How to cache SQL Alchemy calls with Flask-Cache and Redis?

Question

I have a Flask app that takes parameters from a web form, queries a DB with SQL Alchemy and returns Jinja-generated HTML showing a table with the results. I want to cache the calls to the DB. I looked into Redis (Using redis as an LRU cache for postgres), which led me to http://pythonhosted.org/Flask-Cache/.

Now I am trying to use Redis + Flask-Cache to cache the calls to the DB. Based on the Flask-Cache docs, it seems like I need to set up a custom Redis cache.

class RedisCache(BaseCache):
    def __init__(self, servers, default_timeout=500):
        pass

def redis(app, config, args, kwargs):
   args.append(app.config['REDIS_SERVERS'])
   return RedisCache(*args, **kwargs)

From there I would need to something like:

# not sure what to put for args or kwargs
cache = redis(app, config={'CACHE_TYPE': 'redis'})

app = Flask(__name__)
cache.init_app(app)

I have two questions:

1. What do I put for args and kwargs? What do these mean? How do I set up a Redis cache with Flask-Cache?

2. Once the cache is set up, it seems like I would want to somehow "memoize" the calls the DB so that if the method gets the same query it has the output cached. How do I do this? My best guess would be to wrap the call the SQL Alchemy in a method that could then be given memoize decorator? That way if two identical queries were passed to the method, Flask-Cache would recognize this and return to the appropriate response. I'm guessing that it would look like this:

   @cache.memoize(timeout=50)
   def queryDB(q):
       return q.all()
   

This seems like a fairly common use of Redis + Flask + Flask-Cache + SQL Alchemy, but I am unable to find a complete example to follow. If someone could post one, that would be super helpful -- but for me and for others down the line.

Possession

I encountered the problem that passing a query object as argument yielded memoize using the object's memory address. A wrapping class solved this for. Hope this might help somebody.. – Desiccant

Answer 1

You don't need to create custom RedisCache class. The docs is just teaching how you would create new backends that are not available in flask-cache. But RedisCache is already available in werkzeug >= 0.7, which you might have already installed because it is one of the core dependencies of flask.

This is how I could run the flask-cache with redis backend:

import time
from flask import Flask
from flask_cache import Cache

app = Flask(__name__)
cache = Cache(app, config={'CACHE_TYPE': 'redis'})

@cache.memoize(timeout=60)
def query_db():
    time.sleep(5)
    return "Results from DB"

@app.route('/')
def index():
    return query_db()

app.run(debug=True)

The reason you're getting "ImportError: redis is not a valid FlaskCache backend" is probably because you don't have redis (python library) installed which you can simply install by:
pip install redis.

Headlock

pip install redis in my case :) Remember that you need to configure redis connection like Riz described in his answer. – Calise

Note: Importing flask extension using from flask flaskt.ext.cache import Cache has been deprecated use: from flask_cache import Cache instead. – Lashawn

Answer 2

your redis args would look something like this:

cache = Cache(app, config={
    'CACHE_TYPE': 'redis',
    'CACHE_KEY_PREFIX': 'fcache',
    'CACHE_REDIS_HOST': 'localhost',
    'CACHE_REDIS_PORT': '6379',
    'CACHE_REDIS_URL': 'redis://localhost:6379'
    })

Putting the @cache.memoize over a method that grabs the info from the DB should work.

Tiptoe

If I follow this example, I get "ImportError: redis is not a valid FlaskCache backend" – Possession

If I change CACHE_TYPE to 'redis' I get ERROR:flask_cache:Exception possibly due to cache backend....AttributeError: 'Flask' object has no attribute 'mget' – Possession

Make sure you have the redid python library installed. pip install redis – Tiptoe

How do you set up a Redis host for production (not using localhost as the host)? I'm trying to find a good online guide to get the Redis host set up but can't find anything. – Pubes