GitLab utilities

We have developed a number of utilities to help ease development:

MergeHash

Refer to: https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/utils/merge_hash.rb:

  • Deep merges an array of hashes:

    Gitlab::Utils::MergeHash.merge(
      [{ hello: ["world"] },
       { hello: "Everyone" },
       { hello: { greetings: ['Bonjour', 'Hello', 'Hallo', 'Dzien dobry'] } },
        "Goodbye", "Hallo"]
    )
    

    Gives:

    [
      {
        hello:
          [
            "world",
            "Everyone",
            { greetings: ['Bonjour', 'Hello', 'Hallo', 'Dzien dobry'] }
          ]
      },
      "Goodbye"
    ]
    
  • Extracts all keys and values from a hash into an array:

    Gitlab::Utils::MergeHash.crush(
      { hello: "world", this: { crushes: ["an entire", "hash"] } }
    )
    

    Gives:

    [:hello, "world", :this, :crushes, "an entire", "hash"]
    

Override

Refer to https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/utils/override.rb:

  • This utility can help you check if one method would override another or not. It is the same concept as Java’s @Override annotation or Scala’s override keyword. However, we only run this check when ENV['STATIC_VERIFICATION'] is set to avoid production runtime overhead. This is useful for checking:

    • If you have typos in overriding methods.
    • If you renamed the overridden methods, which make the original override methods irrelevant.

      Here’s a simple example:

      class Base
        def execute
        end
      end
      
      class Derived < Base
        extend ::Gitlab::Utils::Override
      
        override :execute # Override check happens here
        def execute
        end
      end
      

      This also works on modules:

      module Extension
        extend ::Gitlab::Utils::Override
      
        override :execute # Modules do not check this immediately
        def execute
        end
      end
      
      class Derived < Base
        prepend Extension # Override check happens here, not in the module
      end
      

      Note that the check will only happen when either:

      • The overriding method is defined in a class, or:
      • The overriding method is defined in a module, and it’s prepended to a class or a module.

      Because only a class or prepended module can actually override a method. Including or extending a module into another cannot override anything.

StrongMemoize

Refer to https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/utils/strong_memoize.rb:

  • Memoize the value even if it is nil or false.

    We often do @value ||= compute. However, this doesn’t work well if compute might eventually give nil and you don’t want to compute again. Instead you could use defined? to check if the value is set or not. It’s tedious to write such pattern, and StrongMemoize would help you use such pattern.

    Instead of writing patterns like this:

    class Find
      def result
        return @result if defined?(@result)
    
        @result = search
      end
    end
    

    You could write it like:

    class Find
      include Gitlab::Utils::StrongMemoize
    
      def result
        strong_memoize(:result) do
          search
        end
      end
    end
    
  • Clear memoization

    class Find
      include Gitlab::Utils::StrongMemoize
    end
    
    Find.new.clear_memoization(:result)
    

RequestCache

Refer to https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/cache/request_cache.rb.

This module provides a simple way to cache values in RequestStore, and the cache key would be based on the class name, method name, optionally customized instance level values, optionally customized method level values, and optional method arguments.

A simple example that only uses the instance level customised values is:

class UserAccess
  extend Gitlab::Cache::RequestCache

  request_cache_key do
    [user&.id, project&.id]
  end

  request_cache def can_push_to_branch?(ref)
    # ...
  end
end

This way, the result of can_push_to_branch? would be cached in RequestStore.store based on the cache key. If RequestStore is not currently active, then it would be stored in a hash, and saved in an instance variable so the cache logic would be the same.

We can also set different strategies for different methods:

class Commit
  extend Gitlab::Cache::RequestCache

  def author
    User.find_by_any_email(author_email)
  end
  request_cache(:author) { author_email }
end

ReactiveCaching

The ReactiveCaching concern is used to fetch some data in the background and store it in the Rails cache, keeping it up-to-date for as long as it is being requested. If the data hasn’t been requested for reactive_cache_lifetime, it will stop being refreshed, and then be removed.

Example of use:

class Foo < ApplicationRecord
  include ReactiveCaching

  after_save :clear_reactive_cache!

  def calculate_reactive_cache
    # Expensive operation here. The return value of this method is cached
  end

  def result
    with_reactive_cache do |data|
      # ...
    end
  end
end

In this example, the first time #result is called, it will return nil. However, it will enqueue a background worker to call #calculate_reactive_cache and set an initial cache lifetime of ten minutes.

The background worker needs to find or generate the object on which with_reactive_cache was called. The default behaviour can be overridden by defining a custom reactive_cache_worker_finder. Otherwise, the background worker will use the class name and primary key to get the object using the ActiveRecord find_by method.

class Bar
  include ReactiveCaching

  self.reactive_cache_key = ->() { ["bar", "thing"] }
  self.reactive_cache_worker_finder = ->(_id, *args) { from_cache(*args) }

  def self.from_cache(var1, var2)
    # This method will be called by the background worker with "bar1" and
    # "bar2" as arguments.
    new(var1, var2)
  end

  def initialize(var1, var2)
    # ...
  end

  def calculate_reactive_cache
    # Expensive operation here. The return value of this method is cached
  end

  def result
    with_reactive_cache("bar1", "bar2") do |data|
      # ...
    end
  end
end

Each time the background job completes, it stores the return value of #calculate_reactive_cache. It is also re-enqueued to run again after reactive_cache_refresh_interval, therefore, it will keep the stored value up to date. Calculations are never run concurrently.

Calling #result while a value is cached will call the block given to #with_reactive_cache, yielding the cached value. It will also extend the lifetime by the reactive_cache_lifetime value.

Once the lifetime has expired, no more background jobs will be enqueued and calling #result will again return nil - starting the process all over again.