Final version of the producer. It can (and does) crash but it's under management of Systemd which restarts it after 5 minutes. The only fanciness is the variable sleep time.

gistfile1.txt

require "dotenv"
require "redis"
require "db"
require "pg"
require "log"
 
module Producer 
  VERSION = "0.1.0" 
  Dotenv.load? "env" 
  Log.setup_from_env(default_level: :debug) 
  Log.info { "Producer started" } 
 
  redis = Redis.new(url: ENV["REDIS"]) 
  pg = DB.open(ENV["POSTGRES"]) 
 
  begin 
    loop do
      server_ids = [] of Int64
      pg.query("SELECT id FROM servers WHERE last_checked_at < now() - interval '1 hour' OR last_checked_at IS NULL") do |rs|
        rs.each do
          server_ids.push(rs.read(Int64))
        end
      end
      Log.debug &.emit("server_ids", server_ids: server_ids)
      redis.rpush("hosts", server_ids)
      closest_to_due = pg.query_one?("SELECT last_checked_at FROM servers WHERE last_checked_at > now() - interval '1 hour' ORDER BY last_ch
      if closest_to_due.nil?
         closest_to_due = Time.utc - 1.hour
      end
      period_to_sleep = Time.utc - (closest_to_due - 10.seconds)
      Log.info &.emit("Sleeping", sleep: period_to_sleep.to_s)
      sleep period_to_sleep
    end
  ensure
    redis.close
    pg.close
  end
end