Monitoring the health of your canary allows CodeDeploy to make a decision to whether a rollback is needed or not. If any of the CloudWatch Alarms specified gets to ALARM status, CodeDeploy rollsback the deployment automatically.

Introduce an error on purpose

Lets break the Lambda function on purpose so that the CanaryErrorsAlarm gets triggered during deployment. Update the lambda code in /sam-app/HelloWorldFunction/src/main/java/helloworld/ to throw an error on every invocation, like this:

package helloworld;

import java.util.HashMap;
import java.util.Map;


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

    public APIGatewayProxyResponseEvent handleRequest(final APIGatewayProxyRequestEvent input, final Context context) {
        throw new Error("This will cause a deployment rollback");
        // Map<String, String> headers = new HashMap<>();
        // headers.put("Content-Type", "application/json");
        // headers.put("X-Custom-Header", "application/json");

        // APIGatewayProxyResponseEvent response = new APIGatewayProxyResponseEvent()
        //         .withHeaders(headers);
        // try {
        //     final String pageContents = this.getPageContents("");
        //     String output = String.format("{ \"message\": \"hello beautiful world\", \"location\": \"%s\" }", pageContents);

        //     return response
        //             .withStatusCode(200)
        //             .withBody(output);
        // } catch (IOException e) {
        //     return response
        //             .withBody("{}")
        //             .withStatusCode(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()));

Make sure to update the unit test, otherwise the build will fail. Comment out every line in the /sam-app/HelloWorldFunction/src/test/java/helloworld/ file:

package helloworld;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertNotNull;
import static org.junit.Assert.assertTrue;
import org.junit.Test;

public class AppTest {
  public void successfulResponse() {
    // App app = new App();
    // APIGatewayProxyResponseEvent result = app.handleRequest(null, null);
    // assertEquals(result.getStatusCode().intValue(), 200);
    // assertEquals(result.getHeaders().get("Content-Type"), "application/json");
    // String content = result.getBody();
    // assertNotNull(content);
    // assertTrue(content.contains("\"message\""));
    // assertTrue(content.contains("\"hello beautiful world\""));
    // assertTrue(content.contains("\"location\""));

Push the changes

In the terminal, run the following commands from the root directory of your sam-app project.

git add .
git commit -m "Breaking the lambda function on purpose"
git push