Re: How to detect if Postgres needs a restart when configuration changes

On Wed, Oct 1, 2014 at 2:18 PM, Stuart Bishop <stuart@stuartbishop.net> wrote:

It is unnecessary maintaining a list of parameters which require a
restart, as you can tell by looking at pg_settings.

The way I do it is by comparing the contents of pg_settings with a
snapshot I made just after (or just before) the server was last
restarted. If any settings have been changed that have a context of
'postmaster', the server needs a restart.

It's not as easy as it sounds to do that with Ansible, since you don't magically get a list of parameters which are changed from last time. All parameters are in a template which generates postgresql.conf, so you just get the information that that file changed, that is why you need to compare each parameter separately. Since it is a lot faster to compare just a few parameters which we set that require a restart, it is worth keeping a list.

If anyone is interested this is how I detect if a server needs a restart. I define a dict with parameters that require a restart and set the keys to have values of the current variables:

postgres__restart_params:
  listen_addresses: "{{ postgres_listen_addresses }}"
  port: "{{ postgres_port }}"
  max_connections: "{{ postgres_max_connections }}"
  superuser_reserved_connections: "{{ postgres_superuser_reserved_connections }}"
  wal_level: "{{ postgres_wal_level }}"
  fsync: "{{ postgres_fsync }}"
  max_wal_senders: "{{ postgres_max_wal_senders }}"
  hot_standby: "{{ postgres_hot_standby }}"
  shared_buffers: "{{ postgres_shared_buffers }}"
  archive_mode: "{{ postgres_archive_mode }}"

then I check this in the following task:

- name: Check if a postgres restart is required on master
  command: psql -p {{ postgres_port }} -At -U postgres -c "SELECT current_setting('{{ item.key }}') <> '{{ item.value }}';"
  with_dict: postgres__restart_params
  register: restart_params
  always_run: yes
  ignore_errors: yes
  changed_when: restart_params.stdout == 't' or restart_params|failed
  notify: restart postgres

ignore_errors is needed since when changing port, psql cannot connect, so when that happens I also restart.

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