Using Consul Template with Nested Values – HashiCorp Help Center

Introduction

Consul-template can be used to dynamically write to files based on the values of keys stored in Consul's built-in KV store. This provides a flexible mechanism for generating configuration files or other artifacts that depend on Consul data.

Expected Outcome

After completing this guide, you will:

Understand how to use Consul-Template to generate files based on Consul KV store keys.
Be able to nest keys within other keys in templates for more advanced use cases.

Prerequisites

A working installation of Consul.
Consul-Template installed.
A Consul KV store populated with the necessary data.
Basic understanding of Go templates (used by Consul-Template).

Use Case

Generate a file dynamically based on values stored in the Consul KV store. For example, writing configuration files that are automatically updated when Consul KV values change.

Procedure

Step 1: Create a Basic Template

Create a file named in.tpl with the following content:

{{ key "foo" }}

This template retrieves the value associated with the key foo in the Consul KV store.

Step 2: Run Consul-Template to Generate the Output

Run the following command to process the template and write the output to a file named out.txt:

consul-template -template "in.tpl:out.txt" -once

Step 3: Verify the Output

If the key foo has a value of bar in the Consul KV store, the contents of out.txt will be:

$ cat out.txt
bar

Step 4: Use Nested Keys

You can also use nested keys within templates. Update in.tpl to the following:

{{ key ( key "muddled" ) }}

Here, the value of the key muddled determines the next key to retrieve, allowing for dynamic key resolution.

Step 5: Run Consul-Template with Nested Keys

Run the following command again to process the updated template:

consul-template -template "in.tpl:out.txt" -once

Step 6: Populate KV Store for Nested Example

To demonstrate the nesting functionality, set the following key-value pairs in the KV store:

$ consul kv put duddled fuddled
Success! Data written to: duddled
$ consul kv put muddled duddled
Success! Data written to: muddled

Now, muddled maps to duddled, and duddled maps to fuddled. The out.txt file will now contain:

Additional Information