Modifying the Application Code

In this step, we are going to define a couple of metrics that we want to capture amongst our three core services, and will instrument the logMetric() method to push them asynchronously to CloudWatch Metrics, having them first logged to CloudWatch Logs and then processed in background by the utility module we deployed in the previous step.

Defining Metrics

Let’s define the following Business & Operational metrics:

  • ColdStart
  • UnsupportedHTTPMethod
  • SuccessfulGetItem
  • FailedGetItem

Modifying the Get Item By ID Function

Importing Dependencies

  1. Open the Lambda function located on /serverless-observability-workshop/code/sample-app/src/handlers/get-by-id.js and start by importing the required MetricUnit and logMetric dependencies in the beginning of the file and initializing a _cold_start variable to capture cold starts in our lambda executions:

    const AWS = require('aws-sdk')
const docClient = new AWS.DynamoDB.DocumentClient()
const { MetricUnit } = require('../lib/helper/models')
const { logMetric } = require('../lib/logging/logger')

let _cold_start = true

    Adding LogMetric

  2. Now, inside the getByIdHandler() we are going to test whether it’s the first execution of a given Lambda container and label it as Cold Start, also pushing this information as a CloudWatch Metric using our logMetric() method. Add and if statement checking whether the _cold_start variable is true or false right after the beginning of the try block.

    exports.getByIdHandler = async (event, context) => {
  let response
  try {
    if (_cold_start) {
        //Metrics
        await logMetric(name = 'ColdStart', unit = MetricUnit.Count, value = 1, { service: 'item_service', function_name: context.functionName })
        _cold_start = false
    }

  3. Now, we are going to capture metrics for UnsupportedHTTPMethod, instrumenting the logMetric() method call if our if (event.httpMethod !== 'GET') evaluates true.

    if (event.httpMethod !== 'GET') {
  await logMetric(name = 'UnsupportedHTTPMethod', unit = MetricUnit.Count, value = 1, { service: 'item_service', operation: 'get-by-id' })
  throw new Error(`getById only accept GET method, you tried: ${event.httpMethod}`)
}

  4. Next, we are ready to add our business metrics for successful and failed item retrievals by a given ID. Still inside your getByIdHandler(), find and add in the end of your try and beginning of your catch blocks the metrics for SuccessfulGetItem and FailedGetItem:

    try{
  //After Sucessful Response Composition
  //Metrics
  await logMetric(name = 'SuccessfulGetItem', unit = MetricUnit.Count, value = 1, { service: 'item_service', operation: 'get-by-id' })
} catch (err) {
  //After Exception Handling
  //Metrics
  await logMetric(name = 'FailedGetItem', unit = MetricUnit.Count, value = 1, { service: 'item_service', operation: 'get-by-id' })
}

  5. Save your changes to the serverless-observability-workshop/code/sample-app/src/handlers/get-by-id.js file.

    Your getByIdHandler method should look like the one below

    Full getByIdHandler method (expand for code)

Deploy the application

cd ~/environment/serverless-observability-workshop/code/sample-app
sam build && sam deploy

Export the stack output variables

To invoke our API’s, we first need to fetch the ApiUrl output variable that our CloudFormation stack gives us. So let us iterate through our stack and export all output variables as environment variables:

export ApiUrl=$(aws cloudformation describe-stacks --stack-name monitoring-app --output json | jq '.Stacks[].Outputs[] | select(.OutputKey=="ApiUrl") | .OutputValue' | sed -e 's/^"//'  -e 's/"$//')
echo "export ApiUrl="$ApiUrl

Test the Get Item By ID operation

curl -X GET $ApiUrl/items/1 | jq