Lambda@Edge custom 'Server-Timing' missing

Question

I have a Lambda@Edge on `origin-request` and it returns a `Server-Timing` header, however the value is ignored or missing.

According to the Cloudfront documentation on [Server-Timing header metrics](https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/understanding-response-headers-policies.html#server-timing-header):

"The response that CloudFront sends to the viewer contains a single Server-Timing header that includes the **value that came from the origin and the metrics that CloudFront added**." (emphasis mine)

I also don't see any documentation saying Lambda@Edge it's not allowed to return a `Server-Timing` header.

Here are the steps I tried:

- Setting the response header policy with a "Custom headers" set to `Server-Timing` and override `false`, the value is always missing.
- Setting the response header with "Server-Timing header" enabled returns **only** Cloudfront metrics and ignores my custom Lambda@Edge response.

Looks like a bug or missing documentation, thank you in advance!

Answer

Is your Lambda function generating and returning the whole response? If it is passing the request through to an origin, I think you'd need to add server-timing on the **Origin Response** trigger, rather than Origin Request. Either way, the server-timing header generated by your Lambda@Edge function or the Origin will only be sent to the user in the event of a cache miss. If your requests are being served from cache, the value generated by the origin or Lambda@Edge for server-timing will not be present.

Lambda@Edge custom 'Server-Timing' missing

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