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Our /file/serve Robot

Serve files to web browsers

🤖/file/serve serves files to web browsers.

When you want Transloadit to tranform files on the fly, you can use this Robot to determine which Step of a Template should be served to the end-user (via a CDN), as well as set extra information on the served files, such as headers. This way you can for instance suggest the CDN for how long to keep cached copies of the result around. By default, as you can see in the headers parameter, we instruct browsers to cache the result for 72h (259200 seconds) and CDNs to cache the content for 24h (86400 seconds). These values should be adjusted to suit your use case.

🤖/file/serve merely acts as the glue layer between our Assembly engine and serving files over HTTP. It let's you pick the proper result of a series of Steps via the use parameter and configure headers on the original content. That is where its responsibilies end, and 🤖/tlcdn/deliver, then takes over to globally distribute this original content across the globe, and make sure that is cached close to your end-users, when they make requests such as https://my-app.tlcdn.com/resize-img/canoe.jpg?w=500, another. 🤖/tlcdn/deliver is not a part of your Assembly Instructions, but it may appear on your invoices as bandwidth charges incur when distributing the cached copies. 🤖/file/serve only charges when the CDN does not have a cached copy and requests to regenerate the original content, which depending on your caching settings could be just once a month, or year, per file/transformation.

While theoretically possible, you could use 🤖/file/serve directly in HTML files, but we strongly recommend against this, because if your site gets popular and the media URL that /file/serve is handling gets hit one million times, that is one million new image resizes. Wrapping it with a CDN (and thanks to the caching that comes with it) makes sure encoding charges stay low, as well as latencies.

More information on:


  • use

    String / Array of Strings / Object required

    Specifies which Step(s) to use as input.

    • You can pick any names for Steps except ":original" (reserved for user uploads handled by Transloadit)

    • You can provide several Steps as input with arrays:

      "use": [

    💡 That’s likely all you need to know about use, but you can view Advanced use cases.

  • headers

    Object ⋅ default: { "Access-Control-Allow-Headers": "X-Requested-With, Content-Type, Cache-Control, Accept, Content-Length, Transloadit-Client, Authorization", "Access-Control-Allow-Methods": "POST, GET, PUT, DELETE, OPTIONS", "Access-Control-Allow-Origin" : "*", "Cache-Control": "public, max-age=259200, s-max-age=86400", "Content-Type": "${file.mime}; charset=utf-8", "Transfer-Encoding": "chunked", "Transloadit-Assembly": "…", "Transloadit-RequestID": "…" }

    An object containing a list of headers to be set for a file as we serve it to a CDN/web browser, such as { FileURL: "${file.url_name}" } which will be merged over the defaults, and can include any available Assembly Variable.

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