discourse/app/models/concerns/cached_counting.rb

# frozen_string_literal: true

module CachedCounting
  extend ActiveSupport::Concern

  EXPIRE_CACHE_AFTER = 4.days.to_i

  LUA_INCR_AND_EXPIRE = DiscourseRedis::EvalHelper.new <<~LUA
    local result = redis.call("INCR", KEYS[1])

    if result == 1 then
      redis.call("EXPIRE", KEYS[1], ARGV[1])
    end

    return result
  LUA

  included do
    class << self
      attr_accessor :autoflush, :autoflush_seconds, :last_flush
    end

    # auto flush if backlog is larger than this
    self.autoflush = 2000

    # auto flush if older than this
    self.autoflush_seconds = 5.minutes

    self.last_flush = Time.now.utc
  end

  class_methods do
    def perform_increment!(key, opts = nil)
      val = DiscourseRedis.ignore_readonly do
        LUA_INCR_AND_EXPIRE.eval(
          Discourse.redis.without_namespace,
          [Discourse.redis.namespace_key(key)],
          [EXPIRE_CACHE_AFTER]
        ).to_i
      end

      # readonly mode it is going to be nil, skip
      return if val.nil?

      autoflush = (opts && opts[:autoflush]) || self.autoflush
      if autoflush > 0 && val >= autoflush
        write_cache!
        return
      end

      if (Time.now.utc - last_flush).to_i > autoflush_seconds
        write_cache!
      end
    end

    def write_cache!(date = nil)
      raise NotImplementedError
    end

    # this may seem a bit fancy but in so it allows
    # for concurrent calls without double counting
    def get_and_reset(key)
      Discourse.redis.set(key, '0', ex: EXPIRE_CACHE_AFTER, get: true).to_i
    end

    def request_id(query_params, retries = 0)
      id = where(query_params).pluck_first(:id)
      id ||= create!(query_params.merge(count: 0)).id
    rescue # primary key violation
      if retries == 0
        request_id(query_params, 1)
      else
        raise
      end
    end
  end
end