ruby 52 lines · 6 steps

Routing heavy reports to a read replica in Rails

A query object and a controller concern push analytical reads onto the reading replica to keep the primary free for writes.

Explained by highlit

Rails Intermediate read-replica database-roles query-object concerns aggregation

1class ReportingQuery
def initialize(scope = ApplicationRecord)
  @scope = scope
end
5 
def on_replica(&block)
  @scope.connected_to(role: :reading) do
    block.call
  end
end
11 
def monthly_revenue(store_id:, month:)
  on_replica do
    Order
      .where(store_id: store_id)
      .where(placed_at: month.all_month)
      .where(status: :fulfilled)
      .group("DATE_TRUNC('day', placed_at)")
      .sum(:total_cents)
  end
end
22 
def top_customers(limit: 25)
  on_replica do
    Customer
      .joins(:orders)
      .select("customers.*, SUM(orders.total_cents) AS lifetime_cents")
      .group("customers.id")
      .order("lifetime_cents DESC")
      .limit(limit)
      .to_a
  end
end
34end
35 
36module ReplicaReporting
extend ActiveSupport::Concern
38 
included do
  around_action :route_reports_to_replica, only: %i[index show export]
end
42 
private
44 
def route_reports_to_replica(&block)
  ActiveRecord::Base.connected_to(role: :reading) do
    ActiveRecord::Base.while_preventing_writes do
      block.call
    end
  end
end
52end

01 / 01

STEP 01

Walkthrough

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Three takeaways

1Isolating expensive aggregate reads on a replica protects the primary database from analytical load.
2Wrapping queries in a single on_replica helper keeps role-switching out of each individual method.
3while_preventing_writes turns replica routing into a safety guarantee, not just a hint.

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