Updating a Nomad job template for running tasks without restarting the containers


We are currently evaluating Nomad for a consolidation project and have encountered an issue we can’t come up with a clean solution for it. For reference we are using standalone Nomad and are not considering using Consul at this time as it would make the project more complicated. We are making use of the Nomad service registry for discovery and dynamic configuration using config templates in the job files.

We are going to be using HAproxy to ingress traffic into the cluster which is a hard requirement as it is what we currently use and are familiar with as well as we need its performance compared to something like Traefik. It has been set up as a system service using docker host networking with static ports and we are using a template to configure it. Here is a cut down job file to show how we are running it:

job "haproxy" {
  region = "global"
  datacenters = ["dc1"]
  type  = "system"

  group "haproxy" {
    network {
      port "http" { static = 80 }
      port "https" { static = 443 }

    task "haproxy" {
      driver = "docker"
      config {
        image = "haproxytech/haproxy-alpine:2.7"
        network_mode = "host"
        volumes = [ "local/haproxy:/usr/local/etc/haproxy" ]

      template {
        change_mode = "signal"
        change_signal = "SIGHUP"
        destination = "local/haproxy/haproxy.cfg"
        data = <<EOH
  mode http
  timeout client  10s
  timeout connect 5s
  timeout server  10s
  timeout http-request 10s

frontend http
  bind :80
  use_backend api if { hdr(host) -i api.example.com }

backend api
{{- range nomadService "api" }}
  server {{ .Address }}:{{ .Port }} {{ .Address }}:{{ .Port }} check
{{- end }}

This works really well, when the api service adds or removes tasks the template updates and HAproxy gets a SIGHUP telling it to hot reload and no client connections are lost (we have shutdown_delay set in our other jobs to give haproxy time to update).

This is great until we want to make a change to the config such as add another frontend+backend or change some config. For example in the above file I may want to handle another host header so I add the following to the existing frontend config:

  use_backend api if { hdr(host) -i api.example.com }
  use_backend api if { hdr(host) -i api.somewhereelse.com }

When I submit the job file with the change Nomad sees it and restarts all my ingress LBs at the same time, causing a short outage. On our existing LB servers we can manually reload haproxy so no connections are lost but in Nomad we don’t seem to have that choice, it seems to always restart the tasks whenever the job spec changes, and I cant find any guides on how we could resolve this.

Is there a way to prevent or work around this issue? I have looked into writing the perfect template that will never need us to restart the HAproxy allocations ever, using the services to build the frontends and backends and keep config in tags and variables, but it gets complicated and I expect that as soon as it goes into production we will find issues that need correcting or config that needs adding, and we can’t afford to lose ingress even for the small amount of time it takes to restart the containers.

Hi @glynnbailey,

Unfortunately, changing the template block within the job specification is classed as a destructive update and therefore will result in a replacement of running allocations of that job version, as you’ve seen.

When I submit the job file with the change Nomad sees it and restarts all my ingress LBs at the same time, causing a short outage.

There are some job spec parameters which can prevent this and make the rollout of new versions easier and without impact on availability. The update block in particular can control how an update is rolled out across all the running allocations. kill_timeout and kill_signal can be used to better control how Nomad stops allocations for replacement. The timeout in particular can ensure HAProxy has the time to gracefully close connections and shutdown, rather than be prematurely killed.

I hope this helps. Please let me know if you have any followup questions.

jrasell and the Nomad team

Thanks for the response. I have found a solution and I thought I would put it here incase anyone has the same issue. I did not realise that consul-template also supported the nomad service discovery.

We can setup haproxy on a server outside the nomad cluster and use consul-template to build the config and SIGHUP haproxy. If we need to change the template we can do so without needing to submit a nomad job so the problem of the haproxy containers restarting goes away. In my testing it works really well, really impressed with it.

Thanks again.