Creating Consul Templates with Chef's Template resource.
# Consul
I won't go into what Consul is in the scope of this post, but it is expected you know the basics.
# consul-template
consul-template can be used to render a template using values stored in Consul. This is often used when creating configuration files which need to be updated dynamically, such as a software load balancer that needs to dynamically load the servers it is balancing.
The Flow
- New server launches, registers with Consul.
- A push is made from Consul to consul-template to render the template with the new value(s).
- If the template was updated, consul-template will restart/reload the desired service.
Example
In this example we will have an auto-scaling web server group behind an HAProxy instance.
How do we update HAProxy when a new web server is launched
- New web server is launched.
- A startup script executes Chef with an environment/role.
- Chef lays down the consul-template .ctpl file.
- The instance registers with the Consul cluster.
- Consul pushes the new web server's data to consul-template on the HAProxy instance.
- consul-template renders a new config with the new web server.
- Since a change was made to the template, consul-template reloads the HAProxy config file.
- Traffic is now being sent to the new web server.
# Chef
We need something to place the consul-template configuration file on the system, as well as the ctpl consul-template templates. This is where Chef comes in and lays down the ctpl file so consul-template can generate the end .conf file for HAProxy, or whatever file you are managing with consul-template.