MemoryMiddleware

memory_middleware.rb

class MemoryMiddleware
  ENV_REQUEST_PATH = 'REQUEST_PATH'.freeze
  ENV_PUMA_AFTER_REPLY = 'rack.after_reply'.freeze
  LOGGER = Logger.new('/tmp/memory.log'.freeze)

  def initialize(app)
    @app = app
  end

  def call(env)
    path = env[ENV_REQUEST_PATH]
    return @app.call(env) if asset_request?(path)

    result = nil
    log(path) { result = @app.call(env) }
    schedule_gc(env)
    result
  end

  private

  def asset_request?(path) # w/o nginx in development env it serves assets as well
    path.match(/\.(js|ico|png|gif|css)\z/.freeze)
  end

  def log(path, &block)
    pid = Process.pid
    heap_before = GC.stat[:heap_live_slots] / 1_000
    usage_before = (Integer(`ps -o rss= -p #{pid}`) * 0.001).to_i

    result = block.call

    usage_after = (Integer(`ps -o rss= -p #{pid}`) * 0.001).to_i
    heap_after = GC.stat[:heap_live_slots] / 1_000

    diff_usage = usage_after - usage_before
    diff_heap = heap_after - heap_before
    msg = "#{diff_usage} mb, #{diff_heap} heap – process [#{pid}] #{path} – #{usage_after} mb, #{heap_after} heap"
    LOGGER.info(msg)
    result
  end

  def schedule_gc(env) # in Ruby >= 2.2 GC won't stop the world (incremental)
    return if env[ENV_PUMA_AFTER_REPLY].nil?
    env[ENV_PUMA_AFTER_REPLY] << -> { GC.start }
  end
end