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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.

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Workflow

  1. Define a template file with placeholders (e.g., app.ctmpl).
  2. Start the Consul agent on your server: 
    consul agent -config-dir=/etc/consul.d
  3. 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
  4. The daemon retrieves key/value pairs from Consul.
  5. Placeholders in the template are replaced with live data.
  6. The rendered file is saved to disk.
  7. An optional command (like a service reload) is executed to apply updates.
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.

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Key Benefits

BenefitDescription
Dynamic UpdatesAutomatically re-renders files when Consul KV entries change.
Automatic ReloadsExecutes custom commands (e.g., systemctl reload) on changes.
Powerful TemplatingLeverages Go templates with conditionals, loops, and functions.

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

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