AWS Cloudwatch Metric and callbackWaitsForEmptyEventLoop do not work together? - node.js

Question

below is a simplification of my code.
const AWS = require('aws-sdk');
exports.handler = async (event, context, callback) => {
context.callbackWaitsForEmptyEventLoop = true;
AWS.config.update({region: 'cn-north-1'});
// Create CloudWatch service object
var cw = new AWS.CloudWatch({apiVersion: '2010-08-01'});
var params = {
MetricData: [
{
MetricName: 'PAGES_VISITED',
Dimensions: [
{
Name: 'UNIQUE_PAGES',
Value: 'URLS'
},
],
Unit: 'None',
Value: 1.0
},
],
Namespace: 'MyNewNameSpace'
};
cw.putMetricData(params, function(err, data) {
if (err) {
console.log("Error", err);
} else {
console.log("Success", JSON.stringify(data));
}
});
callback(null, "the result");
};
It seems that once I set the callbackWaitsForEmptyEventLoop = false then the metric cannot be put up there. I donot understand this conflict.

Answer 1

If you set callbackWaitsForEmptyEventLoop = false then your function execution terminates before all the callbacks are done. In this case, the function terminates before the callback from cw.putMetricData is ever called, so your code is not executed. It is likely that the operation on CloudWatch actually happens, but that you just don't see the callback, as it does not happen.
Here's your function, using the async/await model, without callbacks and without callbackWaitsForEmptyEventLoop:
const AWS = require('aws-sdk');
exports.handler = async event => {
AWS.config.update({region: 'cn-north-1'});
// Create CloudWatch service object
var cw = new AWS.CloudWatch({apiVersion: '2010-08-01'});
var params = {...};
await cw.putMetricData(params)
.promise()
.then(data => {console.log("Success", JSON.stringify(data));})
.catch(err => {console.log("Error", err);})
return "the result";
};