How can I test locally using sam local start-lambda with a Rust lambda?

[shirakaba]

I've set up a lambda with CDK by referring to the repository structure used by rnag/rust.aws-cdk-lambda/cdk-examples/rust-standalone, whilst matching this repo's up-to-date dependencies.

I want to do local testing (run this lambda as a server on localhost and make HTTP GET requests to it), so I've tried running this:

$ sam local start-lambda --port 3000 --template ./cdk.out/RustStandaloneStack.template.json   
Starting the Local Lambda Service. You can now invoke your Lambda Functions defined in your template through the endpoint.
2023-01-23 11:08:56  * Running on http://127.0.0.1:3000/ (Press CTRL+C to quit)

The server successfully starts up, but I can't figure out what sort of request to send it. It just responds with a 404 no matter what path or headers I specify to it with my GET requests.

By comparison, one-off invocations work just fine:

# ./tests/sample.json is an event matching the same format of the one seen in this issue comment:
# https://github.com/awslabs/aws-lambda-rust-runtime/issues/365#issue-1054795934
$ sam local invoke MyRustLambda --event ./tests/sample.json --template ./cdk.out/RustStandaloneStack.template.json

Requesting using the exact same path and headers specified in my ./tests/sample.json file, I am met with a 404 response.

This feels like an underdocumented area in the SAM/CDK/Lambda docs so any pointers would be greatly appreciated.

[calavera]

I'm not familiar with sam local, but these links might help you test locally:

https://www.cargo-lambda.info/commands/watch.html
https://www.cargo-lambda.info/commands/invoke.html
https://www.cargo-lambda.info/guide/screencasts.html#cargo-lambda-watch

[bnusunny]

sam local start-lambda starts up a local Lambda Service endpoint. You can find out the request format from Lambda API doc. You can send POST request to this endpoint via curl or AWS SDK.

AWS CLI is a simpler way to invoke lambda via this endpoint. More details here.

aws lambda invoke --function-name "HelloWorldFunction" --endpoint-url "http://127.0.0.1:3001" --no-verify-ssl out.txt

[shirakaba]

Thank you both very much for the timely response that let me get straight back to work! I'll check through @calavera's resources as well in a moment, but @bnusunny's Lambda API docs look like exactly the piece of the puzzle I've been missing.

Recap

To recap, I'm trying to translate the following sam local invoke to a sam local start-lambda + HTTP request (for testing a web app):

sam local invoke MyRustLambda --event ./tests/sample.json --template ./cdk.out/RustStandaloneStack.template.json

... where tests/sample.json could be as complex as this example or as simple as just:

{
  "requestContext": {
    "http": {
      "method": "GET"
    },
    "timeEpoch": 1674445263
  }
}

Step one: sam local start-lambda

We begin by starting up the lambda as a HTTP server:

sam local start-lambda --port 3000 --template ./cdk.out/RustStandaloneStack.template.json

Step two: the HTTP request

Background

Looking at the Lambda API doc, it says the POST request should be in the following format:

POST /2015-03-31/functions/FunctionName/invocations?Qualifier=Qualifier HTTP/1.1
X-Amz-Invocation-Type: InvocationType
X-Amz-Log-Type: LogType
X-Amz-Client-Context: ClientContext

Conclusion

So in my case, as I can leave X-Amz-Client-Context blank (given that my Lambda doesn't make use of it), my HTTP request would become:

curl -X POST --data @./tests/sample.json \
  --header "X-Amz-Invocation-Type: RequestResponse" \
  --header "X-Amz-Log-Type: None" \
  http://localhost:3000/2015-03-31/functions/MyRustLambda/invocations

And as the headers are inconsequential (the first header asks for the default value and I don't mind whether I get logs or not), I can simplify that further to just:

curl -X POST --data @./tests/sample.json http://localhost:3000/2015-03-31/functions/MyRustLambda/invocations