Writing a Node.js canary script - Amazon CloudWatch
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Writing a Node.js canary script

Creating a CloudWatch Synthetics canary from scratch

Here is an example minimal Synthetics Canary script. This script passes as a successful run, and returns a string. To see what a failing canary looks like, change let fail = false; to let fail = true;.

You must define an entry point function for the canary script. To see how files are uploaded to the Amazon S3 location specified as the canary's ArtifactS3Location, create these files under the /tmp folder. After the script runs, the pass/fail status and the duration metrics are published to CloudWatch and the files under /tmp are uploaded to S3.

const basicCustomEntryPoint = async function () { // Insert your code here // Perform multi-step pass/fail check // Log decisions made and results to /tmp // Be sure to wait for all your code paths to complete // before returning control back to Synthetics. // In that way, your canary will not finish and report success // before your code has finished executing // Throw to fail, return to succeed let fail = false; if (fail) { throw "Failed basicCanary check."; } return "Successfully completed basicCanary checks."; }; exports.handler = async () => { return await basicCustomEntryPoint(); };

Next, we'll expand the script to use Synthetics logging and make a call using the Amazon SDK. For demonstration purposes, this script will create an Amazon DynamoDB client and make a call to the DynamoDB listTables API. It logs the response to the request and logs either pass or fail depending on whether the request was successful.

const log = require('SyntheticsLogger'); const AWS = require('aws-sdk'); // Require any dependencies that your script needs // Bundle additional files and dependencies into a .zip file with folder structure // nodejs/node_modules/additional files and folders const basicCustomEntryPoint = async function () { log.info("Starting DynamoDB:listTables canary."); let dynamodb = new AWS.DynamoDB(); var params = {}; let request = await dynamodb.listTables(params); try { let response = await request.promise(); log.info("listTables response: " + JSON.stringify(response)); } catch (err) { log.error("listTables error: " + JSON.stringify(err), err.stack); throw err; } return "Successfully completed DynamoDB:listTables canary."; }; exports.handler = async () => { return await basicCustomEntryPoint(); };

Packaging your canary files

If you are uploading your canary scripts using an Amazon S3 location, your zip file should include your script under this folder structure: nodejs/node_modules/myCanaryFilename.js file.

If you have more than a single .js file or you have a dependency that your script depends on, you can bundle them all into a single ZIP file that contains the folder structure nodejs/node_modules/myCanaryFilename.js file and other folders and files. If you are using syn-nodejs-puppeteer-3.4 or later, you can optionally put your canary files in another folder and creating your folder structure like this: nodejs/node_modules/myFolder/myCanaryFilename.js file and other folders and files.

Handler name

Be sure to set your canary’s script entry point (handler) as myCanaryFilename.functionName to match the file name of your script’s entry point. If you are using a runtime earlier than syn-nodejs-puppeteer-3.4, then functionName must be handler. If you are using syn-nodejs-puppeteer-3.4 or later, you can choose any function name as the handler. If you are using syn-nodejs-puppeteer-3.4 or later, you can also optionally store the canary in a separate folder such as nodejs/node_modules/myFolder/my_canary_filename. If you store it in a separate folder, specify that path in your script entry point, such as myFolder/my_canary_filename.functionName.

Changing an existing Puppeteer script to use as a Synthetics canary

This section explains how to take Puppeteer scripts and modify them to run as Synthetics canary scripts. For more information about Puppeteer, see Puppeteer API v1.14.0.

We'll start with this example Puppeteer script:

const puppeteer = require('puppeteer'); (async () => { const browser = await puppeteer.launch(); const page = await browser.newPage(); await page.goto('https://example.com'); await page.screenshot({path: 'example.png'}); await browser.close(); })();

The conversion steps are as follows:

  • Create and export a handler function. The handler is the entry point function for the script. If you are using a runtime earlier than syn-nodejs-puppeteer-3.4, the handler function must be named handler. If you are using syn-nodejs-puppeteer-3.4 or later, the function can have any name, but it must be the same name that is used in the script. Also, if you are using syn-nodejs-puppeteer-3.4 or later, you can store your scripts under any folder and specify that folder as part of the handler name.

    const basicPuppeteerExample = async function () {}; exports.handler = async () => { return await basicPuppeteerExample(); };
  • Use the Synthetics dependency.

    var synthetics = require('Synthetics');
  • Use the Synthetics.getPage function to get a Puppeteer Page object.

    const page = await synthetics.getPage();

    The page object returned by the Synthetics.getPage function has the page.on request, response and requestfailed events instrumented for logging. Synthetics also sets up HAR file generation for requests and responses on the page, and adds the canary ARN to the user-agent headers of outgoing requests on the page.

The script is now ready to be run as a Synthetics canary. Here is the updated script:

var synthetics = require('Synthetics'); // Synthetics dependency const basicPuppeteerExample = async function () { const page = await synthetics.getPage(); // Get instrumented page from Synthetics await page.goto('https://example.com'); await page.screenshot({path: '/tmp/example.png'}); // Write screenshot to /tmp folder }; exports.handler = async () => { // Exported handler function return await basicPuppeteerExample(); };

Environment variables

You can use environment variables when creating canaries. This allows you to write a single canary script and then use that script with different values to quickly create multiple canaries that have a similar task.

For example, suppose your organization has endpoints such as prod, dev, and pre-release for the different stages of your software development, and you need to create canaries to test each of these endpoints. You can write a single canary script that tests your software and then specify different values for the endpoint environment variable when you create each of the three canaries. Then, when you create a canary, you specify the script and the values to use for the environment variables.

The names of environment variables can contain letters, numbers, and the underscore character. They must start with a letter and be at least two characters. The total size of your environment variables can't exceed 4 KB. You can't specify any Lambda reserved environment variables as the names of your environment variables. For more information about reserved environment variables, see Runtime environment variables.


The environment variables keys and values are not encrypted. Do not store sensitive information in them.

The following example script uses two environment variables. This script is for a canary that checks whether a webpage is available. It uses environment variables to parameterize both the URL that it checks and the CloudWatch Synthetics log level that it uses.

The following function sets LogLevel to the value of the LOG_LEVEL environment variable.


This function sets URL to the value of the URL environment variable.

const URL = process.env.URL;

This is the complete script. When you create a canary using this script, you specify values for the LOG_LEVEL and URL environment variables.

var synthetics = require('Synthetics'); const log = require('SyntheticsLogger'); const pageLoadEnvironmentVariable = async function () { // Setting the log level (0-3) synthetics.setLogLevel(process.env.LOG_LEVEL); // INSERT URL here const URL = process.env.URL; let page = await synthetics.getPage(); //You can customize the wait condition here. For instance, //using 'networkidle2' may be less restrictive. const response = await page.goto(URL, {waitUntil: 'domcontentloaded', timeout: 30000}); if (!response) { throw "Failed to load page!"; } //Wait for page to render. //Increase or decrease wait time based on endpoint being monitored. await page.waitFor(15000); await synthetics.takeScreenshot('loaded', 'loaded'); let pageTitle = await page.title(); log.info('Page title: ' + pageTitle); log.debug('Environment variable:' + process.env.URL); //If the response status code is not a 2xx success code if (response.status() < 200 || response.status() > 299) { throw "Failed to load page!"; } }; exports.handler = async () => { return await pageLoadEnvironmentVariable(); };

Passing environment variables to your script

To pass environment variables to your script when you create a canary in the console, specify the keys and values of the environment variables in the Environment variables section on the console. For more information, see Creating a canary.

To pass environment variables through the API or Amazon CLI, use the EnvironmentVariables parameter in the RunConfig section. The following is an example Amazon CLI command that creates a canary that uses two environment variables with keys of Environment and Region.

aws synthetics create-canary --cli-input-json '{ "Name":"nameofCanary", "ExecutionRoleArn":"roleArn", "ArtifactS3Location":"s3://cw-syn-results-123456789012-us-west-2", "Schedule":{ "Expression":"rate(0 minute)", "DurationInSeconds":604800 }, "Code":{ "S3Bucket": "canarycreation", "S3Key": "cwsyn-mycanaryheartbeat-12345678-d1bd-1234-abcd-123456789012-12345678-6a1f-47c3-b291-123456789012.zip", "Handler":"pageLoadBlueprint.handler" }, "RunConfig": { "TimeoutInSeconds":60, "EnvironmentVariables": { "Environment":"Production", "Region": "us-west-1" } }, "SuccessRetentionPeriodInDays":13, "FailureRetentionPeriodInDays":13, "RuntimeVersion":"syn-nodejs-2.0" }'

Integrating your canary with other Amazon services

All canaries can use the Amazon SDK library. You can use this library when you write your canary to integrate the canary with other Amazon services.

To do so, you need to add the following code to your canary. For these examples, Amazon Secrets Manager is used as the service that the canary is integrating with.

  • Import the Amazon SDK.

    const AWS = require('aws-sdk');
  • Create a client for the Amazon service that you are integrating with.

    const secretsManager = new AWS.SecretsManager();
  • Use the client to make API calls to that service.

    var params = { SecretId: secretName }; return await secretsManager.getSecretValue(params).promise();

The following canary script code snippet demonstrates an example of integration with Secrets Manager in more detail.

var synthetics = require('Synthetics'); const log = require('SyntheticsLogger'); const AWS = require('aws-sdk'); const secretsManager = new AWS.SecretsManager(); const getSecrets = async (secretName) => { var params = { SecretId: secretName }; return await secretsManager.getSecretValue(params).promise(); } const secretsExample = async function () { let URL = "<URL>"; let page = await synthetics.getPage(); log.info(`Navigating to URL: ${URL}`); const response = await page.goto(URL, {waitUntil: 'domcontentloaded', timeout: 30000}); // Fetch secrets let secrets = await getSecrets("secretname") /** * Use secrets to login. * * Assuming secrets are stored in a JSON format like: * { * "username": "<USERNAME>", * "password": "<PASSWORD>" * } **/ let secretsObj = JSON.parse(secrets.SecretString); await synthetics.executeStep('login', async function () { await page.type(">USERNAME-INPUT-SELECTOR<", secretsObj.username); await page.type(">PASSWORD-INPUT-SELECTOR<", secretsObj.password); await Promise.all([ page.waitForNavigation({ timeout: 30000 }), await page.click(">SUBMIT-BUTTON-SELECTOR<") ]); }); // Verify login was successful await synthetics.executeStep('verify', async function () { await page.waitForXPath(">SELECTOR<", { timeout: 30000 }); }); }; exports.handler = async () => { return await secretsExample(); };

Forcing your canary to use a static IP address

You can set up a canary so that it uses a static IP address.

To force a canary to use a static IP address
  1. Create a new VPC. For more information, see Using DNS with Your VPC.

  2. Create a new internet gateway. For more information, see Adding an internet gateway to your VPC.

  3. Create a public subnet inside your new VPC.

  4. Add a new route table to the VPC.

  5. Add a route in the new route table, that goes from to the internet gateway.

  6. Associate the new route table with the public subnet.

  7. Create an elastic IP address. For more information, see Elastic IP addresses .

  8. Create a new NAT gateway and assign it to the public subnet and the elastic IP address.

  9. Create a private subnet inside the VPC.

  10. Add a route to the VPC default route table, that goes from to the NAT gateway

  11. Create your canary.