AWS APIGW - Access static HTTP Headers for Integration Request in Mapping Template for Lambda

Question

I am trying to modify an API GW config similar to what is found here https://oozio.medium.com/serverless-discord-bot-55f95f26f743

What I need to do is pass to lambda a custom static HTTP Header. I have defined in APIGW -> /{resource} -> Integration Request -> HTTP Headers : a header like {"Name": "application", "Mapped from": "'discord'", "Caching": false} (with single ticks as noted in the documentation for a static value)

This header never appears in the list of headers from the mapping template. I've spent the better part of a day trying to solve this.

Simplified question:

How do you access a static HTTP Header defined in Integration Request, in the Mapping Template?

TIA

Answer 1

After working with AWS support, this is apparently not possible.

Many Thanks for your kind patience. After our call I started to look for a possible
solution to fetch the statically set HTTP header in the mapping template, but
unfortunately could not find any solution. 

I then discussed this scenario with my colleagues and also reached out to the
back-end API Gateway Service Team. It seems that this is something that is not
possible currently. 
We cannot access the static header value defined in the integration request, in
the request body mapping template. If your use case needs to send the static header
in body for non proxy integration then it is recommended that you may define it
statically in the body mapping template directly.

I understand as you explained to me how you need to access the header in the mapping
template but unfortunately this does not seem to have a solution. Although the
statically set header is seen to be passed in the API Gateway logs, there is no
mechanism in the lambda service to fetch these headers. 

Currently, I can ask the Service Team to consider the same as a feature request but
I wont be having a ETA available and also would need a solid reasoning since the
same can be done in the mapping template. 

The workaround at this time (March 2021) has to be expressing it directly in the mapping template itself, or using Proxy for Lambda.

The following was provided as justification for the feature request.

When creating APIGW via a templating process (Such as terraform), being able to
express 'API Wide' variables is a useful tool for sending contextual information
to a backend integration. For example, a Lambda that can process requests from
multiple sources, but it needs to know which source should be used. While the
value can be statically expressed in the mapping template, having it be a
variable as part of the API allows for better visibility and management as its
an interact-able component, where as the mapping template is effectively a
complex string. This also does not need to be a header, it could simply be
"integration variables" that allow you to define simple k:v pairs that are either
passed as part of context, or accessible in $context within the mapping template.
While statically expressing it in the mapping template is a workaround, it is
only that.