class DistributedMemoizer # never wait for longer that 1 second for a cross process lock MAX_WAIT = 2 LOCK = Mutex.new # memoize a key across processes and machines def self.memoize(key, duration = 60 * 60 * 24, redis = nil) redis ||= $redis redis_key = self.redis_key(key) unless result = redis.get(redis_key) redis_lock_key = self.redis_lock_key(key) start = Time.new got_lock = false begin while Time.new < start + MAX_WAIT && !got_lock LOCK.synchronize do got_lock = get_lock(redis, redis_lock_key) end sleep 0.001 end unless result = redis.get(redis_key) result = yield redis.setex(redis_key, duration, result) end ensure # NOTE: delete regardless so next one in does not need to wait MAX_WAIT again redis.del(redis_lock_key) end end result end def self.redis_lock_key(key) "memoize_lock_" << key end def self.redis_key(key) "memoize_" << key end protected def self.get_lock(redis, redis_lock_key) redis.watch(redis_lock_key) current = redis.get(redis_lock_key) return false if current unique = SecureRandom.hex result = redis.multi do redis.setex(redis_lock_key, MAX_WAIT, unique) end redis.unwatch result == ["OK"] end end