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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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