===== API Gateway =====


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






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

REST-Api has more functionality regarding inspection.


===== Lambda behind the API gateway =====

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


====  Fallpit: Invoke Lambda POST ==== 

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/

==== Debugging ====

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

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==== Java Lambda behind the API gateway ====
https://www.baeldung.com/aws-lambda-api-gateway


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

=== Java signature ===
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
<code>

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'
}
</code>



<sxh java>
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();
    }
</sxh>


=== Return format of Lambda ===

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

<sxh java>

// 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();
</sxh>


====  Use Lambda Proxy integration ==== 

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/


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This is the most powerful integration:
  - **\{{proxy}}** sends all resources - to the Lambda
  - **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

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  * <color #ed1c24>Still the API must be deployed to a "stage"</color>
  * <color #ed1c24>Minimum one resource must be present after the stage - or you will get "{"message":"Missing Authentication Token"}"</color>

After that the lambda reacts on
<code>
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/
</code>

==== Mapping Templates ====

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/




==== Token Exchange =====
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



===== A Detailed Overview of AWS API Gateway =====
An overview of the API gateway functions.
https://www.alexdebrie.com/posts/api-gateway-elements/

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