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397a79ba56
As discussed in #214, source_created and source_changed didn't really have clear definitions. In truth, their names and separation betray their origins as model receivers in earlier versions. The "source group" abstraction helped us get away from thinking about things exclusively in terms of models, but these remained as an artifact.
225 lines
8.8 KiB
ReStructuredText
225 lines
8.8 KiB
ReStructuredText
Caching
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*******
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Default Backend Workflow
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================
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``ImageSpec``
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-------------
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At the heart of ImageKit are image generators. These are classes with a
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``generate()`` method which returns an image file. An image spec is a type of
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image generator. The thing that makes specs special is that they accept a source
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image. So an image spec is just an image generator that makes an image from some
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other image.
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``ImageCacheFile``
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------------------
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However, an image spec by itself would be vastly inefficient. Every time an
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an image was accessed in some way, it would have be regenerated and saved.
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Most of the time, you want to re-use a previously generated image, based on the
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input image and spec, instead of generating a new one. That's where
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``ImageCacheFile`` comes in. ``ImageCacheFile`` is a File-like object that
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wraps an image generator. They look and feel just like regular file
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objects, but they've got a little trick up their sleeve: they represent files
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that may not actually exist!
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Cache File Strategy
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-------------------
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Each ``ImageCacheFile`` has a cache file strategy, which abstracts away when
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image is actually generated. It can implement the following four methods:
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* ``on_content_required`` - called by ``ImageCacheFile`` when it requires the
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contents of the generated image. For example, when you call ``read()`` or
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try to access information contained in the file.
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* ``on_existence_required`` - called by ``ImageCacheFile`` when it requires the
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generated image to exist but may not be concerned with its contents. For
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example, when you access its ``url`` or ``path`` attribute.
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* ``on_source_saved`` - called when the source of a spec is saved
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* ``on_source_deleted`` - called when the source of a spec is deleted
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The default strategy only defines the first two of these, as follows:
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.. code-block:: python
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class JustInTime(object):
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def on_content_required(self, file):
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file.generate()
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def on_existence_required(self, file):
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file.generate()
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Cache File Backend
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------------------
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The ``generate`` method on the ``ImageCacheFile`` is further delegated to the
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cache file backend, which abstracts away how an image is generated.
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The cache file backend defaults to the setting
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``IMAGEKIT_DEFAULT_CACHEFILE_BACKEND`` and can be set explicitly on a spec with
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the ``cachefile_backend`` attribute.
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The default works like this:
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* Checks the file storage to see if a file exists
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* If not, caches that information for 5 seconds
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* If it does, caches that information in the ``IMAGEKIT_CACHE_BACKEND``
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If file doesn't exsit, generates it immediately and synchronously
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That pretty much covers the architecture of the caching layer, and its default
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behavior. I like the default behavior. When will an image be regenerated?
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Whenever it needs to be! When will your storage backend get hit? Depending on
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your ``IMAGEKIT_CACHE_BACKEND`` settings, as little as twice per file (once for the
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existence check and once to save the generated file). What if you want to change
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a spec? The generated file name (which is used as part of the cache keys) vary
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with the source file name and spec attributes, so if you change any of those, a
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new file will be generated. The default behavior is easy!
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.. note::
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Like regular Django ImageFields, IK doesn't currently cache width and height
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values, so accessing those will always result in a read. That will probably
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change soon though.
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Optimizing
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==========
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There are several ways to improve the performance (reduce I/O operations) of
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ImageKit. Each has its own pros and cons.
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Caching Data About Generated Files
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----------------------------------
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The easiest, and most significant improvement you can make to improve the
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performance of your site is to have ImageKit cache the state of your generated
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files. The default cache file backend will already do this (if ``DEBUG`` is
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``True``), using your default Django cache backend, but you can make it way
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better by setting ``IMAGEKIT_CACHE_BACKEND``. Generally, once a file is
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generated, you will never be removing it; therefore, if you can, you should set
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``IMAGEKIT_CACHE_BACKEND`` to a cache backend that will cache forever.
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Pre-Generating Images
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---------------------
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The default cache file backend generates images immediately and synchronously.
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If you don't do anything special, that will be when they are first requested—as
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part of request-response cycle. This means that the first visitor to your page
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will have to wait for the file to be created before they see any HTML.
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This can be mitigated, though, by simply generating the images ahead of time, by
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running the ``generateimages`` management command.
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.. note::
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If using with template tags, be sure to read :ref:`source-groups`.
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Deferring Image Generation
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--------------------------
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As mentioned above, image generation is normally done synchronously. through
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the default cache file backend. However, you can also take advantage of
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deferred generation. In order to do this, you'll need to do two things:
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1) install `django-celery`__
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2) tell ImageKit to use the async cachefile backend.
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To do this for all specs, set the ``IMAGEKIT_DEFAULT_CACHEFILE_BACKEND`` in
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your settings
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.. code-block:: python
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IMAGEKIT_DEFAULT_CACHEFILE_BACKEND = 'imagekit.cachefiles.backends.Async'
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Images will now be generated asynchronously. But watch out! Asynchrounous
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generation means you'll have to account for images that haven't been generated
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yet. You can do this by checking the truthiness of your files; if an image
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hasn't been generated, it will be falsy:
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.. code-block:: html
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{% if not profile.avatar_thumbnail %}
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<img src="/path/to/placeholder.jpg" />
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{% else %}
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<img src="{{ profile.avatar_thumbnail.url }}" />
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{% endif %}
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Or, in Python:
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.. code-block:: python
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profile = Profile.objects.all()[0]
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if profile.avatar_thumbnail:
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url = profile.avatar_thumbnail.url
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else:
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url = '/path/to/placeholder.jpg'
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__ https://pypi.python.org/pypi/django-celery
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Removing Safeguards
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-------------------
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Even with pre-generating images, ImageKit will still try to ensure that your
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image exists when you access it by default. This is for your benefit: if you
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forget to generate your images, ImageKit will see that and generate it for you.
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If the state of the file is cached (see above), this is a pretty cheap
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operation. However, if the state isn't cached, ImageKit will need to query the
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storage backend.
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For those who aren't willing to accept that cost (and who never want ImageKit
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to generate images in the request-responce cycle), there's the "optimistic"
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cache file strategy. This strategy only generates a new image when a spec's
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source image is created or changed. Unlike with the "just in time" strategy,
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accessing the file won't cause it to be generated, ImageKit will just assume
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that it already exists.
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To use this cache file strategy for all specs, set the
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``IMAGEKIT_DEFAULT_CACHEFILE_STRATEGY`` in your settings:
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.. code-block:: python
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IMAGEKIT_DEFAULT_CACHEFILE_STRATEGY = 'imagekit.cachefiles.strategies.Optimistic'
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If you have specs that :ref:`change based on attributes of the source
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<dynamic-specs>`, that's not going to cut it, though; the file will also need to
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be generated when those attributes change. Likewise, image generators that don't
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have sources (i.e. generators that aren't specs) won't cause files to be
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generated automatically when using the optimistic strategy. (ImageKit can't know
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when those need to be generated, if not on access.) In both cases, you'll have
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to trigger the file generation yourself—either by generating the file in code
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when necessary, or by periodically running the ``generateimages`` management
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command. Luckily, ImageKit makes this pretty easy:
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.. code-block:: python
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from imagekit.cachefiles import LazyImageCacheFile
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file = LazyImageCacheFile('myapp:profile:avatar_thumbnail', source=source_file)
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file.generate()
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One final situation in which images won't be generated automatically when using
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the optimistic strategy is when you use a spec with a source that hasn't been
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registered with it. Unlike the previous two examples, this situation cannot be
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rectified by running the ``generateimages`` management command, for the simple
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reason that the command has no way of knowing it needs to generate a file for
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that spec from that source. Typically, this situation would arise when using the
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template tags. Unlike ImageSpecFields, which automatically register all the
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possible source images with the spec you define, the template tags
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("generateimage" and "thumbnail") let you use any spec with any source.
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Therefore, in order to generate the appropriate files using the
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``generateimages`` management command, you'll need to first register a source
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group that represents all of the sources you wish to use with the corresponding
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specs. See :ref:`source-groups` for more information.
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