CODEWITHSUNDEEP
javaamazon-web-servicesaws-lambda
user3139545
user3139545

Reputation: 7374

Guaranteed to run function before AWS lambda exits

Is there a way in the JVM to guarantee that some function will run before a AWS lambda function will exit? I would like to flush an internal buffer to stdout as a last action in a lambda function even if some exception is thrown.

Upvotes: 1

Views: 2468

Answers (2)

Antarr Byrd
Antarr Byrd

Reputation: 26061

aws-samples shows how to do it here

package helloworld;

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;
import java.net.URL;
import java.util.HashMap;
import java.util.Map;
import java.util.stream.Collectors;

import com.amazonaws.services.lambda.runtime.Context;
import com.amazonaws.services.lambda.runtime.RequestHandler;

/**
 * Handler for requests to Lambda function.
 */
public class App implements RequestHandler<Object, Object> {

    static {
        Runtime.getRuntime().addShutdownHook(new Thread() {
            @Override
            public void run() {
                System.out.println("[runtime] ShutdownHook triggered");

                System.out.println("[runtime] Cleaning up");
                // perform actual clean up work here.
                try {
                    Thread.sleep(200);
                 } catch (Exception e) {
                    System.out.println(e);
                 }

                System.out.println("[runtime] exiting");
                System.exit(0);
            }
        });
    }

    public Object handleRequest(final Object input, final Context context) {
        Map<String, String> headers = new HashMap<>();
        headers.put("Content-Type", "application/json");
        headers.put("X-Custom-Header", "application/json");
        try {
            final String pageContents = this.getPageContents("https://checkip.amazonaws.com");
            String output = String.format("{ \"message\": \"hello world\", \"location\": \"%s\" }", pageContents);
            return new GatewayResponse(output, headers, 200);
        } catch (IOException e) {
            return new GatewayResponse("{}", headers, 500);
        }
    }

    private String getPageContents(String address) throws IOException {
        URL url = new URL(address);
        try (BufferedReader br = new BufferedReader(new InputStreamReader(url.openStream()))) {
            return br.lines().collect(Collectors.joining(System.lineSeparator()));
        }
    }
}

Upvotes: 0

s.hesse
s.hesse

Reputation: 2060

As far as I understand you want to execute some code before your Lambda function is stopped, regardless what your execution state is (running/waiting/exception handling/etc).

This is not possible out of the box with Lambda, i.e. there is no event fired or something similar which can be identified as a shutdown hook. The JVM will be freezed as soon as you hit the timeout. However, you can observe the remaining execution time by using the method getRemainingTimeInMillis() from the Context object. From the docs:

Returns the number of milliseconds left before the execution times out.

So, when initializing your function you can schedule a task which is regularly checking how much time is left until your Lambda function reaches the timeout. Then, if only less than X (milli-)seconds are left, you do Y.

Upvotes: 1

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