Caching¶
Default Backend Workflow¶
ImageSpec
¶
At the heart of ImageKit are image generators. These are classes with a
generate()
method which returns an image file. An image spec is a type of
image generator. The thing that makes specs special is that they accept a source
image. So an image spec is just an image generator that makes an image from some
other image.
ImageCacheFile
¶
However, an image spec by itself would be vastly inefficient. Every time an
an image was accessed in some way, it would have be regenerated and saved.
Most of the time, you want to re-use a previously generated image, based on the
input image and spec, instead of generating a new one. That’s where
ImageCacheFile
comes in. ImageCacheFile
is a File-like object that
wraps an image generator. They look and feel just like regular file
objects, but they’ve got a little trick up their sleeve: they represent files
that may not actually exist!
Cache File Strategy¶
Each ImageCacheFile
has a cache file strategy, which abstracts away when
image is actually generated. It can implement the following three methods:
on_content_required
- called byImageCacheFile
when it requires the contents of the generated image. For example, when you callread()
or try to access information contained in the file.on_existence_required
- called byImageCacheFile
when it requires the generated image to exist but may not be concerned with its contents. For example, when you access itsurl
orpath
attribute.on_source_saved
- called when the source of a spec is saved
The default strategy only defines the first two of these, as follows:
class JustInTime(object):
def on_content_required(self, file):
file.generate()
def on_existence_required(self, file):
file.generate()
Cache File Backend¶
The generate
method on the ImageCacheFile
is further delegated to the
cache file backend, which abstracts away how an image is generated.
The cache file backend defaults to the setting
IMAGEKIT_DEFAULT_CACHEFILE_BACKEND
and can be set explicitly on a spec with
the cachefile_backend
attribute.
The default works like this:
- Checks the file storage to see if a file exists
- If not, caches that information for 5 seconds
- If it does, caches that information in the
IMAGEKIT_CACHE_BACKEND
If file doesn’t exist, generates it immediately and synchronously
That pretty much covers the architecture of the caching layer, and its default
behavior. I like the default behavior. When will an image be regenerated?
Whenever it needs to be! When will your storage backend get hit? Depending on
your IMAGEKIT_CACHE_BACKEND
settings, as little as twice per file (once for the
existence check and once to save the generated file). What if you want to change
a spec? The generated file name (which is used as part of the cache keys) vary
with the source file name and spec attributes, so if you change any of those, a
new file will be generated. The default behavior is easy!
Note
Like regular Django ImageFields, IK doesn’t currently cache width and height values, so accessing those will always result in a read. That will probably change soon though.
Optimizing¶
There are several ways to improve the performance (reduce I/O operations) of ImageKit. Each has its own pros and cons.
Caching Data About Generated Files¶
The easiest, and most significant improvement you can make to improve the
performance of your site is to have ImageKit cache the state of your generated
files. The default cache file backend will already do this (if DEBUG
is
False
), using your default Django cache backend, but you can make it way
better by setting IMAGEKIT_CACHE_BACKEND
. Generally, once a file is
generated, you will never be removing it; therefore, if you can, you should set
IMAGEKIT_CACHE_BACKEND
to a cache backend that will cache forever.
Pre-Generating Images¶
The default cache file backend generates images immediately and synchronously. If you don’t do anything special, that will be when they are first requested—as part of request-response cycle. This means that the first visitor to your page will have to wait for the file to be created before they see any HTML.
This can be mitigated, though, by simply generating the images ahead of time, by
running the generateimages
management command.
Note
If using with template tags, be sure to read Source Groups.
Deferring Image Generation¶
As mentioned above, image generation is normally done synchronously. through the default cache file backend. However, you can also take advantage of deferred generation. In order to do this, you’ll need to do two things:
- install celery (or django-celery if you are bound to Celery<3.1)
- tell ImageKit to use the async cachefile backend.
To do this for all specs, set the
IMAGEKIT_DEFAULT_CACHEFILE_BACKEND
in your settings
IMAGEKIT_DEFAULT_CACHEFILE_BACKEND = 'imagekit.cachefiles.backends.Async'
Images will now be generated asynchronously. But watch out! Asynchrounous generation means you’ll have to account for images that haven’t been generated yet. You can do this by checking the truthiness of your files; if an image hasn’t been generated, it will be falsy:
{% if not profile.avatar_thumbnail %}
<img src="/path/to/placeholder.jpg" />
{% else %}
<img src="{{ profile.avatar_thumbnail.url }}" />
{% endif %}
Or, in Python:
profile = Profile.objects.all()[0]
if profile.avatar_thumbnail:
url = profile.avatar_thumbnail.url
else:
url = '/path/to/placeholder.jpg'
Note
If you are using an “async” backend in combination with the “optimistic” cache file strategy (see Removing Safeguards below), checking for thruthiness as described above will not work. The “optimistic” backend is very optimistic so to say, and removes the check. Create and use the following strategy to a) have images only created on save, and b) retain the ability to check whether the images have already been created:
class ImagekitOnSaveStrategy(object):
def on_source_saved(self, file):
file.generate()
Removing Safeguards¶
Even with pre-generating images, ImageKit will still try to ensure that your image exists when you access it by default. This is for your benefit: if you forget to generate your images, ImageKit will see that and generate it for you. If the state of the file is cached (see above), this is a pretty cheap operation. However, if the state isn’t cached, ImageKit will need to query the storage backend.
For those who aren’t willing to accept that cost (and who never want ImageKit to generate images in the request-responce cycle), there’s the “optimistic” cache file strategy. This strategy only generates a new image when a spec’s source image is created or changed. Unlike with the “just in time” strategy, accessing the file won’t cause it to be generated, ImageKit will just assume that it already exists.
To use this cache file strategy for all specs, set the
IMAGEKIT_DEFAULT_CACHEFILE_STRATEGY
in your settings:
IMAGEKIT_DEFAULT_CACHEFILE_STRATEGY = 'imagekit.cachefiles.strategies.Optimistic'
If you have specs that change based on attributes of the source, that’s not going to cut it, though; the file will also need to
be generated when those attributes change. Likewise, image generators that don’t
have sources (i.e. generators that aren’t specs) won’t cause files to be
generated automatically when using the optimistic strategy. (ImageKit can’t know
when those need to be generated, if not on access.) In both cases, you’ll have
to trigger the file generation yourself—either by generating the file in code
when necessary, or by periodically running the generateimages
management
command. Luckily, ImageKit makes this pretty easy:
from imagekit.cachefiles import LazyImageCacheFile
file = LazyImageCacheFile('myapp:profile:avatar_thumbnail', source=source_file)
file.generate()
One final situation in which images won’t be generated automatically when using
the optimistic strategy is when you use a spec with a source that hasn’t been
registered with it. Unlike the previous two examples, this situation cannot be
rectified by running the generateimages
management command, for the simple
reason that the command has no way of knowing it needs to generate a file for
that spec from that source. Typically, this situation would arise when using the
template tags. Unlike ImageSpecFields, which automatically register all the
possible source images with the spec you define, the template tags
(“generateimage” and “thumbnail”) let you use any spec with any source.
Therefore, in order to generate the appropriate files using the
generateimages
management command, you’ll need to first register a source
group that represents all of the sources you wish to use with the corresponding
specs. See Source Groups for more information.