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

On Wed, Oct 1, 2014 at 2:18 PM, Stuart Bishop <stuart(at)stuartbishop(dot)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.