# 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 by `ImageCacheFile` when it requires the contents of the generated image. For example, when you call `read()` or try to access information contained in the file.

- `on_existence_required` - called by `ImageCacheFile` when it requires the generated image to exist but may not be concerned with its contents. For example, when you access its `url` or `path` 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

Generally, once a file is generated, you will never be removing it, so by default ImageKit will use default cache to cache the state of generated files “forever” (or only 5 minutes when `DEBUG = True`).

The time for which ImageKit will cache state is configured with `IMAGEKIT_CACHE_TIMEOUT`. If set to `None` this means “never expire” (default when `DEBUG = False`). You can reduce this timeout if you want or set it to some numeric value in seconds if your cache backend behaves differently and for example do not cache values if timeout is `None`.

If you clear your cache durring deployment or some other reason probably you do not want to lose the cache for generated images especcialy if you are using some slow remote storage (like Amazon S3). Then you can configure seprate cache (for example redis) in your `CACHES` config and tell ImageKit to use it instead of the default cache by setting `IMAGEKIT_CACHE_BACKEND`.

### 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:

1. install celery (or django-celery if you are bound to Celery<3.1)

2. 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()

Note

If you use custom storage backend for some specs, (storage passed to the field different than configured one) it’s required the storage to be pickleable

### 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.