API Gateway with SAM isn't updated correctly - amazon-web-services

Question

We use Cloud Formation for define a bunch of Lambda functions:
AWSTemplateFormatVersion: '2010-09-09'
Transform:
- 'AWS::Serverless-2016-10-31'
Resources:
MyLambda:
Type: 'AWS::Serverless::Function'
Properties:
Handler: com.handler::MyLambda
Runtime: java8
CodeUri: .
Description: Some desc
MemorySize: 512
Timeout: 15
Role: !Ref LambdaRole
FunctionName: MyLambda
Events:
MyLambdaEvt:
Type: Api
Properties:
RestApiId: !Ref MyApiDef
Path: /lambda/my
Method: get
MyApiDef:
Type: AWS::Serverless::Api
Properties:
DefinitionUri: s3://a-bucket/api-gateway.yml
StageName: prod
Outputs:
ApiUrl:
Description: URL of your API endpoint
Value: !Join
- ''
- - https://
- !Ref MyApiDef
- '.execute-api.'
- !Ref 'AWS::Region'
- '.amazonaws.com/prod'
A CodePipeline generate a changeset and execute it.
In this way all the Lambda function are correctly updated but the API Gateway endpoint are not update correctly and we need to import and deploy the YML in s3://a-bucket/api-gateway.yml manually.

Answer 1

Why the API doesn't update (an educated guess)
In order for a change to be added to a a change set, CloudFormation has to detect a change. If the only thing that changes (for MyApiDef) between deployments is the contents of the .yaml file out on S3, CloudFormation isn't going to detect a change that it needs to add to the change set.
If this API definition lived in the CF template, rather than a file on S3, CF would (obviously) detect every change and update the API for you.
Since the definition lives in S3, and the file name hasn't changed, no change is detected, so nothing gets updated.
Possible work arounds
You have to convince CloudFormation that something has changed with your API definition. These two things worked for me:
Updating the MyApiDef key itself each run works. (MyApiDefv2,
MyApiDefv3, etc)
Updating the DefinitionUri works. (i.e. version the
filename in S3).
Neither of these is great, but appending a version to the filename in S3 seems more reasonable than the other option.
There are probably other ways to convince CloudFormation a change has taken place. Notably, I could not get Variables to work for this purpose.