prometheus_decorated_repository.rb

require "benchmark"
require "prometheus/client"

# XXX: I've actually not run this code, and my understanding of the Prometheus
# API for Ruby is from a quick perusal of how things are implemented, so this
# _might_ not actually work. It's meant to be more illustrative than anything,
# so if it's wrong, feel free to walk away with the glee that you understand
# what you're doing.
class PrometheusInstrumentedPostsRepository
  include IPostsRepository

  # @param [IPostsRepository] repository
  # @param [::Prometheus::Client::Counter] request_counter
  # @param [::Prometheus::Client::Histogram] request_histogram
  def initialize(repository, request_counter, request_histogram)
    @repository = repository
    @request_counter = request_counter
    @request_histogram = request_histogram
  end

  # @return [Array<::Domain::Post>]
  def all
    all_posts = []

    @request_histogram.observe(
      Benchmark.realtime do
        all_posts = @repository.all
      end,
      labels: [:all_posts]
    )

    all_posts
  ensure
    @request_counter.increment(labels: [:all_posts])
  end

  # @return [Array<::Domain::Post>]
  def all_published
    all_published = []

    @request_histogram.observe(
      Benchmark.realtime do
        all_published ||= @repository.all_published
      end,
      labels: [:all_published]
    )

    all_published
  ensure
    @request_counter.increment(labels: [:all_published_posts])
  end

  # @param [Integer] post_id
  # @return [::Domain::Post]
  def by_id(post_id)
    by_id = ::Domain::Post.new

    @request_histogram.observe(
      Benchmark.realtime do
        by_id ||= @repository.by_id(post_id)
      end,
      labels: [:by_id]
    )

    by_id
  ensure
    @request_counter.increment(labels: [:by_id])
  end

  # @param [Array<::Domain::Post>] posts
  # @return [Boolean]
  # @raise [PostsRepositoryError]
  def store(*posts)
    result = false

    @request_histogram.observe(
      Benchmark.realtime do
        result ||= @repository.store(*posts)
      end,
      labels: [:store]
    )

    result
  ensure
    @request_counter.increment(labels: [:store])
  end
end

# @return [Integer]
def main
  # ...
  
  posts_repo_req_count = ::Prometheus::Client::Counter.new(
    :posts_repository_requests_total,
    docstring: "...",
    labels: [:repository],
  )

  posts_repo_latency = ::Prometheus::Client::Histogram.new(
    :posts_repository_latency_seconds,
    docstring: "...",
    labels: [:repository]
  )

  repository = ActiveRecordPostsRepository.new(::Models::Post)
  repository ||= PrometheusInstrumentedPostsRepository.new(
    repository,
    posts_repo_req_count,
    posts_repo_latency,
  )
  
  # ...
end