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<h1 id="url-previews"><a class="header" href="#url-previews">URL Previews</a></h1>
<p>The <code>GET /_matrix/media/r0/preview_url</code> endpoint provides a generic preview API
for URLs which outputs <a href="https://ogp.me/">Open Graph</a> responses (with some Matrix
specific additions).</p>
<p>This does have trade-offs compared to other designs:</p>
<ul>
<li>Pros:
<ul>
<li>Simple and flexible; can be used by any clients at any point</li>
</ul>
</li>
<li>Cons:
<ul>
<li>If each homeserver provides one of these independently, all the HSes in a
room may needlessly DoS the target URI</li>
<li>The URL metadata must be stored somewhere, rather than just using Matrix
itself to store the media.</li>
<li>Matrix cannot be used to distribute the metadata between homeservers.</li>
</ul>
</li>
</ul>
<p>When Synapse is asked to preview a URL it does the following:</p>
<ol>
<li>Checks against a URL blacklist (defined as <code>url_preview_url_blacklist</code> in the
config).</li>
<li>Checks the in-memory cache by URLs and returns the result if it exists. (This
is also used to de-duplicate processing of multiple in-flight requests at once.)</li>
<li>Kicks off a background process to generate a preview:
<ol>
<li>Checks the database cache by URL and timestamp and returns the result if it
has not expired and was successful (a 2xx return code).</li>
<li>Checks if the URL matches an <a href="https://oembed.com/">oEmbed</a> pattern. If it
does, update the URL to download.</li>
<li>Downloads the URL and stores it into a file via the media storage provider
and saves the local media metadata.</li>
<li>If the media is an image:
<ol>
<li>Generates thumbnails.</li>
<li>Generates an Open Graph response based on image properties.</li>
</ol>
</li>
<li>If the media is HTML:
<ol>
<li>Decodes the HTML via the stored file.</li>
<li>Generates an Open Graph response from the HTML.</li>
<li>If an image exists in the Open Graph response:
<ol>
<li>Downloads the URL and stores it into a file via the media storage
provider and saves the local media metadata.</li>
<li>Generates thumbnails.</li>
<li>Updates the Open Graph response based on image properties.</li>
</ol>
</li>
</ol>
</li>
<li>If the media is JSON and an oEmbed URL was found:
<ol>
<li>Convert the oEmbed response to an Open Graph response.</li>
<li>If a thumbnail or image is in the oEmbed response:
<ol>
<li>Downloads the URL and stores it into a file via the media storage
provider and saves the local media metadata.</li>
<li>Generates thumbnails.</li>
<li>Updates the Open Graph response based on image properties.</li>
</ol>
</li>
</ol>
</li>
<li>Stores the result in the database cache.</li>
</ol>
</li>
<li>Returns the result.</li>
</ol>
<p>The in-memory cache expires after 1 hour.</p>
<p>Expired entries in the database cache (and their associated media files) are
deleted every 10 seconds. The default expiration time is 1 hour from download.</p>
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