Caching and Cache Invalidation

Sam edited this page May 20, 2017 · 14 revisions

Glide V4

For information on Glide v4, see the documentation. This wiki covers Glide v3 only.


Cache invalidation is a relatively complex topic and ideally you have to think about it as little as possible. The goal of this page is to give you a rough idea of how cache keys are generated in Glide and some hints as to how to make the cache work best for you.

Cache Keys

Cache keys used for DiskCacheStrategy.RESULT in Glide are comprised of four main parts:

  • The String returned from the getId() method in DataFetcher. Typically DataFetchers simply return the result of calling toString() on your data model, so the string version of your URL if you provide a URL, or the file path if you provide a File etc.

  • The width and height passed in to the override(int, int) method if called, or by default the width and height provided by your Target's getSize() method.

  • The Strings returned from the getId() methods of the various decoders and encoders used to load and cache your image. Only the decoders and encoders that affect the retrieved bytes include ids. For example if you have an encoder that simply writes a byte array to disk, that encoder will not have an id because it does not affect the data in any way.

  • An optional signature you may apply to each load (see Custom cache invalidation below).

All of these keys are hashed in a particular order to create a unique and safe File name to save a particular image on disk.

Cache Invalidation

Because File names are hashed keys, there is no good way to simply delete all of the cached files on disk that correspond to a particular url or file path. The problem would be simpler if you were only ever allowed to load or cache the original image, but since Glide also caches thumbnails and provides various transformations, each of which will result in a new File in the cache, tracking down and deleting every cached version of an image is difficult.

In practice, the best way to invalidate a cache file is to change your identifier when the content changes (url, uri, file path etc).

Custom cache invalidation

Since it's often difficult or impossible to change identifiers, Glide also offers the signature() API to mix in additional data that you control into your cache key. Signatures work well for media store content, as well as any content you can maintain some versioning metadata for.

  • Media store content - For media store content, you can use Glide's MediaStoreSignature class as your signature. MediaStoreSignature allows you to mix the date modified time, mime type, and orientation of a media store item into the cache key. These three attributes reliably catch edits and updates allowing you to cache media store thumbs.

  • Files - You can use StringSignature to mix in the File's date modified time.

  • Urls - Although the best way to invalidate urls is to make sure the server changes the url and updates the client when the content at the url changes, you can also use StringSignature to mix in arbitrary metadata (such as a version number) instead.

Passing in string signatures to loads is simple:

Glide.with(yourFragment)
    .load(yourFileDataModel)
    .signature(new StringSignature(yourVersionMetadata))
    .into(yourImageView);

The media store signature is also straightforward data from the MediaStore:

Glide.with(fragment)
    .load(mediaStoreUri)
    .signature(new MediaStoreSignature(mimeType, dateModified, orientation))
    .into(view);

You can also define your own signature by implementing the Key interface. Be sure to implement equals(), hashCode() and the updateDiskCacheKey() method:

public class IntegerVersionSignature implements Key {
    private int currentVersion;

    public IntegerVersionSignature(int currentVersion) {
         this.currentVersion = currentVersion;
    }
   
    @Override
    public boolean equals(Object o) {
        if (o instanceof IntegerVersionSignature) {
            IntegerVersionSignature other = (IntegerVersionSignature) o;
            return currentVersion = other.currentVersion;
        }
        return false;
    }
 
    @Override
    public int hashCode() {
        return currentVersion;
    }

    @Override
    public void updateDiskCacheKey(MessageDigest md) {
        messageDigest.update(ByteBuffer.allocate(Integer.SIZE).putInt(signature).array());
    }
}

Keep in mind that to avoid degrading performance, you will want to batch load any versioning metadata in the background so that it is available when you want to load your image.

If all else fails and you can neither change your identifier nor keep track of any reasonable version metadata, you can also disable disk caching entirely using diskCacheStrategy() and DiskCacheStrategy.NONE.

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