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-URL Previews
-============
-
-The `GET /_matrix/media/r0/preview_url` endpoint provides a generic preview API
-for URLs which outputs [Open Graph](https://ogp.me/) responses (with some Matrix
-specific additions).
-
-This does have trade-offs compared to other designs:
-
-* Pros:
- * Simple and flexible; can be used by any clients at any point
-* Cons:
- * If each homeserver provides one of these independently, all the HSes in a
- room may needlessly DoS the target URI
- * The URL metadata must be stored somewhere, rather than just using Matrix
- itself to store the media.
- * Matrix cannot be used to distribute the metadata between homeservers.
-
-When Synapse is asked to preview a URL it does the following:
-
-1. Checks against a URL blacklist (defined as `url_preview_url_blacklist` in the
- config).
-2. 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.)
-3. Kicks off a background process to generate a preview:
- 1. Checks the database cache by URL and timestamp and returns the result if it
- has not expired and was successful (a 2xx return code).
- 2. Checks if the URL matches an [oEmbed](https://oembed.com/) pattern. If it
- does, update the URL to download.
- 3. Downloads the URL and stores it into a file via the media storage provider
- and saves the local media metadata.
- 4. If the media is an image:
- 1. Generates thumbnails.
- 2. Generates an Open Graph response based on image properties.
- 5. If the media is HTML:
- 1. Decodes the HTML via the stored file.
- 2. Generates an Open Graph response from the HTML.
- 3. If a JSON oEmbed URL was found in the HTML via autodiscovery:
- 1. Downloads the URL and stores it into a file via the media storage provider
- and saves the local media metadata.
- 2. Convert the oEmbed response to an Open Graph response.
- 3. Override any Open Graph data from the HTML with data from oEmbed.
- 4. If an image exists in the Open Graph response:
- 1. Downloads the URL and stores it into a file via the media storage
- provider and saves the local media metadata.
- 2. Generates thumbnails.
- 3. Updates the Open Graph response based on image properties.
- 6. If the media is JSON and an oEmbed URL was found:
- 1. Convert the oEmbed response to an Open Graph response.
- 2. If a thumbnail or image is in the oEmbed response:
- 1. Downloads the URL and stores it into a file via the media storage
- provider and saves the local media metadata.
- 2. Generates thumbnails.
- 3. Updates the Open Graph response based on image properties.
- 7. Stores the result in the database cache.
-4. Returns the result.
-
-The in-memory cache expires after 1 hour.
-
-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.
|
