overlord is a lightweight configuration management tool focused on:

• keeping local configuration files up-to-date reagarding AWS Autoscaling Group (ASG).
• reloading applications to pick up new config file changes

It's heavily inspired by confd.

1. Download latest release of overlord binaries: here.
2. Create a configuration file for your application in /etc/overlord/resources/. This toml file (it has to have the .toml extension) describes the ASG to monitor, the configuration file to keep up-to-date and how to restart the application.
3. Create a template of your application's configuration file in /etc/overlord/temlates/ in golang format.

Here is an example with an HAProxy configuration:

/etc/overlord/resources/haproxy.toml:

[template]
src = "haproxy.cfg.tmpl" #template used to generate configuration file (located in /etc/overseer/temlates/)
dest = "/etc/haproxy/haproxy.cfg" #file to generate from the template
hosts = ["my-asg"] #ASG to monitor
reload_cmd = "touch /var/run/haproxy.pid; haproxy -D -f /etc/haproxy/haproxy.cfg -p /var/run/haproxy.pid -sf $(cat /var/run/haproxy.pid)" #command to reload the configuration

/etc/overlord/temlates/haproxy.cf.tmpl:

defaults
    mode http
    timeout connect 5000ms
    timeout client 50000ms
    timeout server 50000ms

listen  my-app
	bind *:80
	bind *:443
	balance roundrobin
	{{range $index, $ip := index . "my-asg"}}server my-backend-{{$index}} {{$ip}}
	{{end}}

In addition to IP addresses, overlord now provides detailed instance information including lifecycle state, health status, and more. This allows for more sophisticated configuration logic.

Templates now receive a data structure with two main sections:

• .ips - Backward compatible IP address lists (same as before)
• .instances - Detailed instance information

Each instance in the .instances array provides:

• InstanceID - AWS instance ID
• PrivateIP - Private IPv4 address
• IPv6Address - IPv6 address
• LifecycleState - ASG lifecycle state (InService, Terminating, etc. https://docs.aws.amazon.com/autoscaling/ec2/userguide/ec2-auto-scaling-lifecycle.html or empty for non-ASG instances)
• HealthStatus - Instance health status (Healthy, Unhealthy or empty for non-ASG instances)
• InstanceState - EC2 instance state (running, stopped, etc. https://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_InstanceState.html)
• AvailabilityZone - AWS availability zone
• InstanceType - EC2 instance type
• GetIP(ipv6 bool) - Method to get appropriate IP address
• IsHealthy() - Method to check if instance is in a healthy state

# Backward compatible IP access
{{range index .ips "my-asg"}}
  IP: {{.}}
{{end}}

# New instance details access
{{range index .instances "my-asg"}}
  Instance: {{.InstanceID}}
  IP: {{.GetIP false}}
  Lifecycle State: {{.LifecycleState}}
  Health Status: {{.HealthStatus}}
  Is Healthy: {{.IsHealthy}}
{{end}}

# Conditional logic based on lifecycle state
{{range index .instances "my-asg"}}
  {{if eq .LifecycleState "InService"}}
    server backend-{{.InstanceID}} {{.GetIP false}} check
  {{else if eq .LifecycleState "Terminating"}}
    # Instance is being terminated, exclude from config
  {{end}}
{{end}}

# Only include healthy instances
{{range index .instances "my-asg"}}
  {{if .IsHealthy}}
    server healthy-backend-{{.InstanceID}} {{.GetIP false}} check
  {{end}}
{{end}}

This enhanced functionality allows for more sophisticated load balancer configurations, health-aware routing, and better monitoring integration.