gitlab-org--gitlab-foss/doc/development/utilities.md

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

This doc refers to https://gitlab.com/gitlab-org/gitlab/blob/master/app/models/concerns/reactive_caching.rb.

The ReactiveCaching concern is used for fetching 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.

Examples

class Foo < ApplicationRecord
  include ReactiveCaching

  after_save :clear_reactive_cache!

  def calculate_reactive_cache(param1, param2)
    # Expensive operation here. The return value of this method is cached
  end

  def result
    # Any arguments can be passed to `with_reactive_cache`. `calculate_reactive_cache`
    # will be called with the same arguments.
    with_reactive_cache(param1, param2) 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 10 min.

How it works

The first time #with_reactive_cache is called, a background job is enqueued and with_reactive_cache returns nil. The background job calls #calculate_reactive_cache and stores its return value. It also re-enqueues the background job to run again after reactive_cache_refresh_interval. Therefore, it will keep the stored value up to date. Calculations never run concurrently.

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

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

When to use

• If we need to make a request to an external API (for example, requests to the k8s API). It is not advisable to keep the application server worker blocked for the duration of the external request.
• If a model needs to perform a lot of database calls or other time consuming calculations.

How to use

In models and services

The ReactiveCaching concern can be used in models as well as project_services (app/models/project_services).

1. Include the concern in your model or service.

   When including in a model:

   include ReactiveCaching
   

   or when including in a project_service:

   include ReactiveService
   

2. Implement the calculate_reactive_cache method in your model/service.

3. Call with_reactive_cache in your model/service where the cached value is needed.

In controllers

Controller endpoints that call a model or service method that uses ReactiveCaching should not wait until the background worker completes.

• An API that calls a model or service method that uses ReactiveCaching should return 202 accepted when the cache is being calculated (when #with_reactive_cache returns nil).
• It should also set the polling interval header with Gitlab::PollingInterval.set_header.
• The consumer of the API is expected to poll the API.
• You can also consider implementing ETag caching to reduce the server load caused by polling.

Methods to implement in a model or service

These are methods that should be implemented in the model/service that includes ReactiveCaching.

#calculate_reactive_cache (required)

• This method must be implemented. Its return value will be cached.
• It will be called by ReactiveCaching when it needs to populate the cache.
• Any arguments passed to with_reactive_cache will also be passed to calculate_reactive_cache.

#reactive_cache_updated (optional)

• This method can be implemented if needed.
• It is called by the ReactiveCaching concern whenever the cache is updated. If the cache is being refreshed and the new cache value is the same as the old cache value, this method will not be called. It is only called if a new value is stored in the cache.
• It can be used to perform an action whenever the cache is updated.

Methods called by a model or service

These are methods provided by ReactiveCaching and should be called in the model/service.

#with_reactive_cache (required)

• with_reactive_cache must be called where the result of calculate_reactive_cache is required.

• A block can be given to with_reactive_cache. with_reactive_cache can also take any number of arguments. Any arguments passed to with_reactive_cache will be passed to calculate_reactive_cache. The arguments passed to with_reactive_cache will be appended to the cache key name.

• If with_reactive_cache is called when the result has already been cached, the block will be called, yielding the cached value and the return value of the block will be returned by with_reactive_cache. It will also reset the timeout of the cache to the reactive_cache_lifetime value.

• If the result has not been cached as yet, with_reactive_cache will return nil. It will also enqueue a background job, which will call calculate_reactive_cache and cache the result.

• Once the background job has completed and the result is cached, the next call to with_reactive_cache will pick up the cached value.

• In the example below, data is the cached value which is yielded to the block given to with_reactive_cache.

  class Foo < ApplicationRecord
    include ReactiveCaching
  
    def calculate_reactive_cache(param1, param2)
      # Expensive operation here. The return value of this method is cached
    end
  
    def result
      with_reactive_cache(param1, param2) do |data|
        # ...
      end
    end
  end
  

#clear_reactive_cache! (optional)

• This method can be called when the cache needs to be expired/cleared. For example, it can be called in an after_save callback in a model so that the cache is cleared after the model is modified.
• This method should be called with the same parameters that are passed to with_reactive_cache because the parameters are part of the cache key.

#without_reactive_cache (optional)

• This is a convenience method that can be used for debugging purposes.
• This method calls calculate_reactive_cache in the current process instead of in a background worker.

Configurable options

There are some class_attribute options which can be tweaked.

self.reactive_cache_key

• The value of this attribute is the prefix to the data and alive cache key names. The parameters passed to with_reactive_cache form the rest of the cache key names.

• By default, this key uses the model's name and the ID of the record.

  self.reactive_cache_key = -> (record) { [model_name.singular, record.id] }
  

• The data and alive cache keys in this case will be "ExampleModel:1:arg1:arg2" and "ExampleModel:1:arg1:arg2:alive" respectively, where ExampleModel is the name of the model, 1 is the ID of the record, arg1 and arg2 are parameters passed to with_reactive_cache.

• If you're including this concern in a service instead, you will need to override the default by adding the following to your service:

  self.reactive_cache_key = ->(service) { [service.class.model_name.singular, service.project_id] }
  

  If your reactive_cache_key is exactly like the above, you can use the existing ReactiveService concern instead.

self.reactive_cache_lease_timeout

• ReactiveCaching uses Gitlab::ExclusiveLease to ensure that the cache calculation is never run concurrently by multiple workers.
• This attribute is the timeout for the Gitlab::ExclusiveLease.
• It defaults to 2 minutes, but can be overriden if a different timeout is required.

self.reactive_cache_lease_timeout = 2.minutes

self.reactive_cache_refresh_interval

• This is the interval at which the cache is refreshed.
• It defaults to 1 minute.

self.reactive_cache_lease_timeout = 1.minute

self.reactive_cache_lifetime

• This is the duration after which the cache will be cleared if there are no requests.
• The default is 10 minutes. If there are no requests for this cache value for 10 minutes, the cache will expire.
• If the cache value is requested before it expires, the timeout of the cache will be reset to reactive_cache_lifetime.

self.reactive_cache_lifetime = 10.minutes

self.reactive_cache_worker_finder

• This is the method used by the background worker to find or generate the object on which calculate_reactive_cache can be called.

• By default it uses the model primary key to find the object:

  self.reactive_cache_worker_finder = ->(id, *_args) do
    find_by(primary_key => id)
  end
  

• The default behaviour can be overridden by defining a custom reactive_cache_worker_finder.

  class Foo < ApplicationRecord
    include ReactiveCaching
  
    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(var1, var2)
      # Expensive operation here. The return value of this method is cached
    end
  
    def result
      with_reactive_cache("bar1", "bar2") do |data|
        # ...
      end
    end
  end
  

  • In this example, the primary key ID will be passed to reactive_cache_worker_finder along with the parameters passed to with_reactive_cache.
  • The custom reactive_cache_worker_finder calls .from_cache with the parameters passed to with_reactive_cache.