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Consul-Template is the best tool ever

If you ever want to use some of Consul's amazing features with a non-consul aware application, consul-template can be a fantastic way to do it. Some examples of ways that I've used it:

  1. Dynamically configure an always-up-to-date list of haproxy backends as different services join and leave the consul cluster.
  2. Dynamically configure secrets or configuration values for your running applications -- one change in the consul KV can reconfigure all running services that use that config value. See this example service killswitch.
  3. Configure a dynamically updated upstream list for nginx -- e.g. scale your WordPress php-fpm pools up or down as needed, with no service interruption.

See the official site and documentation for details and the most up-to-date version number.

Install consul-template

export CURRENT_VERSION=0.25.1
wget https://releases.hashicorp.com/consul-template/$CURRENT_VERSION/consul-template_$CURRENT_VERSION_linux_amd64.zip
unzip consul-template_$CURRENT_VERSION_linux_amd64.zip
sudo cp consul-template /usr/local/bin/

Create a systemd service

cat << EOF > /etc/systemd/system/consul-template.service
[Unit]
Description=the consul-template service
# presumes that consul is installed as a service (won't work with `consul agent -dev`)
After=consul.service

[Service]
ExecStart=/usr/local/bin/consul-template -syslog -config /etc/consul-template.d/
Restart=always
KillSignal=SIGINT
StandardOutput=null # using syslog

[Install]
WantedBy=multi-user.target

EOF

Create config directories/files

mkdir /etc/consul-template.d/

Example template

If you store the example configuration below in a file, /etc/consul-template.d/tutorialinux, it would automatically be picked up when the consul-template service starts up (due to the -config flag in the ExecStart statement in the unit file above). Here's what the consul-template process would then do:

  • Look for the file defined as the source.

  • Interpret that file as a consul-template.

  • Render the resulting content at the destination.

  • KEEP Re-rendering it any time the content would change based on consul or vault key changes, service catalog changes, address changes, etc.

    template { source = "/etc/tutorialinux/config-example-consultemplate.tpl destination = "/etc/tutorialinux/config.json command = "bash -c 'systemctl reload-or-restart --no-block tutorialinux.service'" }

Additional References

https://learn.hashicorp.com/tutorials/consul/consul-template