Not all storage is created equal: introducing Storage Buckets

The Storage Standard defines an API for persistent storage and quota estimates, and the platform storage architecture. We're experimenting with an API for making persistent storage eviction under heavy memory pressure more predictable. It's available behind the #enable-experimental-web-platform-features flag in Chromium 106, and we'd love your feedback.

# What problem does the storage standard aim to solve?

Traditionally, as the user runs out of storage space on their device, the data stored with APIs like IndexedDB or localStorage gets lost without the user being able to intervene. A way to make storage persistent is through invoking the persist() method of the StorageManager interface. It simultaneously requests the end user for permission and changes the storage to be persistent once granted:

const persisted = await navigator.storage.persist();
if (persisted) {
  /* Storage will not be cleared except by explicit user action. */
}

This method of asking for storage to be persisted is all or nothing. There's no way to express more fine-grained persistence needs. It's all one storage bucket.

# The Storage Buckets proposal

The core idea of the Storage Buckets proposal is granting sites the ability to create multiple storage buckets, where the browser may choose to delete each bucket independently of other buckets. This allows developers to specify eviction prioritization to make sure the most valuable data doesn't get deleted.

# Use case example

To illustrate where storage buckets would come in handy, imagine an email application. It would be unforgivable if the app lost the user's unsent drafts that only exist on the client. In contrast, if they are stored on a server, the user would probably be fine with some of their oldest inbox emails to be removed from the client if their browser is under heavy storage pressure.

# Using the Storage Buckets API

# Creating a new storage bucket

A new storage bucket can be created with the open() method on the StorageBucketManager interface.

// Create a storage bucket for emails that are synchronized with the
// server.
const inboxBucket = await navigator.storageBuckets.open('inbox');

# Creating a persisted new storage bucket

To ensure the storage bucket is persisted, you can pass durability and persisted option arguments to the open() method:

• persisted determines if the storage bucket should be persisted or not. The allowed values are either false (default) or true.

• durability provides a hint to the browser that helps it trade off write performance against a reduced risk of data loss in the event of power failures. The allowed values are 'relaxed' (default) or 'strict':

  • 'strict' buckets attempt to minimize the risk of data loss on power failure. This may come at the cost of reduced performance, meaning that writes may take longer to complete, might impact overall system performance, may consume more battery power, and may wear out the storage device faster.
  • 'relaxed' buckets may "forget" writes that were completed in the last few seconds, when a power loss occurs. In return, writing data to these buckets may have better performance characteristics, and may allow a battery charge to last longer, and may result in longer storage device lifetime. Also, power failures will not lead to data corruption at a higher rate than for 'strict' buckets.

// Create a storage bucket for email drafts that only exist on the client.
const draftsBucket = await navigator.storageBuckets.open('drafts', {
  durability: 'strict', // Or `'relaxed'`.
  persisted: true, // Or `false`.
});

# Accessing the storage APIs from a storage bucket

Each storage bucket is associated with storage APIs, for example, IndexedDB, the Cache interface, or the File interface. These storage APIs work as per the usual, just that the entry point is from the StorageBucket interface, for example, StorageBucket.indexedDB.

const inboxDb = await new Promise(resolve => {
  const request = inboxBucket.indexedDB.open('messages');
  request.onupgradeneeded = () => { /* migration code */ };
  request.onsuccess = () => resolve(request.result);
  request.onerror = () => reject(request.error);
});

# Feedback

The Chrome team wants to hear what you think of storage buckets. Your feedback on the Storage Buckets proposal is warmly welcomed. Please provide feedback by commenting on existing or filing new GitHub Issues.

# Useful resources

• Storage Buckets explainer
• Storage Buckets repository
• ChromeStatus entry
• Chromium bug
• Blink component Blink > Storage > Buckets
• Mozilla Standards Position
• TAG review