Real-time applications with API Gateway WebSockets and AWS Lambda https://serverless.com/blog/api-gateway-websockets-support/

Calling API gateways via generated SDKs https://docs.aws.amazon.com/apigateway/latest/developerguide/how-to-generate-sdk-javascript.html

https://docs.aws.amazon.com/apigateway/latest/developerguide/http-api-vs-rest.html

REST-Api has more functionality regarding inspection.

https://docs.aws.amazon.com/lambda/latest/dg/API_Invoke.html#API_Invoke_RequestSyntax

The API must invoke the backend Lambda function using the HTTP method POST. https://docs.aws.amazon.com/lambda/latest/dg/API_Invoke.html#API_Invoke_RequestSyntax

Otherwise you will get

• a 500 error on the Gateway
• a 403 error on the Lambda, behind the Gateway https://aws.amazon.com/premiumsupport/knowledge-center/api-gateway-lambda-template-invoke-error/

If a Lambda does not behave as intended - use the Test button. ONe can see much more exceptions in there.

https://www.baeldung.com/aws-lambda-api-gateway

Example: https://github.com/skipidar/aws-javalambda-behind-apigateway

There are obligations on the Java-Lambda signature, when executed behind Gateway.

It is important to have

• InputStream inputStream,
• OutputStream outputStream

in the interface of the Lambda - otherwise the API Gateway will throw exceptions

Dependencies

dependencies {
    implementation 'com.amazonaws:aws-lambda-java-core:1.2.1'
    implementation 'com.google.code.gson:gson:2.8.6'
    testImplementation 'org.apache.logging.log4j:log4j-api:2.13.0'
    testImplementation 'org.apache.logging.log4j:log4j-core:2.13.0'
    testImplementation 'org.apache.logging.log4j:log4j-slf4j18-impl:2.13.0'
    testImplementation 'org.junit.jupiter:junit-jupiter-api:5.6.0'
    testRuntimeOnly 'org.junit.jupiter:junit-jupiter-engine:5.6.0'

    compile 'com.googlecode.json-simple:json-simple:1.1.1'
}

package example;

import com.amazonaws.services.lambda.runtime.Context;
import com.amazonaws.services.lambda.runtime.LambdaLogger;
import com.amazonaws.services.lambda.runtime.RequestStreamHandler;
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.nimbusds.oauth2.sdk.TokenRequest;
import no.nav.security.mock.oauth2.MockOAuth2Server;
import no.nav.security.mock.oauth2.token.OAuth2TokenCallback;
import org.jetbrains.annotations.NotNull;
import org.jetbrains.annotations.Nullable;
import org.json.simple.JSONObject;
import org.json.simple.parser.JSONParser;
import org.json.simple.parser.ParseException;

import java.io.*;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.Map;
import java.util.concurrent.TimeUnit;


// Handler value: example.Handler
public class Handler implements RequestStreamHandler {

    /**
     * The handleRequest interface, as below,
     * with InputStream, OutputStream
     *
     * when Lambda is used with the API Gateway - is required.
     *
     * https://www.baeldung.com/aws-lambda-api-gateway
     *
     * @param inputStream
     * @param outputStream
     * @param context
     * @throws IOException
     */
    public void handleRequest(InputStream inputStream, OutputStream outputStream, Context context) throws IOException {
        JSONParser parser = new JSONParser();
        BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));
        JSONObject responseJson = new JSONObject();

        JSONObject event = null;
        try {
            event = (JSONObject) parser.parse(reader);

            handleEvent(event,
                    responseJson,
                    inputStream,
                    outputStream,
                    context);

        } catch (ParseException e) {
            responseJson.put("statusCode", 400);
            responseJson.put("exception", e);
        }
        
        // example response
        JSONObject obj2 = new JSONObject();
        obj2.put("Content-Type", "application/json");
        responseJson.put("statusCode", 200);
        responseJson.put("headers", obj2);
        responseJson.put("body", "hello world");

        // writing the result into the
        OutputStreamWriter writer = new OutputStreamWriter(outputStream, "UTF-8");
        writer.write(responseJson.toString());
        writer.close();
    }

Lambdas needs to return JSON, to be capable to be used from the gateway. https://docs.amazonaws.cn/en_us/apigateway/latest/developerguide/set-up-lambda-proxy-integrations.html#api-gateway-simple-proxy-for-lambda-output-format

// Handler value: example.Handler
public class Handler implements RequestStreamHandler {
    public void handleRequest(InputStream inputStream, OutputStream outputStream, Context context) throws IOException {
        // example response
        JSONObject obj2 = new JSONObject();
        obj2.put("Content-Type", "application/json");
        responseJson.put("statusCode", 200);
        responseJson.put("headers", obj2);
        responseJson.put("body", "hello world");

        // writing the result into the
        OutputStreamWriter writer = new OutputStreamWriter(outputStream, "UTF-8");
        writer.write(responseJson.toString());
        writer.close();

When a client submits an API request, API Gateway passes to the integrated Lambda function the raw request as-is:

That means, that the option `Use Lambda Proxy integration`

• deactivates the “Mapping Templates”
• https://aws.amazon.com/premiumsupport/knowledge-center/custom-headers-api-gateway-lambda/

This is the most powerful integration:

1. \proxy sends all resources - to the Lambda
2. ANY - accepts all methods

https://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-set-up-simple-proxy.html#api-gateway-proxy-resource%3Ficmpid=docs_apigateway_console

• Still the API must be deployed to a “stage”
• Minimum one resource must be present after the stage - or you will get “{“message”:“Missing Authentication Token”}”

After that the lambda reacts on

curl -X POST https://s5oi8r61hk.execute-api.eu-west-1.amazonaws.com/stage/someresourcehere/1/2/bla/blub/enigma21/
curl -X GET https://s5oi8r61hk.execute-api.eu-west-1.amazonaws.com/stage/2/

Can be used to add custom headers from the API gateway to the Lambda

https://aws.amazon.com/premiumsupport/knowledge-center/custom-headers-api-gateway-lambda/

The token exchange is possible with 2 pieces:

• obtain a new token at the lambda authorizer
• set the header via VLT as in

https://stackoverflow.com/questions/40585016/is-it-possible-to-add-an-http-header-from-aws-custom-auth-on-api-gateway

An overview of the API gateway functions. https://www.alexdebrie.com/posts/api-gateway-elements/