Micronaut ❤️ AWS Lambda 🛠

CloudSpout

Jun 3, 2020·2 min read

Micronaut is a great way to combine Java with AWS’ Lambda — but the context remains a challenge:

Micronaut provides a lot of pluming and guides on how to use it with AWS Lambda: It provides facilities for regular Lambdas as also API-Gatway integration in proxy mode. 😃

While the guides work great and any Java-Spring developer has a ‘Home sweet home’ feeling right from the get-go; there remain one challenge:

Challenge

While the concept of an (application-) context is second nature for (nearly) every Java developer it is still hard to figure out what can be @Injected in a Micronaut application — especially in in the AWS Lambda world: Each call to the handler method carries context which needs to be accessed.

This becomes especially important for any non-trivial application that needs access to the authentication details or the Cognito identity (while being integrated via API gateway)

Actually, the javadoc documentation is the best point to figure out which types can be autowired:

Solution

1. To figure out which ‘beans’ can be injected head over to the API summary at
   https://micronaut-projects.github.io/micronaut-aws/1.4.1/api/index.html?overview-summary.html
2. Search for the types you are interested in. Ie. the AwsProxyRequestContext
3. Confirm that there is a matching ArgumentBinder type — actually something that implements io.micronaut.http.bind.bindesr.TypedRequestArgumentBinder
4. Use such a type in your Micronaut Lambda:

@Put("/{id}")
public HttpResponse update(
    @PathVariable("id") String id,
    @Body MyRequest request,
    AwsProxyRequestContext context) {

In this example, the AwsProxyRequestContext is the actual AWS’ Lamda SDK type itself but there exists numerous views on the current request to inject.

As of Micronaut 2.0.0-M3 the following types can be injected as argument:

• com.amazonaws.serverless.proxy.model.AwsProxyRequestContext
• io.micronaut.http.BasicAuth
• com.amazonaws.services.lambda.runtime.Context
• For the body:
  - io.reactivex.Maybe
  - java.util.concurrent.CompletableFuture
  - io.micronaut.http.server.multipart.MultipartBody
  - io.reactivex.Observable
  - org.reactivestreams.Publisher
  - io.reactivex.Single

Regular @Introspected/ @Singular still can be @Inject ed on a class level into the controller:

@Controller("/api/db/")
public class ServerController {
  @Inject private DynamoDBServiceImpl dynamoDBService;