Hey folks, I’d appreciate a sanity check – I feel like what we need is achievable, but I’m missing something along the way.
Our database has been horizontally sharded for close to a decade, and in order to accomplish that we’ve been monkeypatching ActiveRecord in a bunch of ways. With Rails shards, I’d like to finally remove that monkeypatch and come back to a convention – it will allow us to implement better caching, connection pooling, etc etc.
We have a database architecture that resembles this fictional list of databases and tables:
- National Database
- Table: Presidents (name: String)
- Geographical Database Shards
- Table: Students (name: String)
- US East Shard
- US West Shard
I’ve mirrored that setup in a dummy Rails app like this:
# database.yml
default: &default
adapter: postgresql
pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
timeout: 5000
development:
national:
<<: *default
database: national
migrations_paths: db/national_migrations
students_us_east:
<<: *default
database: students_us_east
migrations_paths: db/geographical_migrations
students_us_west:
<<: *default
database: students_us_west
migrations_paths: db/geographical_migrations
# models/application_record.rb
class ApplicationRecord < ActiveRecord::Base
primary_abstract_class
end
# models/national_record.rb
class NationalRecord < ApplicationRecord
self.abstract_class = true
connects_to database: { writing: :national }
end
# models/geographical_record.rb
class GeographicalRecord < ApplicationRecord
self.abstract_class = true
connects_to shards: {
us_east: { writing: :students_us_east },
us_west: { writing: :students_us_west }
}
end
# models/student.rb
class Student < GeographicalRecord
end
# models/president.rb
class President < NationalRecord
end
Now, I would think I could create and query records from the three databases, like this:
> rails db:setup db:migrate
Running via Spring preloader in process 82253
Created database 'national'
Created database 'students_us_east'
Created database 'students_us_west'
> rails c
Running via Spring preloader in process 95494
Loading development environment (Rails 7.2.1.1)
vanilla-rails-app(dev)* ActiveRecord::Base.connected_to(shard: :us_east, role: :writer) do
vanilla-rails-app(dev)* Student.create(name: "bobby tables")
vanilla-rails-app(dev)* puts Student.first.name
vanilla-rails-app(dev)> end
But alas, I’ve done something wrong. The error message I get is:
/.asdf/installs/ruby/3.3.0/lib/ruby/gems/3.3.0/gems/irb-1.14.1/lib/irb.rb:1260:in `full_message’: No connection pool for ‘GeographicalRecord’ found for the ‘us_east’ shard. (ActiveRecord::ConnectionNotEstablished)
Here’s what I’ve tried so far:
-
GeographicalRecord.connected_to(shard: :us_east, role: :writer) do(as above) -
`ActiveRecord::Base.connected_to(…) (calling connected_to on ActiveRecord::Base instead of my own parent class)
-
I’ve read and inserted debugger statements in a ton of the connection lookup and handling code for active_record, and I can see a lot of what’s going on but I’m still missing something.
-
Attempted to solve my problem by re-creating a dedicated brand new application and building everything with generators:
rails new vanilla_rails_app --database=postgresql- Edit database.yml to above
rails generate scaffold presidents name --database nationalrails generate scaffold students name --database=students_us_east --parent=GeographicalRecord- The generator kind of fell over here, not knowing what to do with students_us_east and GeographicalRecord. I edited files to match above.
I summarised what I found in