503 Service Temporarily Unavailable Load Balancer

Question

Hi AWS, I was working on writing the Infrastructure code using Terraform for my web application.

This is the code for snippet for **security group**:

```
resource "aws_security_group" "client_alb" {
  name_prefix = "${var.default_tags.project_name}-alb"
  description = "security group for web application load balancer"
  vpc_id      = aws_vpc.main.id
  tags = {
    Name = "${var.default_tags.project_name}-sg"
  }
}

resource "aws_security_group_rule" "client_alb_allow_80" {
  security_group_id = aws_security_group.client_alb.id
  type              = "ingress"
  protocol          = "tcp"
  from_port         = 80
  to_port           = 80
  cidr_blocks       = ["0.0.0.0/0"]
  ipv6_cidr_blocks  = ["::/0"]
  description       = "Allow HTTP traffic."
}

resource "aws_security_group_rule" "client_alb_allow_443" {
  security_group_id = aws_security_group.client_alb.id
  type              = "ingress"
  protocol          = "tcp"
  from_port         = 443
  to_port           = 443
  cidr_blocks       = ["0.0.0.0/0"]
  ipv6_cidr_blocks  = ["::/0"]
  description       = "Allow HTTP traffic."
}

resource "aws_security_group_rule" "client_alb_allow_outbound" {
  security_group_id = aws_security_group.client_alb.id
  type              = "egress"
  protocol          = "-1"
  from_port         = 0
  to_port           = 0
  cidr_blocks       = ["0.0.0.0/0"]
  ipv6_cidr_blocks  = ["::/0"]
  description       = "Allow any outbound traffic."
}

```

This is the code for **ALB**:

```
# User Facing Client Application Load Balancer
resource "aws_lb" "client_alb" {
  name_prefix        = "cl-"
  load_balancer_type = "application"
  security_groups    = [aws_security_group.client_alb.id]
  subnets            = aws_subnet.public_subnet.*.id
  idle_timeout       = 60
  ip_address_type    = "dualstack"

tags = { "Name" = "${var.default_tags.project_name}-client-alb" }
}

# User Facing Client Target Group
resource "aws_lb_target_group" "client_alb_targets" {
  name_prefix          = "cl-"
  port                 = 9090
  protocol             = "HTTP"
  vpc_id               = aws_vpc.main.id
  deregistration_delay = 30
  target_type          = "ip"

health_check {
    enabled  = true
    interval = 60
    protocol = "HTTP"
  }

tags = { "Name" = "${var.default_tags.project_name}-client-tg" }
}

# User Facing Client ALB Listeners
resource "aws_lb_listener" "client_alb_http_80" {
  load_balancer_arn = aws_lb.client_alb.arn
  port              = 80
  protocol          = "HTTP"

default_action {
    type             = "forward"
    target_group_arn = aws_lb_target_group.client_alb_targets.arn
  }
}
```

When I am hitting the DNS name for ALB, I am getting this error which you can see in the screenshot provided.

![ALB 503 ERROR](/media/postImages/original/IM-aYAoiFuQqSACMSTH0d9Dg)

As per the AWS docs, this error comes when **the target groups for the load balancer have no registered targets**. https://docs.aws.amazon.com/elasticloadbalancing/latest/application/load-balancer-troubleshooting.html#http-503-issues

I have added the code for the target group but I am confused why it is not working. Do I need to create an EC2 instance and add it as a target for ALB. Please help.

Answer

Hello, 
You must add an ec2 as a listener in your target group and add that target group to your ALB with the correct health check.

Thanks

Answer

Hello.

You must use "aws_lb_target_group_attachment" to attach EC2 to a target group.  
In your code, the target group is created, but EC2 is not attached, which is probably causing this error.  
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/lb_target_group_attachment.html  
```
resource "aws_lb_target_group_attachment" "test" {
  target_group_arn = aws_lb_target_group.test.arn
  target_id        = aws_instance.test.id
  port             = 80
}
```

Answer

The issue reside in  the targets behind the ALB .
Double check your target maybe you have forgotten  to register EC2 instances
Check that the instances are in the AZ of ALB

503 Service Temporarily Unavailable Load Balancer

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