storage.bs

<pre class=metadata>
Group: WHATWG
H1: Storage
Shortname: storage
Text Macro: TWITTER storagestandard
Text Macro: LATESTRD 2020-08
Abstract: The Storage Standard defines an API for persistent storage and quota estimates, as well as the platform storage architecture.
Translation: ja https://triple-underscore.github.io/storage-ja.html
</pre>

<pre class=anchors>
urlPrefix: https://tc39.github.io/ecma262/; spec: ECMASCRIPT
    text: agent; url: #sec-agents; type: dfn
    text: agent cluster; url: #sec-agent-clusters; type: dfn
</pre>



<h2 id=introduction>Introduction</h2>

<p>Over the years the web has grown various APIs that can be used for storage, e.g., IndexedDB,
<code>localStorage</code>, and <code>showNotification()</code>. The Storage Standard consolidates
these APIs by defining:

<ul class=brief>
 <li>A bucket, the primitive these APIs store their data in
 <li>A way of making that bucket persistent
 <li>A way of getting usage and quota estimates for an <a for=/>origin</a>
</ul>

<p>Traditionally, as the user runs out of storage space on their device, the data stored with these
APIs gets lost without the user being able to intervene. However, persistent buckets cannot be
cleared without consent by the user. This thus brings data guarantees users have enjoyed on native
platforms to the web.

<div class="example" id=example-3a7051a8>
 <p>A simple way to make storage persistent is through invoking the {{persist()}} method. It
 simultaneously requests the end user for permission and changes the storage to be persistent once
 granted:</p>

 <pre><code class="lang-javascript">
navigator.storage.persist().then(persisted => {
  if (persisted) {
    /* &hellip; */
  }
});
</code></pre>

 <p>To not show user-agent-driven dialogs to the end user unannounced slightly more involved code
 can be written:</p>

 <pre><code class="lang-javascript">
Promise.all([
  navigator.storage.persisted(),
  navigator.permissions.query({name: "persistent-storage"})
]).then(([persisted, permission]) => {
  if (!persisted &amp;&amp; permission.status == "granted") {
    navigator.storage.persist().then( /* &hellip; */ );
  } else if (!persisted &amp;&amp; permission.status == "prompt") {
    showPersistentStorageExplanation();
  }
});
</code></pre>

 <p>The {{estimate()}} method can be used to determine whether there is enough space left to
 store content for an application:

 <pre><code class="lang-javascript">
function retrieveNextChunk(nextChunkInfo) {
  return navigator.storage.estimate().then(info => {
    if (info.quota - info.usage > nextChunkInfo.size) {
      return fetch(nextChunkInfo.url);
    } else {
      throw new Error("insufficient space to store next chunk");
    }
  }).then( /* &hellip; */ );
}
</code></pre>

</div>



<h2 id=terminology>Terminology</h2>

<p>This specification depends on the Infra Standard. [[!INFRA]]

<p>This specification uses terminology from the HTML, IDL, and Permissions Standards.
[[!HTML]] [[!WEBIDL]] [[!PERMISSIONS]]



<h2 id=infrastructure>Lay of the land</h2>

<p>A <a for=/>user agent</a> has various kinds of semi-persistent state:

<dl>
 <dt>Credentials
 <dd><p>End-user credentials, such as username and passwords submitted through HTML forms
 <dt>Permissions
 <dd><p>Permissions for various features, such as geolocation
 <dt>Network
 <dd><p>HTTP cache, cookies, authentication entries, TLS client certificates
 <dt id=site-storage>Storage
 <dd>Indexed DB, Cache API, service worker registrations, <code>localStorage</code>,
 <code>sessionStorage</code>, application caches, notifications, etc.
</dl>

<p>This standard primarily concerns itself with storage.



<h2 id=model>Model</h2>

<p>Standards defining local or session storage APIs will define a <a>storage endpoint</a> and
<a lt="registered storage endpoints">register</a> it by changing this standard. They will invoke
either the <a>obtain a local storage bottle map</a> or the
<a>obtain a session storage bottle map</a> algorithm, which will give them:

<ul>
 <li><p>Failure, which might mean the API has to throw or otherwise indicate there is no storage
 available for that <a for=/>environment settings object</a>.

 <li><p>A <a>storage proxy map</a> that operates analogously to a <a for=/>map</a>, which can be
 used to store data in a manner that suits the API. This standard takes care of isolating that data
 from other APIs, <a>storage keys</a>, and <a>storage types</a>.
</ul>

<p class=note>If you are defining a standard for such an API, consider filing an issue against this
standard for assistance and review.

<p>To isolate this data this standard defines a <a for=/>storage shed</a> which segments
<a>storage shelves</a> by a <a>storage key</a>. A <a>storage shelf</a> in turn consists of a
<a>storage bucket</a> and will likely consist of multiple <a>storage buckets</a> in the future to
allow for different storage policies. And lastly, a <a>storage bucket</a> consists of
<a>storage bottles</a>, one for each <a>storage endpoint</a>.


<h3 id=storage-endpoints>Storage endpoints</h3>

<p>A <dfn export>storage endpoint</dfn> is a <a lt="local storage">local</a> or
<a>session storage</a> API that uses the infrastructure defined by this standard, most notably
<a for=/>storage bottles</a>, to keep track of its storage needs.

<p>A <a>storage endpoint</a> has an <dfn for="storage endpoint">identifier</dfn>, which is a
<a>storage identifier</a>.

<p>A <a>storage endpoint</a> also has <dfn for="storage endpoint">types</dfn>, which is a
<a for=/>set</a> of <a>storage types</a>.

<p>A <a>storage endpoint</a> also has a <dfn for="storage endpoint">quota</dfn>, which is null or a
number representing a recommended <a for="storage bottle">quota</a> (in bytes) for each
<a for=/>storage bottle</a> corresponding to this <a>storage endpoint</a>.

<p>A <dfn>storage identifier</dfn> is an <a for=/>ASCII string</a>.

<p>A <dfn>storage type</dfn> is "<code>local</code>" or "<code>session</code>".

<hr>

<p>The <dfn>registered storage endpoints</dfn> are a <a for=/>set</a> of <a>storage endpoints</a>
defined by the following table:
<!-- Note, these will be exposed more generally through https://github.com/whatwg/storage/pull/69 so
     please keep https://w3ctag.github.io/design-principles/#casing-rules in mind. -->

<table>
 <tr>
  <th><a for="storage endpoint">Identifier</a>
  <th><a for="storage endpoint">Type</a>
  <th><a for="storage endpoint">Quota</a>
 <tr>
  <td>"<code>caches</code>"
  <td>« "<code>local</code>" »
  <td>null
 <tr>
  <td>"<code>indexedDB</code>"
  <td>« "<code>local</code>" »
  <td>null
 <tr>
  <td>"<code>localStorage</code>"
  <td>« "<code>local</code>" »
  <td>5 &times; 2<sup>20</sup> (i.e., 5 mebibytes)
 <tr>
  <td>"<code>serviceWorkerRegistrations</code>"
  <td>« "<code>local</code>" »
  <td>null
 <tr>
  <td>"<code>sessionStorage</code>"
  <td>« "<code>session</code>" »
  <td>5 &times; 2<sup>20</sup> (i.e., 5 mebibytes)
</table>

<p class=note>As mentioned, standards can use these <a>storage identifiers</a> with
<a>obtain a local storage bottle map</a> and <a>obtain a session storage bottle map</a>. It is
anticipated that some APIs will be applicable to both <a>storage types</a> going forward.
<!-- If that does not happen by 2024 we probably ought to admit defeat and simplify this a bit. -->


<h3 id=storage-keys>Storage keys</h3>

<p>A <dfn>storage key</dfn> is an <a for=/>origin</a>. [[!HTML]]

<p class=XXX>This is expected to change; see
<a href="https://privacycg.github.io/storage-partitioning/">Client-Side Storage Partitioning</a>.

<p>To <dfn export>obtain a storage key</dfn>, given an <a>environment settings object</a>
<var>environment</var>, run these steps:

<ol>
 <li><p>Let <var>key</var> be <var>environment</var>'s
 <a for="environment settings object">origin</a>.

 <li><p>If <var>key</var> is an <a>opaque origin</a>, then return failure.

 <li><p>If the user has disabled storage, then return failure.

 <li><p>Return <var>key</var>.
</ol>


<h3 id=storage-sheds>Storage sheds</h3>

<p>A <dfn>storage shed</dfn> is a <a for=/>map</a> of <a>storage keys</a> to <a>storage shelves</a>.
It is initially empty.

<hr>

<p>A <a for=/>user agent</a> holds a <dfn for="user agent">storage shed</dfn>, which is a
<a for=/>storage shed</a>. A user agent's <a for="user agent">storage shed</a> holds all
<dfn>local storage</dfn> data.

<p>A <a for=/>browsing session</a> holds a <dfn for="browsing session">storage shed</dfn>, which is
a <a for=/>storage shed</a>. A <a for=/>browsing session</a>'s
<a for="browsing session">storage shed</a> holds all <dfn>session storage</dfn> data.

<p>To <dfn export>legacy-clone a browsing session storage shed</dfn>, given a
<a for=/>browsing session</a> <var>A</var> and a <a for=/>browsing session</a> <var>B</var>, run
these steps:

<ol>
 <li>
  <p><a for=map>For each</a> <var>key</var> → <var>shelf</var> of <var>A</var>'s
  <a for="browsing session">storage shed</a>:

  <ol>
   <li><p>Let <var>newShelf</var> be the result of running <a>create a storage shelf</a> with
   "<code>session</code>".

   <li><p>Set <var>newShelf</var>'s <a>bucket map</a>["<code>default</code>"]'s
   <a>bottle map</a>["<code>sessionStorage</code>"]'s <a for="storage bottle">map</a> to a
   <a for=map>clone</a> of <var>shelf</var>'s <a>bucket map</a>["<code>default</code>"]'s
   <a>bottle map</a>["<code>sessionStorage</code>"]'s <a for="storage bottle">map</a>.

   <li><p>Set <var>B</var>'s <a for="browsing session">storage shed</a>[<var>key</var>] to
   <var>newShelf</var>.
  </ol>
</ol>

<p class="note">This is considered legacy as the benefits, if any, do not outweigh the
implementation complexity. And therefore it will not be expanded or used outside of
<cite>HTML</cite>. [[HTML]]


<h3 id=storage-shelves>Storage shelves</h3>

<p>A <dfn oldids=site-storage-unit lt="storage shelf|storage shelves">storage shelf</dfn> exists for
each <a>storage key</a> within a <a for=/>storage shed</a>. It holds a <dfn>bucket map</dfn>,
which is a <a for=/>map</a> of <a for=/>strings</a> to <a>storage buckets</a>.

<p class=note>For now "<code>default</code>" is the only <a for=map>key</a> that exists in a
<a>bucket map</a>. See <a href="https://github.com/whatwg/storage/issues/2">issue #2</a>. It is
given a <a for=map>value</a> when a <a>storage shelf</a> is
<a lt="obtain a storage shelf">obtained</a> for the first time.

<p>To <dfn>obtain a storage shelf</dfn>, given a <a for=/>storage shed</a> <var>shed</var>, an
<a>environment settings object</a> <var>environment</var>, and a <a>storage type</a>
<var>type</var>, run these steps:

<ol>
 <li><p>Let <var>key</var> be the result of running <a>obtain a storage key</a> with
 <var>environment</var>.

 <li><p>If <var>key</var> is failure, then return failure.

 <li><p>If <var>shed</var>[<var>key</var>] does not <a for=map>exist</a>, then set
 <var>shed</var>[<var>key</var>] to the result of running <a>create a storage shelf</a> with
 <var>type</var>.

 <li><p>Return <var>shed</var>[<var>key</var>].
</ol>

<p>To <dfn>obtain a local storage shelf</dfn>, given an <a>environment settings object</a>
<var>environment</var>, return the result of running <a>obtain a storage shelf</a> with the user
agent's <a for="user agent">storage shed</a>, <var>environment</var>, and "<code>local</code>".

<p>To <dfn>create a storage shelf</dfn>, given a <a>storage type</a> <var>type</var>, run these
steps:

<ol>
 <li><p>Let <var>shelf</var> be a new <a>storage shelf</a>.

 <li><p>Set <var>shelf</var>'s <a>bucket map</a>["<code>default</code>"] to the result of running
 <a>create a storage bucket</a> with <var>type</var>.

 <li><p>Return <var>shelf</var>.
</ol>


<h3 id=buckets oldids=boxes>Storage buckets</h3>

<p>A <dfn>storage bucket</dfn> is a place for <a>storage endpoints</a> to store data.

<p>A <a>storage bucket</a> has a <dfn>bottle map</dfn> of <a>storage identifiers</a> to
<a>storage bottles</a>.

<hr>

<p>A <dfn id=bucket oldids=box>local storage bucket</dfn> is a <a>storage bucket</a> for
<a>local storage</a> APIs.

<p>A <a>local storage bucket</a> has a
<dfn for="local storage bucket" id=bucket-mode oldids=box-mode>mode</dfn>, which is
"<code>best-effort</code>" or "<code>persistent</code>". It is initially "<code>best-effort</code>".

<hr>

<p>A <dfn>session storage bucket</dfn> is a <a>storage bucket</a> for <a>session storage</a> APIs.

<hr>

<p>To <dfn>create a storage bucket</dfn>, given a <a>storage type</a> <var>type</var>, run these
steps:

<ol>
 <li><p>Let <var>bucket</var> be null.

 <li><p>If <var>type</var> is "<code>local</code>", then set <var>bucket</var> to a new
 <a>local storage bucket</a>.

 <li>
  <p>Otherwise:

  <ol>
   <li><p>Assert: <var>type</var> is "<code>session</code>".

   <li><p>Set <var>bucket</var> to a new <a>session storage bucket</a>.
  </ol>

 <li><p><a for=set>For each</a> <var>endpoint</var> of <a>registered storage endpoints</a> whose
 <a for="storage endpoint">types</a> <a for=set>contain</a> <var>type</var>, set <var>bucket</var>'s
 <a>bottle map</a>[<var>endpoint</var>'s <a for="storage endpoint">identifier</a>] to a new
 <a>storage bottle</a> whose <a for="storage bottle">quota</a> is <var>endpoint</var>'s
 <a for="storage endpoint">quota</a>.

 <li><p>Return <var>bucket</var>.
</ol>


<h3 id=storage-bottles>Storage bottles</h3>

<p>A <dfn>storage bottle</dfn> is a part of a <a>storage bucket</a> carved out for a single
<a>storage endpoint</a>. A <a>storage bottle</a> has a <dfn for="storage bottle">map</dfn>, which is
initially an empty <a for=/>map</a>. A <a>storage bottle</a> also has a
<dfn for="storage bottle">proxy map reference set</dfn>, which is initially an empty
<a for=/>set</a>. A <a>storage bottle</a> also has a <dfn for="storage bottle">quota</dfn>, which is
null or a number representing a conservative estimate of the total amount of bytes it can hold. Null
indicates the lack of a limit. <span class=note>It is still bound by the <a>storage quota</a> of its
encompassing <a for=/>storage shelf</a>.

<p>A <a>storage bottle</a>'s <a for="storage bottle">map</a> is where the actual data meant to be
stored lives. User agents are expected to store this data, and make it available across <a>agent</a>
and even <a>agent cluster</a> boundaries, in an <a>implementation-defined</a> manner, so that this
standard and standards using this standard can access the contents.

<hr>

<p>To <dfn>obtain a storage bottle map</dfn>, given a <a>storage type</a> <var>type</var>,
<a for=/>environment settings object</a> <var>environment</var>, and <a>storage identifier</a>
<var>identifier</var>, run these steps:</p>

<ol>
 <li><p>Let <var>shed</var> be null.

 <li><p>If <var>type</var> is "<code>local</code>", then set <var>shed</var> to the user agent's
 <a for="user agent">storage shed</a>.

 <li>
  <p>Otherwise:

  <ol>
   <li><p>Assert: <var>type</var> is "<code>session</code>".

   <li><p>Set <var>shed</var> to <var>environment</var>'s
   <a for="environment settings object">browsing session</a>'s
   <a for="browsing session">storage shed</a>.
  </ol>

 <li><p>Let <var>shelf</var> be the result of running <a>obtain a storage shelf</a>, with
 <var>shed</var>, <var>environment</var>, and <var>type</var>.

 <li><p>If <var>shelf</var> is failure, then return failure.

 <li><p>Let <var>bucket</var> be <var>shelf</var>'s <a>bucket map</a>["<code>default</code>"].

 <li><p>Let <var>bottle</var> be <var>bucket</var>'s <a>bottle map</a>[<var>identifier</var>].

 <li><p>Let <var>proxyMap</var> be a new <a>storage proxy map</a> whose
 <a for="storage proxy map">backing map</a> is <var>bottle</var>'s <a for="storage bottle">map</a>.

 <li><p><a for=set>Append</a> <var>proxyMap</var> to <var>bottle</var>'s
 <a for="storage bottle">proxy map reference set</a>.

 <li><p>Return <var>proxyMap</var>.
</ol>

<p>To <dfn export>obtain a local storage bottle map</dfn>, given an
<a for=/>environment settings object</a> <var>environment</var> and <a>storage identifier</a>
<var>identifier</var>, return the result of running <a>obtain a storage bottle map</a> with
"<code>local</code>", <var>environment</var>, and <var>identifier</var>.

<p>To <dfn export>obtain a session storage bottle map</dfn>, given an
<a for=/>environment settings object</a> <var>environment</var> and <a>storage identifier</a>
<var>identifier</var>, return the result of running <a>obtain a storage bottle map</a> with
"<code>session</code>", <var>environment</var>, and <var>identifier</var>.


<h3 id=storage-proxy-maps>Storage proxy maps</h3>

<p>A <dfn>storage proxy map</dfn> is equivalent to a <a for=/>map</a>, except that all operations
are instead performed on its <dfn for="storage proxy map">backing map</dfn>.

<p class="XXX">This allows for the <a for="storage proxy map">backing map</a> to be replaced. This
is needed for <a href="https://github.com/whatwg/storage/issues/4">issue #4</a> and potentially the
<a href="https://privacycg.github.io/storage-access/">Storage Access API</a>.



<h2 id=persistence>Persistence permission</h2>

<p>A <a>local storage bucket</a> can only have its <a for="local storage bucket">mode</a> change to
"<code>persistent</code>" if the user (or user agent on behalf of the user) has granted permission
to use the {{"persistent-storage"}} feature.

<p class="note">When granted to an <a for=/>origin</a>, the persistence permission can be used to
protect storage from the user agent's clearing policies. The user agent cannot clear storage marked
as persistent without involvement from the <a for=/>origin</a> or user. This makes it particularly
useful for resources the user needs to have available while offline or resources the user creates
locally.

<p>The <dfn for="PermissionName" enum-value>"<code>persistent-storage</code>"</dfn>
<a>powerful feature</a>'s permission-related flags, algorithms, and types are defaulted, except for:

<dl>
 <dt><a>permission state</a>
 <dd><p>{{"persistent-storage"}}'s <a>permission state</a> must have the same value for all
 <a>environment settings objects</a> with a given <a for="environment settings object">origin</a>.

 <dt><a>permission revocation algorithm</a>
 <dd algorithm="permission-revocation">
  <ol>
   <li><p>If {{"persistent-storage"}}'s <a>permission state</a> is {{"granted"}}, then return.

   <li><p>Let <var>shelf</var> be the result of running <a>obtain a local storage shelf</a> with
   <a>current settings object</a>.

   <li><p>Set <var>shelf</var>'s <a>bucket map</a>["<code>default</code>"]'s
   <a for="local storage bucket">mode</a> to "<code>best-effort</code>".
  </ol>
</dl>



<h2 id=usage-and-quota>Usage and quota</h2>

<p>The <dfn export>storage usage</dfn> of a <a>storage shelf</a> is an <a>implementation-defined</a>
rough estimate of the amount of bytes used by it.

<p class=note>This cannot be an exact amount as user agents might, and are encouraged to, use
deduplication, compression, and other techniques that obscure exactly how much bytes a
<a>storage shelf</a> uses.

<p tracking-vector>The <dfn export>storage quota</dfn> of a <a>storage shelf</a> is an
<a>implementation-defined</a> conservative estimate of the total amount of bytes it can hold. This
amount should be less than the total storage space on the device. It must not be a function of the
available storage space on the device.

<div class=note>
 <p>User agents are strongly encouraged to consider navigation frequency, recency of visits,
 bookmarking, and <a href="#persistence">permission</a> for {{"persistent-storage"}} when
 determining quotas.

 <p>Directly or indirectly revealing available storage space can lead to fingerprinting and leaking
 information outside the scope of the <a for=/>origin</a> involved.
</div>



<h2 id=management>Management</h2>

<p>Whenever a <a>storage bucket</a> is cleared by the user agent, it must be cleared in its
entirety. User agents should avoid clearing <a>storage buckets</a> while script that is able to
access them is running, unless instructed otherwise by the user.

<p>If removal of <a>storage buckets</a> leaves the encompassing <a>storage shelf</a>'s
<a>bucket map</a> <a for=map lt="is empty">empty</a>, then <a for=map>remove</a> that
<a>storage shelf</a> and corresponding <a>storage key</a> from the encompassing
<a for=/>storage shed</a>.


<h3 id=storage-pressure>Storage pressure</h3>

<p>A user agent that comes under storage pressure should clear network state and
<a>local storage buckets</a> whose <a for="local storage bucket">mode</a> is
"<code>best-effort</code>", ideally prioritizing removal in a manner that least impacts the user.

<p>If a user agent continues to be under storage pressure, then the user agent should inform the
user and offer a way to clear the remaining <a>local storage buckets</a>, i.e., those whose
<a for="local storage bucket">mode</a> is "<code>persistent</code>".

<p><a>Session storage buckets</a> must be cleared as <a for=/>browsing sessions</a> are closed.

<p class="note">If the user agent allows for revival of <a for=/>browsing sessions</a>, e.g.,
through reopening <a for=/>browsing sessions</a> or continued use of them after restarting the user
agent, then clearing necessarily involves a more complex set of heuristics.


<h3 id=ui-guidelines>User interface guidelines</h3>

<p>User agents should offer users the ability to clear network state and storage for individual
websites. User agents should not distinguish between network state and storage in their user
interface. This ensures network state cannot be used to revive storage and reduces the number of
concepts users need to be mindful of.
<!-- This intentionally does not define individual website as the specific UI is still in flux. -->

<p>Credentials should be separated as they contain data the user might not be able to revive, such
as an autogenerated password. Permissions are best separated too to avoid inconveniencing the user.



<h2 id=api>API</h2>

<pre class=idl>
[SecureContext]
interface mixin NavigatorStorage {
  [SameObject] readonly attribute StorageManager storage;
};
Navigator includes NavigatorStorage;
WorkerNavigator includes NavigatorStorage;
</pre>

<p>Each <a>environment settings object</a> has an associated {{StorageManager}} object. [[!HTML]]

<p>The <dfn attribute for=NavigatorStorage><code>storage</code></dfn> getter steps are to return
<a>this</a>'s <a>relevant settings object</a>'s {{StorageManager}} object.

<pre class=idl>
[SecureContext,
 Exposed=(Window,Worker)]
interface StorageManager {
  Promise&lt;boolean> persisted();
  [Exposed=Window] Promise&lt;boolean> persist();

  Promise&lt;StorageEstimate> estimate();
};

dictionary StorageEstimate {
  unsigned long long usage;
  unsigned long long quota;
};
</pre>

<p>The <dfn method for=StorageManager><code>persisted()</code></dfn> method steps are:

<ol>
 <li><p>Let <var>promise</var> be a new promise.

 <li><p>Let <var>shelf</var> be the result of running <a>obtain a local storage shelf</a> with
 <a>this</a>'s <a>relevant settings object</a>.

 <li><p>If <var>shelf</var> is failure, then reject <var>promise</var> with a {{TypeError}}.

 <li>
  <p>Otherwise, run these steps <a>in parallel</a>:

  <ol>
   <li>
    <p>Let <var>persisted</var> be true if <var>shelf</var>'s
    <a>bucket map</a>["<code>default</code>"]'s <a for="local storage bucket">mode</a> is
    "<code>persistent</code>"; otherwise false.

    <p class=note>It will be false when there's an internal error.

   <li><p><a>Queue a task</a> to resolve <var>promise</var> with <var>persisted</var>.
  </ol>

 <li><p>Return <var>promise</var>.
</ol>

<p>The <dfn method for=StorageManager><code>persist()</code></dfn> method steps are:

<ol>
 <li><p>Let <var>promise</var> be a new promise.

 <li><p>Let <var>shelf</var> be the result of running <a>obtain a local storage shelf</a> with
 <a>this</a>'s <a>relevant settings object</a>.

 <li><p>If <var>shelf</var> is failure, then reject <var>promise</var> with a {{TypeError}}.

 <li>
  <p>Otherwise, run these steps <a>in parallel</a>:

  <ol>
   <li>
    <p>Let <var>permission</var> be the result of <a>requesting permission to use</a>
    {{"persistent-storage"}}.

    <p class="note">User agents are encouraged to not let the user answer this question twice for
    the same <a for=/>origin</a> around the same time and this algorithm is not equipped to handle
    such a scenario.

   <li><p>Let <var>bucket</var> be <var>shelf</var>'s <a>bucket map</a>["<code>default</code>"].

   <li>
    <p>Let <var>persisted</var> be true if <var>bucket</var>'s
    <a for="local storage bucket">mode</a> is "<code>persistent</code>"; otherwise false.

    <p class=note>It will be false when there's an internal error.

   <li>
    <p>If <var>persisted</var> is false and <var>permission</var> is {{"granted"}}, then:

    <ol>
     <li><p>Set <var>bucket</var>'s <a for="local storage bucket">mode</a> to
     "<code>persistent</code>".

     <li><p>If there was no internal error, then set <var>persisted</var> to true.
    </ol>

   <li><p><a>Queue a task</a> to resolve <var>promise</var> with <var>persisted</var>.
  </ol>

 <li><p>Return <var>promise</var>.
</ol>

<p>The <dfn method for=StorageManager><code>estimate()</code></dfn> method steps are:

<ol>
 <li><p>Let <var>promise</var> be a new promise.

 <li><p>Let <var>shelf</var> be the result of running <a>obtain a local storage shelf</a> with
 <a>this</a>'s <a>relevant settings object</a>.

 <li><p>If <var>shelf</var> is failure, then reject <var>promise</var> with a {{TypeError}}.

 <li>
  <p>Otherwise, run these steps <a>in parallel</a>:

  <ol>
   <li><p>Let <var>usage</var> be <a>storage usage</a> for <var>shelf</var>.

   <li><p>Let <var>quota</var> be <a>storage quota</a> for <var>shelf</var>.

   <li><p>Let <var>dictionary</var> be a new {{StorageEstimate}} dictionary whose {{usage}} member
   is <var>usage</var> and {{quota}} member is <var>quota</var>.

   <li>
    <p>If there was an internal error while obtaining <var>usage</var> and <var>quota</var>, then
    <a>queue a task</a> to reject <var>promise</var> with a {{TypeError}}.

    <p class=note>Internal errors are supposed to be extremely rare and indicate some kind of
    low-level platform or hardware fault. However, at the scale of the web with the diversity of
    implementation and platforms, the unexpected does occur.

   <li><p>Otherwise, <a>queue a task</a> to resolve <var>promise</var> with <var>dictionary</var>.
  </ol>

 <li><p>Return <var>promise</var>.
</ol>



<h2 class=no-num id="acks">Acknowledgments</h2>

<p>With that, many thanks to
Adrian Bateman,
Aislinn Grigas,
Alex Russell,
Ali Alabbas,
Andrew Sutherland,
Ben Kelly,
Ben Turner,
Dale Harvey,
David Grogan,
Domenic Denicola,
fantasai,
Jake Archibald<!-- technically B.J. Archibald -->,
Jeffrey Yasskin,
Jesse Mykolyn,
Jinho Bang,
Jonas Sicking,
Joshua Bell,
Kenji Baheux,
Kinuko Yasuda,
Luke Wagner,
Michael Nordman,
Mounir Lamouri,
Shachar Zohar,
黃強 (Shawn Huang),
簡冠庭 (Timothy Guan-tin Chien), and
Victor Costan
for being awesome!

<p>This standard is written by <a lang=nl href=https://annevankesteren.nl/>Anne van Kesteren</a>
(<a href=https://www.mozilla.org/>Mozilla</a>,
<a href=mailto:annevk@annevk.nl>annevk@annevk.nl</a>).