HTML5 Rocks

There’s various offline related features introduced to modern browsers through HTML5. While offline is convenient, its concept of quota has been left untouched for a long time. The latest version of Chrome browser has the first concept and its implementation of Quota Management API . It handles quota for AppCache, IndexedDB, WebSQL and File System API. Here’s a list of things you should keep in mind when working with Quota Management API in the latest Chrome.

(The specific numbers and details noted below are not a part of HTML5 but the facts in the current Chrome implementation. The API and Quota management in Chrome is still evolving and the details may change over the time.)

• On HTML5 storage, there’s a notion of TEMPORARY storage and PERSISTENT storage.
  • TEMPORARY storage can be used without requesting quota, but may be deleted at the browser’s discretion.
  • PERSISTENT storage is never deleted without the user’s instruction, but usually requires up-front quota request to use.

• For TEMPORARY storage, it is shared between all applications and websites run in the browser.
  • TEMPORARY storage has a default quota of 50% of available disk as a shared pool. (50GB => 25GB) (Not restricted to 1GB anymore)
    [To be more specific, TEMP quota is calucalated by (remaining disk space + TEMP storage space) * 50%. Therefore if apps are using 25GB TEMP storage in total and the current remaining disk space is 25GB that’s already full.]
  • Each application has a limitation to have 20% of the available TEMPORARY storage pool (i.e. 20% of 50% of available disk). (Not restricted to 5Mb anymore)
  • When TEMPORARY storage quota is exceeded, all the data (incl. AppCache, IndexedDB, WebSQL, File System API) stored for oldest used origin gets deleted .
    [Note: since each app can only use up to 20% of the pool, this won’t happen very frequently unless the user has more than 5 active offline apps.]
  • If app tries to make a modification which will result in exceeding TEMPORARY storage quota (20% of pool), an error will be thrown.
  • Each application can query how much data is stored or how much more space is available for the app by calling queryUsageAndQuota() method of Quota API.
  • Requesting more quota against TEMPORARY storage doesn’t do anything.
  • Requesting quota for TEMPORARY storage using webkitRequestFileSystem() doesn’t actually allocate / change quota.

• webkitStorageInfo.queryUsageAndQuota( 
      webkitStorageInfo.TEMPORARY,   // or PERSISTENT 
      usageCallback, 
      errorCallback);

• For PERSISTENT storage, its default quota is 0 and it needs to be explicitly requested per application using requestQuota() of Quota API.
  • The allocated space can be used only by File System API (for PERSISTENT type filesystem) and there’s no such thing like PERSISTENT storage on IndexedDB, WebSQL DB or AppCache (yet).

• webkitStorageInfo.requestQuota( 
      webkitStorageInfo.PERSISTENT
      newQuotaInBytes,
      quotaCallback,
      errorCallback);

• unlimitedStorage on manifest.json of Chrome Web App has been brought as a temporary solution for apps to work without Quota API. So, there’s no guarantee that Chrome will support this feature forever.

• The API is described in WebIDL here: https://groups.google.com/a/chromium.org/group/chromium-html5/msg/5261d24266ba4366?pli=1

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