# Consul Template — Dynamic Configuration Rendering from HashiCorp Consul

> A daemon that watches HashiCorp Consul and Vault for changes and renders Go templates into config files, then optionally reloads services.

## Install

Save as a script file and run:

# Consul Template — Dynamic Configuration Rendering from HashiCorp Consul

## Quick Use
```bash
# Install
curl -O https://releases.hashicorp.com/consul-template/0.39.1/consul-template_0.39.1_linux_amd64.zip
unzip consul-template_0.39.1_linux_amd64.zip
# Render a template once
./consul-template -consul-addr="127.0.0.1:8500" 
  -template="nginx.ctmpl:nginx.conf" -once
```

## Introduction
Consul Template is a standalone daemon from HashiCorp that queries Consul, Vault, and Nomad for live data, renders Go templates into configuration files, and optionally executes a reload command. It bridges the gap between dynamic service discovery and static config files.

## What Consul Template Does
- Watches Consul's service catalog and KV store for real-time changes
- Renders Go templates into config files for Nginx, HAProxy, and any other service
- Fetches secrets from HashiCorp Vault and injects them into rendered output
- Executes a configurable command after rendering (e.g., systemctl reload nginx)
- Supports multiple templates and destinations in a single daemon instance

## Architecture Overview
Consul Template runs as a long-lived process that maintains blocking queries against the Consul HTTP API. When a watched value changes, it re-evaluates the associated Go template, writes the result to disk atomically, and fires the specified reload command. It uses a dependency graph to batch multiple near-simultaneous changes into a single render cycle, avoiding unnecessary restarts.

## Self-Hosting & Configuration
- Download a static binary from HashiCorp releases or install via package managers
- Point -consul-addr at your Consul cluster and -vault-addr at Vault if using secrets
- Define templates with -template="source:dest:command" on the CLI or in an HCL config file
- Use -once for one-shot rendering in CI or -retry for persistent daemon mode
- Run as a systemd service for production deployments

## Key Features
- Atomic file writes prevent services from reading partially rendered configs
- Quiescence timers batch rapid changes to avoid reload storms
- Native Vault integration for secret injection without custom scripts
- Template functions for service health filtering, KV lookups, and environment variables
- Supports Consul Connect service mesh metadata and intentions

## Comparison with Similar Tools
- **confd** — similar template rendering tool; Consul Template has deeper Consul and Vault integration
- **envsubst** — simple environment variable substitution; Consul Template offers live watching and complex logic
- **Ansible templates** — rendered at deploy time; Consul Template updates continuously at runtime
- **Consul Connect / Envoy** — handles service mesh routing directly; Consul Template generates config files for legacy apps
- **gomplate** — standalone Go template CLI; Consul Template adds the watching and reload loop

## FAQ
**Q: Can I use Consul Template without HashiCorp Consul?**
A: Consul is the primary data source. For Vault-only use cases, you can point it solely at Vault, but most deployments pair it with Consul.

**Q: How does it handle rapid changes without thrashing restarts?**
A: It uses quiescence timers (min/max wait) to batch multiple changes into a single render-and-reload cycle.

**Q: Is it safe to inject secrets into config files?**
A: Consul Template writes files with configurable permissions. Combine with short-lived Vault leases and restricted file ACLs for a secure setup.

**Q: Can it render multiple templates?**
A: Yes. Define multiple -template flags or template blocks in the HCL config file. Each template can have its own reload command.

## Sources
- https://github.com/hashicorp/consul-template
- https://developer.hashicorp.com/consul/tutorials/developer-configuration/consul-template

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Source: https://tokrepo.com/en/workflows/asset-05d09c45
Author: Script Depot