Testing Zero Downtime Upgrade Process

Zero Downtime upgrade eliminates any disruption to nova API service during upgrade.

Nova API services are upgraded at the end. The basic idea of the zero downtime upgrade process is to have the connections drain from the old API before being upgraded. In this process, new connections go to the new API nodes while old connections slowly drain from the old nodes. This ensures that the user sees the max_supported API version as a monotonically increasing number. There might be some performance degradation during the process due to slow HTTP responses and delayed request handling, but there is no API downtime.

This page describes how to test the zero downtime upgrade process.


  • Multinode devstack environment with 2 nodes:
    • controller - All services (N release)

    • compute-api - Only n-cpu and n-api services (N release)

  • Highly available load balancer (HAProxy) on top of the n-api services. This is required for zero downtime upgrade as it allows one n-api service to run while we upgrade the other. See instructions to setup HAProxy below.

Instructions to setup HAProxy

Install HAProxy and Keepalived on both nodes.

# apt-get install haproxy keepalived

Let the kernel know that we intend to bind additional IP addresses that won’t be defined in the interfaces file. To do this, edit /etc/sysctl.conf and add the following line:


Make this take effect without rebooting.

# sysctl -p

Configure HAProxy to add backend servers and assign virtual IP to the frontend. On both nodes add the below HAProxy config:

# cd /etc/haproxy
# cat >> haproxy.cfg <<EOF

      chroot /var/lib/haproxy
      user haproxy
      group haproxy
      log local0
      pidfile  /var/run/haproxy.pid
      stats socket /var/run/haproxy.sock mode 600 level admin
      stats timeout 2m
      maxconn 4000

      log  global
      maxconn  8000
      mode  http
      option  redispatch
      retries  3
      stats  enable
      timeout  http-request 10s
      timeout  queue 1m
      timeout  connect 10s
      timeout  client 1m
      timeout  server 1m
      timeout  check 10s

   frontend nova-api-vip
      bind             <<ha proxy virtual ip>>
      default_backend nova-api

   backend nova-api
      balance  roundrobin
      option  tcplog
      server  controller  check
      server  apicomp  check



Just change the IP for log in the global section on each node.

On both nodes add keepalived.conf:

# cd /etc/keepalived
# cat >> keepalived.conf <<EOF

   global_defs {
      router_id controller
   vrrp_script haproxy {
      script "killall -0 haproxy"
      interval 2
      weight 2
   vrrp_instance 50 {
      virtual_router_id 50
      advert_int 1
      priority 101
      state MASTER
      interface eth0
      virtual_ipaddress { dev eth0
      track_script {



Change priority on node2 to 100 ( or vice-versa). Add HAProxy virtual IP.

Restart keepalived service.

# service keepalived restart

Add ENABLED=1 in /etc/default/haproxy and then restart HAProxy service.

# service haproxy restart

When both the services have restarted, node with the highest priority for keepalived claims the virtual IP. You can check which node claimed the virtual IP using:

# ip a

Zero Downtime upgrade process

General rolling upgrade process: Minimal Downtime Upgrade Process.

Before Upgrade

  • Change nova-api endpoint in keystone to point to the HAProxy virtual IP.

  • Run tempest tests

  • Check if n-api services on both nodes are serving the requests.

Before maintenance window

  • Start the upgrade process with controller node.

  • Follow the steps from the general rolling upgrade process to install new code and sync the db for schema changes.

During maintenance window

  • Set compute option in upgrade_levels to auto in nova.conf.

    compute = auto
  • Starting with n-cond restart all services except n-api and n-cpu.

  • In small batches gracefully shutdown nova-cpu, then start n-cpu service with new version of the code.

  • Run tempest tests.

  • Drain connections on n-api while the tempest tests are running. HAProxy allows you to drain the connections by setting weight to zero:

    # echo "set weight nova-api/<<server>> 0" | sudo socat /var/run/haproxy.sock stdio
  • OR disable service using:

    # echo "disable server nova-api/<<server>>" | sudo socat /var/run/haproxy.sock stdio
  • This allows the current node to complete all the pending requests. When this is being upgraded, other api node serves the requests. This way we can achieve zero downtime.

  • Restart n-api service and enable n-api using the command:

    # echo "enable server nova-api/<<server>>" | sudo socat /var/run/haproxy.sock stdio
  • Drain connections from other old api node in the same way and upgrade.

  • No tempest tests should fail since there is no API downtime.

After maintenance window

  • Follow the steps from general rolling upgrade process to clear any cached service version data and complete all online data migrations.