MediaTailor pre-fetching on Live content

Hi,

I have a Live stream source (HLS) broadcasting 24/24h 7/7d where a want to use Ad Replacement capability of MediaTailor. My Ads are related to the content of the stream and will never be displayed twice in the stream lifespan (but to multiple users, yes). The way MediaTailor works is by transcoding the ads at the first VAST call and inserting the ad only at the second one. In my case it doesn't works due to the changing nature of my ads.

I saw the pre-fetching feature but I don't understand how I can use it to solve my challenge. Pre-fetching Retrieval and Consomption both have "start date" end "end date" and I don't know how to fill these.

My two ideas are:

• Skip MediaTailor transcoding by providing HLS (matching content stream codecs, resolution and bitrate) in the VAST
• Use the pre-fetch feature to call my ad server 3mins before the placements to stack all the transcoding job (on MediaTailor side) before the ad must be displayed (knowing that I will have multiple campaign running on the same placement)

Someone know how I can setup one or the other?

Thanks, Mathieu