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Caching

Inevitably, there will be occasions during applications development or deployment when some task is revealed to be taking a significant amount of time to complete. When this occurs, the best way to speed things up is with caching.

Caching is enabled in Pylons using Beaker, the same package that provides session handling. Beaker supports a variety of caching backends: in-memory, database, Google Datastore, filesystem, and memcached. Additional extensions are available that support Tokyo Cabinet, Redis, Dynomite, and Ringo. Back-ends can be added with Beaker’s extension system.

Types of Caching

Pylons offers a variety of caching options depending on the granularity of caching desired. Fine-grained caching down to specific sub-sections of a template, arbitrary Python functions, all the way up to entire controller actions and browser-side full-page caching are available.

Available caching options (ordered by granularity, least to most specific):

  • Browser-side - HTTP/1.1 supports the ETag caching system that allows the browser to use its own cache instead of requiring regeneration of the entire page. ETag-based caching avoids repeated generation of content but if the browser has never seen the page before, the page will still be generated. Therefore using ETag caching in conjunction with one of the other types of caching listed here will achieve optimal throughput and avoid unnecessary calls on resource-intensive operations.
  • Controller Actions - A Pylons controller action can have its entire result cached, including response headers if desired.
  • Templates - The results of an entire rendered template can be cached using the 3 cache keyword arguments to the render calls. These render commands can also be used inside templates.
  • Arbitrary Functions - Any function can be independently cached using Beaker’s cache decorators. This allows fine-grained caching of just the parts of the code that can be cached.
  • Template Fragments - Built-in caching options are available for both Mako and Myghty template engines. They allow fine-grained caching of only certain sections of the template. This is also sometimes called fragment caching since individual fragments of a page can be cached.

Namespaces and Keys

Beaker is used for caching arbitrary Python functions, template results, and in Mako for caching individual <def> blocks. Browser-side caching does not utilize Beaker.

The two primary concepts to bear in mind when caching with Beaker are:

  1. Caches have a namespace, this is to organize a cache such that variations of the same thing being cached are associated under a single place.
  2. Variations of something being cached, are keys which are under that namespace.

For example, if we want to cache a function, the function name along with a unique name for it would be considered the namespace. The arguments it takes to differentiate the output to cache, are the keys.

An example of caching with the cache_region() decorator:

@cache_region('short_term', 'search_func')
def get_results(search_param):
    # do something to retrieve data
    data = get_data(search_param)
    return data

results = get_results('gophers')

In this example, the namespace will be the function name + module + ‘search_func’. Since a single module might have multiple methods of the same name you wish to cache, the cache_region() decorator takes another argument in addition to the region to use, which is added to the namespace.

The key in this example is the search_param value. For each value of it, a separate result will be cached.

See also

Stephen Pierzchala’s Caching for Performance (stephen@pierzchala.com) Beaker Caching Docs

Configuring

Beaker‘s cache options can be easily configured in the project’s INI file. Beaker’s configuration documentation explains how to setup the most common options.

The cache options specified will be used in the absence of more specific keyword arguments to individual cache functions. Functions that support Cache Regions will use the settings for that region.

Cache Regions

Cache regions are named groupings of related options. For example, in many web applications, there might be a few cache strategies used in a company, with short-term cached objects ending up in Memcached, and longer-term cached objects stored in the filesystem or a database.

Using cache regions makes it easy to declare the cache strategies in one place, then use them throughout the application by referencing the cache strategy name.

Cache regions should be setup in the development.ini file, but can also be configured and passed directly into the CacheManager instance that is created in the lib/app_globals.py file.

Example INI section for two cache regions (put these under your [app:main] section):

beaker.cache.regions = short_term, long_term
beaker.cache.short_term.type = ext:memcached
beaker.cache.short_term.url = 127.0.0.1:11211
beaker.cache.short_term.expire = 3600

beaker.cache.long_term.type = ext:database
beaker.cache.long_term.url = mysql://dbuser:dbpass@127.0.0.1/cache_db
beaker.cache.long_term.expire = 86400

This sets up two cache regions, short_term and long_term.

Browser-Side

Browser-side caching can utilize one of several methods. The entire page can have cache headers associated with it to indicate to the browser that it should be cached. Or, using the ETag Cache header, a page can have more fine-grained caching rules applied.

Cache Headers

Cache headers may be set directly on the Response object by setting the headers directly using the headers() property, or by using the cache header helpers.

To ensure pages aren’t accidentally cached in dynamic web applications, Pylons default behavior sets the Pragma and Cache-Control headers to no-cache. Before setting cache headers, these default values should be cleared.

Clearing the default no-cache response headers:

class SampleController(BaseController):
    def index(self):
        # Clear the default cache headers
        del response.headers['Cache-Control']
        del response.headers['Pragma']

        return render('/index.html)

Using the response cache helpers:

# Set an action response to expires in 30 seconds
class SampleController(BaseController):
    def index(self):
        # Clear the default cache headers
        del response.headers['Cache-Control']
        del response.headers['Pragma']

        response.cache_expires(seconds=30)
        return render('/index.html')

# Set the cache-control to private with a max-age of 30 seconds
class SampleController(BaseController):
    def index(self):
        # Clear the default cache headers
        del response.headers['Cache-Control']
        del response.headers['Pragma']

        response.cache_control = {'max-age': 30, 'public': True}
        return render('/index.html')

All of the values that can be passed to the cache_control property dict, also may be passed into the cache_expires function call. It’s recommended that you use the cache_expires helper as it also sets the Last-Modified and Expires headers to the second interval as well.

E-Tag Caching

Caching via ETag involves sending the browser an ETag header so that it knows to save and possibly use a cached copy of the page from its own cache, instead of requesting the application to send a fresh copy.

Because the ETag cache relies on sending headers to the browser, it works in a slightly different manner to the other caching mechanisms.

The etag_cache() function will set the proper HTTP headers if the browser doesn’t yet have a copy of the page. Otherwise, a 304 HTTP Exception will be thrown that is then caught by Paste middleware and turned into a proper 304 response to the browser. This will cause the browser to use its own locally-cached copy.

ETag-based caching requires a single key which is sent in the ETag HTTP header back to the browser. The RFC specification for HTTP headers indicates that an ETag header merely needs to be a string. This value of this string does not need to be unique for every URL as the browser itself determines whether to use its own copy, this decision is based on the URL and the ETag key.

def my_action(self):
    etag_cache('somekey')
    return render('/show.myt', cache_expire=3600)

Or to change other aspects of the response:

def my_action(self):
    etag_cache('somekey')
    response.headers['content-type'] = 'text/plain'
    return render('/show.myt')

The frequency with which an ETag cache key is changed will depend on the web application and the developer’s assessment of how often the browser should be prompted to fetch a fresh copy of the page.

Controller Actions

The beaker_cache() decorator is for caching the results of a complete controller action.

Example:

from pylons.decorators.cache import beaker_cache

class SampleController(BaseController):

    # Cache this controller action forever (until the cache dir is
    # cleaned)
    @beaker_cache()
    def home(self):
        c.data = expensive_call()
        return render('/home.myt')

    # Cache this controller action by its GET args for 10 mins to memory
    @beaker_cache(expire=600, type='memory', query_args=True)
    def show(self, id):
        c.data = expensive_call(id)
        return render('/show.myt')

By default the decorator uses a composite of all of the decorated function’s arguments as the cache key. It can alternatively use a composite of the request.GET query args as the cache key when the query_args option is enabled.

The cache key can be further customized via the key argument.

Warning

By default, the beaker_cache() decorator will cache the entire response object. This means the headers that were generated during the action will be cached as well. This can be disabled by providing cache_response = False to the decorator.

Templates

All render commands have caching functionality built in. To use it, merely add the appropriate cache keyword to the render call.

class SampleController(BaseController):
    def index(self):
        # Cache the template for 10 mins
        return render('/index.html', cache_expire=600)

    def show(self, id):
        # Cache this version of the template for 3 mins
        return render('/show.html', cache_key=id, cache_expire=180)

    def feed(self):
        # Cache for 20 mins to memory
        return render('/feed.html', cache_type='memory', cache_expire=1200)

    def home(self, user):
        # Cache this version of a page forever (until the cache dir
        # is cleaned)
        return render('/home.html', cache_key=user, cache_expire='never')

Note

At the moment, these functions do not support the use of cache region pre-defined argument sets.

Arbitrary Functions

Any Python function that returns a pickle-able result can be cached using Beaker. The recommended way to cache functions is to use the cache_region() decorator. This decorator requires the Cache Regions to be configured.

Using the cache_region() decorator:

@cache_region('short_term', 'search_func')
def get_results(search_param):
    # do something to retrieve data
    data = get_data(search_param)
    return data

results = get_results('gophers')

Invalidating

A cached function can be manually invalidated by using the region_invalidate() function.

Example:

region_invalidate(get_results, None, 'search_func', search_param)

Fragments

Individual template files, and <def> blocks within them can be independently cached. Since the caching system utilizes Beaker, any available Beaker back-ends are present in Mako as well.

Example:

<%def name="mycomp" cached="True" cache_timeout="30" cache_type="memory">
    other text
</%def>