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