KodeKloud Notes

Consul Template is a HashiCorp tool designed to render configuration files dynamically using data from a Consul cluster. Running as a standalone daemon alongside a Consul agent, it continuously watches key/value entries and regenerates files when values change. This ensures seamless configuration updates without manual restarts.

Workflow

Define a template file with placeholders (e.g., app.ctmpl).
Start the Consul agent on your server:
```
consul agent -config-dir=/etc/consul.d
```

Launch the consul-template process, specifying the template and target:

consul-template \
  -template "/etc/templates/app.ctmpl:/etc/app/config.yaml" \
  -config /etc/consul-template/config.hcl

The daemon retrieves key/value pairs from Consul.
Placeholders in the template are replaced with live data.
The rendered file is saved to disk.
An optional command (like a service reload) is executed to apply updates.

Note

By default, Consul Template polls Consul every two seconds. Adjust the interval with the -wait flag or in HCL.

The image is a flowchart explaining the process of using a consul-template, starting from a VM launch, executing the consul-template, creating a config file, and launching an application, with interactions with a Consul Cluster.

Key Benefits

Benefit	Description
Dynamic Updates	Automatically re-renders files when Consul KV entries change.
Automatic Reloads	Executes custom commands (e.g., `systemctl reload`) on changes.
Powerful Templating	Leverages Go templates with conditionals, loops, and functions.

Example HCL Configuration

Create /etc/consul-template/config.hcl:

template {
  source      = "/etc/templates/app.ctmpl"
  destination = "/etc/app/config.yaml"
  command     = "systemctl reload my-app.service"
  left_delimiter  = "{{"
  right_delimiter = "}}"
}

Run the daemon:

consul-template -config /etc/consul-template/config.hcl

Warning

Ensure the Consul agent is running and reachable. When ACLs are enabled, include the token parameter in your HCL or CLI invocation.

Links and References

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