reactive_caching.md

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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.

1 MB hard limit

ReactiveCaching has a 1 megabyte default limit. This value is configurable.

If the data we're trying to cache has over 1 megabyte, it will not be cached and a handled ReactiveCaching::ExceededReactiveCacheLimit will be notified on Sentry.

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.

4. Set the reactive_cache_work_type accordingly.

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 overridden 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_hard_limit

• This is the maximum data size that ReactiveCaching allows to be cached.
• The default is 1 megabyte. Data that goes over this value will not be cached and will silently raise ReactiveCaching::ExceededReactiveCacheLimit on Sentry.

self.reactive_cache_hard_limit = 5.megabytes

self.reactive_cache_work_type

• This is the type of work performed by the calculate_reactive_cache method. Based on this attribute, it's able to pick the right worker to process the caching job. Make sure to set it as :external_dependency if the work performs any external request (e.g. Kubernetes, Sentry); otherwise set it to :no_dependency.

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 behavior 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.