up it's time to create the databases. Rails 6.0 ships with all the rails tasks you need to use multiple databases in Rails.

You can run rails -T to see all the commands you're able to run. You should see the following:

$ rails -T
rails db:create                          # Creates the database from DATABASE_URL or config/database.yml for the ...
rails db:create:animals                  # Create animals database for current environment
rails db:create:primary                  # Create primary database for current environment
rails db:drop                            # Drops the database from DATABASE_URL or config/database.yml for the cu...
rails db:drop:animals                    # Drop animals database for current environment
rails db:drop:primary                    # Drop primary database for current environment
rails db:migrate                         # Migrate the database (options: VERSION=x, VERBOSE=false, SCOPE=blog)
rails db:migrate:animals                 # Migrate animals database for current environment
rails db:migrate:primary                 # Migrate primary database for current environment
rails db:migrate:status                  # Display status of migrations
rails db:migrate:status:animals          # Display status of migrations for animals database
rails db:migrate:status:primary          # Display status of migrations for primary database

Running a command like rails db:create will create both the primary and animals databases. Note that there is no command for creating the users and you'll need to do that manually to support the readonly users for your replicas. If you want to create just the animals database you can run rails db:create:animals.

2 Migrations

Migrations for multiple databases should live in their own folders prefixed with the name of the database key in the configuration.

You also need to set the migrations_paths in the database configurations to tell Rails where to find the migrations.

For example the animals database would look in the db/animals_migrate directory and primary would look in db/migrate. Rails generators now take a --database option so that the file is generated in the correct directory. The command can be run like so:

$ rails g migration CreateDogs name:string --database animals

3 Activating automatic connection switching

Finally, in order to use the read-only replica in your application you'll need to activate the middleware for automatic switching.

Automatic switching allows the application to switch from the primary to replica or replica to primary based on the HTTP verb and whether there was a recent write.

If the application is receiving a POST, PUT, DELETE, or PATCH request the application will automatically write to the primary. For the specified time after the write the application will read from the primary. For a GET or HEAD request the application will read from the replica unless there was a recent write.

To activate the automatic connection switching middleware, add or uncomment the following lines in your application config.

config.active_record.database_selector = { delay: 2.seconds }
config.active_record.database_resolver = ActiveRecord::Middleware::DatabaseSelector::Resolver
config.active_record.database_resolver_context = ActiveRecord::Middleware::DatabaseSelector::Resolver::Session

Rails guarantees "read your own write" and will send your GET or HEAD request to the primary if it's within the delay window. By default the delay is set to 2 seconds. You should change this based on your database infrastructure. Rails doesn't guarantee "read a recent write" for other users within the delay window and will send GET and HEAD requests to the replicas unless they wrote recently.

The automatic connection switching in Rails is relatively primitive and deliberately doesn't do a whole lot. The goal was a system that demonstrated how to do automatic connection switching that was flexible enough to be customizable by app developers.

The setup in Rails allows you to easily change how the switching is done and what parameters it's based on. Let's say you want to use a cookie instead of a session to decide when to swap connections. You can write your own class:

class MyCookieResolver
  # code for your cookie class
end

And then pass it to the middleware:

config.active_record.database_selector = { delay: 2.seconds }
config.active_record.database_resolver = ActiveRecord::Middleware::DatabaseSelector::Resolver
config.active_record.database_resolver_context = MyCookieResolver

4 Using manual connection switching

There are some cases where you may want your application to connect to a primary or a replica and the automatic connection switching isn't adequate. For example, you may know that for a particular request you always want to send the request to a replica, even when you are in a POST request path.

To do this Rails provides a connected_to method that will switch to the connection you need.

ActiveRecord::Base.connected_to(role: :reading) do
  # all code in this block will be connected to the reading role
end

The "role" in the connected_to call looks up the connections that are connected on that connection handler (or role). The reading connection handler will hold all the connections that were connected via connects_to with the role name of reading.

Note that connected_to with a role will look up an existing connection and switch using the connection specification name. This means that if you pass an unknown role like connected_to(role: :nonexistent) you will get an error that says ActiveRecord::ConnectionNotEstablished (No connection pool with 'AnimalsBase' found for the 'nonexistent' role.)

5 Caveats

5.1 Sharding

As noted at the top, Rails doesn't (yet) support sharding. We had to do a lot of work to support multiple databases for Rails 6.0. The lack of support for sharding isn't an oversight, but does require additional work that didn't make it in for 6.0. For now if you need sharding it may be advisable to continue using one of the many gems that supports this.

5.2 Load Balancing Replicas

Rails also doesn't support automatic load balancing of replicas. This is very dependent on your infrastructure. We may implement basic, primitive load balancing in the future, but for an application at scale this should be something your application handles outside of Rails.

5.3 Joining Across Databases

Applications cannot join across databases. Rails 6.1 will support using has_many relationships and creating 2 queries instead of joining, but Rails 6.0 will require you to split the joins into 2 selects manually.

5.4 Schema Cache

If you use a schema cache and multiple databases you'll need to write an initializer that loads the schema cache from your app. This wasn't an issue we could resolve in time for Rails 6.0 but hope to have it in a future version soon.

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