title: find a data type suitable for my cache
body: <p>i want to cache my object in the memory for performance,but also need to recycle them if it's too long not accessed. </p>
  
  <p>every 30 minutes a function would be called , check every cache object , like a file's attributes in windows operation system, each object should have two attributes:last modified time stamp(or a flag) and last accessed time stamp, if the last modified time is great than zero, create a sql for update them in datebase ,and reset the modified time to 0, if the last accessed time  is larger than 30 minutes, then delete them from the cache system.</p>
  
  <p>What is the best way to implement them?and is there already a similar system in python so i don't have to reinvent the wheel.</p>
  
  <p>ps. no Memcached. the object should be accessed directly, no serialization and deserialization .</p>