connection to external server (mongodb server) fails from fargate container deployed using cdk - amazon-ecs

Question

I created a simple node.js/express app and created docker image and successfully pushed it to aws ecr.
Next, I created a cdk project to deploy this container to fargate with public application load balancer. ecs_patterns.ApplicationLoadBalancedFargateService
Although the deployment cmd (cdk deploy) was successful, cluster page in aws console shows "No tasks running" and Services tab within the cluster shows red bar with "0/1 Tasks running" and Tasks tab within cluster shows tasks are getting created and stopped (every 1 or 2 min, a task is created and eventually stopped and a new one is created and this keeps on going forever)
Going inside a stopped task and its Log tab shows
ERROR: Connecting to MongoDB failed. Please check if MongoDB server is running at the correct host/port.. This is the error message I have in my app when connection to mongodb fails when the server is initialized.
The DB credentials and connection url are valid (see below) and it runs in a separate EC2 instance with EIP and domain name. In fact, I can connect to the DB from my dev machine which is outside aws.
Also, just for trial, I created a stack manually through console by creating security groups (for load balancer and service), target group, application load balancer, listener (port 80 HTTP), cluster, task definition (with correct db credentials set in env var), service, etc., it's working without any issue.
All I want is to create similar stack using cdk (I don't want to manually create/maintain it)
Any clue on why connection to external server/db is failing from a fargate container would be very useful. I'm unable to compare the "cdk created cloudformation template" (that's not working) with the "manually created stack" (that's working) as there are too many items in the autogenerated template.
Here is the cdk code based on aws sample code:
const vpc = new ec2.Vpc(this, "MyVpc", { maxAzs: 2 });
const cluster = new ecs.Cluster(this, "MyCluster", { vpc });
const logDriver = ecs.LogDriver.awsLogs({ streamPrefix: "api-log" });
const ecrRepo = ecr.Repository.fromRepositoryName(this, "app-ecr", "abcdef");
new ecs_patterns.ApplicationLoadBalancedFargateService(
this, "FargateService", {
assignPublicIp: true,
cluster,
desiredCount: 1,
memoryLimitMiB: 1024,
cpu: 512,
taskImageOptions: {
containerName: "api-container",
image: ecs.ContainerImage.fromEcrRepository(ecrRepo),
enableLogging: true,
logDriver,
environment: { MONGO_DB_URL: process.env.DB_URL as string }
},
publicLoadBalancer: true,
loadBalancerName: "api-app-lb",
serviceName: "api-service"
}
);

Answer 1

It turned out to be a silly mistake! Instead of MONGO_DB_URL it should be DB_URL because that's what my node.js/express server in the container is using.