Counter

Counting and aggregation library for Rails.

• Counter
  • Main concepts
  • Defining a counter
  • Accessing counter values
  • Anonymous counters
  • Defining a conditional counter
  • Aggregating a value (e.g. sum of order revenue)
  • Recalculating a counter
  • Reset a counter
  • Verify a counter
  • Hooks
  • Testing the counters in production
  • TODO
  • Usage
  • Installation
  • Contributing
  • License

By the time you need Rails counter_caches you probably have other needs too. You probably want to sum column values and you probably have enough throughput that updating a single column value will cause lock contention problems too.

Counter is different from other solutions like Rails counter caches and counter_culture:

• Counters are objects. This makes it possible for them to have an API that allows you to define them, reset, and recalculate them. The definition of a counter is seperate from the value
• Counters are persisted as a ActiveRecord models (not a column of an existing model)
• Incrementing counters can be safely performed in a background job via a change event/deferred reconciliation pattern
• Avoids lock-contention found in other solutions. By storing the value in another object we reduce the contention on the main e.g. User instance. This is only a small improvement though. By using the background change event pattern, we can batch perform the updates reducing the number of processes requiring a lock.
• Counters can also perform aggregation (e.g. sum of column values instead of counting rows)

Main concepts

Counter::Definition defines what the counter is, what model it's connected to, what association it counts, how the count is performed etc. You create a subclass of Counter::Definition and call a few class methods to configure it. The definition is available through counter.definition for any counter value…

Counter::Value is the value of a counter. So, for example, a User might have many Posts, so a User would have a counters association containing a Counter::Value for the number of posts. Counters can be accessed via their name user.posts_counter or via the find_counter method on the association, e.g. user.counters.find_counter PostCounter

Counter::Change is a temporary record that records a change to a counter. Instead of updating a counter directly, which requires obtaining a lock on it to perform it safely and atomically, a new Change event is inserted into the table. On regular intervals, the Counter::Value is updated by incrementing the value by the sum of all outstanding changes. This requires much less frequent locks at the expense of eventual consistency.

For example, you might have many background jobs running concurrently, inserting hundreds/thousands of rows. The would not need to fight for a lock to update the counter and would only need to insert Counter::Change rows. The counter would then be updated, in a single operation, by summing all the persisted change values.

Basically updating a counter value requires this SQL:

UPDATE counter_values
-- Update the counter with the sum of pending changes
SET value = value + changes.sum
FROM (
  -- Find the pending changes for the counter
  SELECT sum(value) as sum
  FROM counter_changes
  WHERE counter_id = 100
) as changes
WHERE id = 100

Or even reconcile all pending counters in a single statement:

UPDATE counter_values
SET value = value + changes.sum
FROM (
  SELECT sum(value)
  FROM counter_changes
  GROUP BY counter_id
) as changes
WHERE counters.id = counter_id

Defining a counter

Counters are defined in a seperate class using a small DSL.

Given a Store with many Orders, it would be defined as…

class OrderCounter < Counter::Definition
  count :orders
end

class Store < ApplicationRecord
  include Counter::Counters

  has_many :orders
  counter OrderCounter
end

First we define the counter class itself using count to specify the association we're counting, then "attach" it to the parent Store model.

By default, the counter will be available as <association>_counter, e.g. store.orders_counter. To customise this, pass a as parameter:

class OrderCounter < Counter::Definition
  include Counter::Counters
  count :orders, as: :total_orders
end

store.total_orders

The counter's value with be stored as a Counter::Value with the name prefixed by the model name. e.g. store_total_orders

Accessing counter values

Since counters are represented as objects, you need to call value on them to retrieve the count.

store.total_orders        #=> Counter::Value
store.total_orders.value  #=> 200

Anonymous counters

Most counters are associated with a model instance and association. These counters are automatically incremented when the associated collection changes but sometimes you just need a global counter that you can increment.

class GlobalOrderCounter < Counter::Definition
  global :my_custom_counter_name
end

GlobalOrderCounter.counter.value #=> 5
GlobalOrderCounter.counter.increment! #=> 6

Defining a conditional counter

Consider this model that we'd like to count but we don't want to count all products, just the premium ones with a price >= 1000

class Product < ApplicationRecord
  include Counter::Counters
  include Counter::Changable

  belongs_to :user

  scope :premium, -> { where("price >= 1000") }

  def premium?
    price >= 1000
  end
end

Here's the counter to do that:

class PremiumProductCounter < Counter::Definition
  # Define the association we're counting
  count :premium_products

  on :create do
    increment_if ->(product) { product.premium? }
  end

  on :delete do
    decrement_if ->(product) { product.premium? }
  end

  on :update do
    increment_if ->(product) {
      product.has_changed? :price, from: ->(price) { price < 1000 }, to: ->(price) { price >= 1000 }
    }

    decrement_if ->(product) {
      product.has_changed? :price, from: ->(price) { price >= 1000 }, to: ->(price) { price < 1000 }
    }
  end
end

There is a lot going on here!

First, we define the counter on a scoped association. This ensures that when we call counter.recalc() we will count using the association's SQL.

We also define several conditions that operate on the instance level, i.e. when we create/update/delete an instance. On create and delete we define a block to determine if the counter should be updated. In this case, we only increment the counter when a premium product is created, and only decrement it when a premium product is deleted.

update is more complex because there are two scenarios: either a product has been updated to make it premium or downgrade from premium to some other state. On update, we increment the counter if the price has gone above 1000; and decrement is the price has now gone below 1000.

We use the has_changed? helper to query the ActiveRecord previous_changes hash and check what has changed. You can specify either Procs or values for from/to. If you only specify a from value, to will default to "any value" (Counter::Any.instance)

Conditional counters work best with a single attribute. If the counter is conditional on e.g. confirmed and subscribed, the update tracking logic becomes very complex especially if the values are both updated at the same time. The solution to this is hopefully Rails generated columns in 7.1 so you can store a "subscribed_and_confirmed" column and check the value of that instead. Rails dirty tracking will need to work with generated columns though; see this PR.

Aggregating a value (e.g. sum of order revenue)

Given an ActiveRecord model Order, we can count a storefront's revenue like so

class Store < ApplicationRecord
  include Counter::Counters

  counter OrderRevenue
end

Define the counter like so

class OrderRevenue < Counter::Definition
  count :orders
  sum :total_price
end

and access it like

  store.orders.create total_price: 100
  store.orders.create total_price: 100
  store.order_revenue.value #=> 200

Recalculating a counter

Counters have a habit of drifting over time, particularly if ActiveRecords hooks aren't run (e.g. with a pure SQL data migration) so you need a method of re-counting the metric. Counters make this easy because they are objects in their own right.

You could refresh a store's revenue stats with:

store.order_revenue.recalc!

this would use the definition of the counter, including any option to sum a column. In the case of conditional counters, they are expected to be attached to an association which matched the conditions so the recalculated count remains accurate.

Reset a counter

You can also reset a counter by calling reset. Since counters are ActiveRecord objects, you could also reset them using

store.order_revenue.reset
Counter::Value.update value: 0

Verify a counter

You might like to check if a counter is correct

store.product_revenue.correct? #=> false

This will re-count / re-calculate the value and compare it to the current one. If you wish to also update the value when it's not correct, use correct!:

store.product_revenue #=>200
store.product_revenue.reset!
store.product_revenue #=>0
store.product_revenue.correct? #=> false
store.product_revenue.correct! #=> false
store.product_revenue #=>200

Hooks

You can add an after_change hook to your counter definition to perform some action when the counter is updated. For example, you might want to send a notification when a counter reaches a certain value.

class OrderRevenueCounter < Counter::Definition
  count :orders, as: :order_revenue
  sum :price

  after_change :send_congratulations_email

  def send_congratulations_email counter, from, to
    return unless from < 1000 && to >= 1000
    send_email "Congratulations! You've made #{to} dollars!"
  end
end

Testing the counters in production

It may be useful to verify the accuracy of the counters in production, especially if you are concerned about conditional counters etc causing counter drift over time.

This form of script takes a sampling approach suitable for large collections. It will randomly select a record and verify that the counter value is correct; if it's not, it stops giving you a chance to investigate.

site_range = Site.minimum(:id)..Site.maximum(:id)

1000.times do
  random_id = rand(site_range)
  site = Site.where("id >= ?", random_id).limit(1).first
  next if site.nil?
  if site.confirmed_subscribers_counter.correct?
    puts "✅ site #{site.id} has correct counter value"
  else
    puts "❌ site #{site.id} has incorrect counter value. Expected #{site.confirmed_subscribers_counter.value} but got #{site.confirmed_subscribers_counter.count_by_sql}"
    break
  end
  sleep 0.1
end

---

TODO

See the asociated project in Github but roughly I'm thinking:

• Hierarchical counters. For example, a Site sends many Newsletters and each Newsletter results in many EmailMessages. Each EmailMessage can be marked as spam. How do you create counters for how many spam emails were sent at the Newsletter level and the Site level?
• Time-based counters for analytics. Instead of a User having one OrderRevenue counter, they would have an OrderRevenue counter for each day. These counters would then be used to produce a chart of their product revenue over the month. Not sure if these are just special counters or something else entirely? Do they use the same ActiveRecord model?
• Can we support floating point values? Sounds useful but don't have a use case for it right now. Would they need to be a different ActiveRecord table?
• In a similar vein of supporting different value types, can we support HLL values? Instead of increment an integer we add the items hash to a HyperLogLog so we can count unique items. An example would be counting site visits in a time-based daily counter, then combine the daily counts and still obtain an estimated number of monthly unique visits. Again, not sure if this is the same ActiveRecord model or something different.
• Actually start running this in production for basic use cases

Usage

No one has used this in production yet.

You probably shouldn't right now unless you're the sort of person that checks if something is poisonous by licking it.

Installation

Add this line to your application's Gemfile:

gem 'counter'

And then execute:

$ bundle

Install the model migrations:

$ rails counter:install:migrations

Contributing

Contribution directions go here.

License

The gem is available as open source under the terms of the MIT License.