In python 3 you can use decorator @lru_cache from functools module. It stores a result of decorated function inside the cache. Imagine that you have simple flask application:
from flask import Flask, jsonify from functools import lru_cache app = Flask(__name__) @app.route('/') def main(): store_to_cache() return jsonify({'message': 'Stored'}) @lru_cache(maxsize=2) def store_to_cache(): return {'this_goes_to_cache': 'and_this_too'}You enter the root URL and you store dictionary to cache. Cache is setup to have only 2 elements inside. Then you have a helper function for getting info about an object that is inside that cache:
@app.route('/get_cache_info') def get_cache_info(): cache_info = store_to_cache.cache_info() return jsonify({ 'Hits': cache_info.hits, 'Misses': cache_info.misses, 'Maxsize': cache_info.maxsize, 'Currsize': cache_info.currsize })When you run this application in development mode - without gunicorn everything works as expected - you store to cache and receive proper information:
$ curl -X GET http://127.0.0.1:8000 { "message": "Stored" } $ curl -X GET http://127.0.0.1:8000/get_cache_info { "Currsize": 1, "Hits": 0, "Maxsize": 2, "Misses": 1 }Let's run the same code but with using gunicorn with two workers:
$ gunicorn --workers=2 application:app $ curl -X GET http://127.0.0.1:8000 $ curl -X GET http://127.0.0.1:8000/get_cache_info { "Currsize": 1, "Hits": 0, "Maxsize": 2, "Misses": 1 } curl -X GET http://127.0.0.1:8000/get_cache_info { "Currsize": 0, "Hits": 0, "Maxsize": 2, "Misses": 0 }Sometimes request returns that there is one item inside cache and other times that there are no items in the cache. Why is that? Because LRU cache is using cache per worker . It means that when user enters your site cache is stored but it is stored only on this worker! The same user enters another time and his request is handled by the second worker which doesn't have anything stored in the cache!
For this reason, it's not a good idea to use cache per worker in your web application. What can you use instead? Use centrally stored cache like Memcached. You will thank yourself in the future.
That's all for today! Feel free to comment - maybe you have a better idea which cache use to avoid pitfalls?
Example of how LRU cache works is based upon this article .
The code that I have made so far is available on github . Stay tuned for next blog post from this series.
Cover image by Tim Green under CC BY-SA 2.0 .