In a serverless.yml file, how does one customize the default http response template? - aws-lambda

Question

Looking at the documentation here https://serverless.com/framework/docs/providers/aws/events/apigateway/#custom-response-templates, there doesn't seem to be much detail at all about setting up these templates.
I'm currently looking to remove the default application/json content-type, which is generated during creation of the handler's integration response (pictured below), and replace it with text/html. Is there defined syntax for how to do this hidden somewhere? Or is this level of customization not possible within the current scope of the framework?
Here is my endpoint as defined in serverless.yml
events:
- http:
method: any
path: /
integration: lambda
request:
region: ${env:AwsRegion}
response:
headers:
Content-Type: "'text/html'"
template: $input.path('body')
- http:
method: any
path: /{proxy+}
Which produces the following configuration on AWS Api Gateway:
I did try modifying the specification like this, as a guess, but it threw a syntax error:
template:
text\html: $input.path('body')

Answer 1

Make It Work
It looks like this isn't really supported by the framework, but it can be hacked together by (ab)using statusCodes in your serverless template.
Moving the response template to under a status code, and providing a pattern for that status code, accomplishes what I think you are after. The syntax:
- http:
method: any
path: /
integration: lambda
request:
region: us-east-1
response:
headers:
Content-Type: "'text/html'"
statusCodes:
200:
pattern: ""
template:
text/html: $input.path('body')
Note: Both the pattern and the template must be present.
Is It Really That Bad?
That's up to you, ultimately. I'm calling it a hack because:
It would be much nicer to be able to supply this at the response.template level, rather than at response.statusCodes.200.template.
Specifying one or more statusCodes removes the set of default lambda error regexes that you get when you don't specify any.
It feels like working around the fact that response.template will only accept a string, whereas it could (should?) accept a string or an object (like it does under statusCodes).
Fix It?
The offending code, from /lib/plugins/aws/package/compile/events/apiGateway/lib/method/integration.js:
if (http.response.template) {
_.merge(integrationResponse.ResponseTemplates, {
'application/json': http.response.template,
});
}
if (config.template) {
const template =
typeof config.template === 'string'
? { 'application/json': config.template }
: config.template;
_.merge(integrationResponse.ResponseTemplates, template);
}
I think for this to work under response.template, the code in the first if() would need to behave more like the code in the second if().