CacheControl 0.6 (beta)

httplib2 caching for requests

Usage

There are two ways to use CacheControl. The first is by using the CacheControl wrapper object.

import requests

from cachecontrol import CacheControl


sess = requests.session()
cached_sess = CacheControl(sess)

response = cached_sess.get('http://google.com')

If the URL contains any caching based headers, it will cache the result in a simple dictionary.

The second way to use CacheControl is by the CacheControlAdapter. Requests in v1.x introduced the concept of a Transport Adapter. This allows you to configure different behaviors based on the endpoint you are using. For more info check the the docs.

Here is how you can use the adapter:

import requests

from cachecontrol import CacheControlAdapter

sess = requests.session()
sess.mount('http://', CacheControlAdapter())

response = sess.get('http://google.com')

Cache Implementations

CacheControl comes with a simple, threadsafe, in memory dictionary like cache. Implementing a cache interface is simply a matter of inheriting from the BaseCache or implementing the necessary methods.

To make it clear how simple this is, here is the base class:

class BaseCache(object):

    def get(self, key):
        raise NotImplemented()

    def set(self, key, value):
        raise NotImplemented()

    def delete(self, key):
        raise NotImplemented()

See? Simple.

Design

The CacheControl object's main task is to wrap the GET call of the session object. The caching takes place by examining the request to see if it should try to ue the cache. For example, if the request includes a 'no-cache' or 'max-age=0' Cache-Control header, it will not try to cache the request. If there is an cached value and its value has been deemed fresh, the it will return the cached response.

If the request cannot be cached, the actual request is peformed. At this point we then analyze the response and see if we should add it to the cache. For example, if the request contains a 'max-age=3600' in the 'Cache-Control' header, it will cache the response before returning it to the caller.

ETags and If-* Headers

httplib2 handles etags and if-* headers according to Editing the Web. I made an effort to include this functionality in CacheControl, but decided against it. The use of ETags is primarily described in terms of detecting a lost update. As such, even though it uses the data store's cache, it does not impact when and what is stored.

For example, if you wanted to use the cache'd value when doing an update (PUT), you could still use the storage object directly:

import json
import requests

from mycache import CacheStore
from cachecontrol import CacheControl


sess = CacheControl(requests.session(),
                    CacheStore())


url = 'http://host.com/my/file/foo'

# see if it exists
resp = sess.head(url)

# It exists so try to update it
if resp.status == 200:

    do_update = True

    # See if we have an etag of the old content
    old_resp = sess.cache.get(url)
    if old_resp and 'etag' in old_resp.headers:
        headers = {'Content-Type': 'application/json',
                   'expect': '100-continue',
                   'if-match': old_resp.headers['etag']}

        # see if we need to do the update
        resp = sess.put(url, headers=headers)
        if resp.status != 100:
            do_update = False

    if do_update:
        headers = {'Content-Type': 'application/json'}
        data = json.dumps({'foo': 'bar'})
        sess.put(url, headers=headers, data=data)

As you can see the actual decision to use PUT and perform an update is most likely application specific and falls outside the responsibilities of cache management, which is what CacheControl is designed to do.

Tests

The tests are all in cachecontrol/tests and is runnable by py.test.

TODO

• Support the Vary header (only match when all headers are the same)

Disclaimers

CacheControl is brand new and maybe totally broken. I have some tests and it is a pretty direct port of httplib2 caching, which I've found to be very reliable. With that in mind, it hasn't been used in a production environment just yet. If you check it out and find bugs, let me know.