Michal ZimmermannPieces of knowledge from the world of GIS.

PostgreSQL Backup and Recovery Orchestration: systemd Automation

Posts in this series have described the basic automation of PostgreSQL backup/recovery strategy. The process itself consists of different periodic tasks that shouldn’t be executed manually. There are essentially two tools dedicated to periodic task running in Linux: cron and systemd.

Cron used to be my first choice of automation in Linux, as it’s very easy to use. On the other hand, it’s quite messy (running crontab -e under different users to find out which one has the job defined) and a bit difficult to test - many times I ran into a situation when underlying bash script executed just fine, while cron job kept failing for reason unknown.

My own cron experience together with a few words from a workmate brought me into the arms of systemd, which is a Linux system and service manager. It’s capable of running periodic tasks just like cron, yet making it more transparent.

Important bits

Understanding the whole systemd is way out of scope of a poor GIS guy, yet I managed to tame three important parts of the ecosystem:

Services

Service is a configuration saved inside “.service” file specifying what you want systemd to do. Following code shows how you can tell systemd to vacuum your database once in a while.

[Unit]
Description=CR vacuumdb
OnFailure=[email protected]%n.service [email protected]%n.service
Wants=cr-sunday.timer

[Service]
User=postgres
Group=postgres
Type=simple
ExecStart=/bin/bash /usr/local/sbin/pgsql-vacuumdb.sh --port %i

[Install]
WantedBy=cr-sunday.target

Unit files come with several handy features. First of all, they are orchestrated with systemctl. Second, any service configuration file containing @ in its filename might be symlinked/copied and run for different instances. Third, notice OnFailure directive in the code above. If anything goes wrong, systemd might serve as a postman delivering the bad news. I set up both e-mail and Slack notifications and they’ve been working like a charm ever since.

On top of that, I find systemd orchestration much easier to test and maintain compared to cron.

With the above code saved in /lib/systemd/system/[email protected], you can copy the file to /lib/systemd/system/[email protected], /lib/systemd/system/[email protected] etc. If you look at ExecStart part of the service file, you’ll notice %i being used at the end - a placeholder replaced with the string between @ and .service in the filename.

This systemd service file is no more than a simple wrapper around the following bash code. We run three different database clusters on one machine and this approach makes their maintenance pretty comfortable.

#!/bin/bash
#
# @author: Michal Zimmermann <[email protected]>
# Vacuums the whole database cluster running on a given port.

while [[ $# > 0 ]]
do
    key="$1"

    case $key in
        -p|--port)
            PORT="$2"
            shift
            ;;
        *)
            echo "Usage: `basename $0` --port|-p [port_number]"
            exit 1
            ;;
    esac
    shift
done

if [[ -z "$PORT" ]]
then
    echo "Port not provided!"
    $0 *
    exit 2
fi

/usr/bin/vacuumdb -U postgres -p $PORT --all --full --analyze

What you get so far is the possibility to run systemctl start [email protected] instead of calling the underlying bash code manually. Not much, really. That’s where timers come to the party.

Timers

Timer files ends with “.timer” and are responsible for running services on given time. The code below, coming from /lib/systemd/system/cr-sunday.timer file runs the pgsql-vacuumdb service every Sunday at 3:45 am.

[Unit]
Description=CR Sunday timer

[Timer]
OnCalendar=Sun *-*-* 03:45
Persistent=yes
Unit=cr-sunday.target

[Install]
WantedBy=multi-user.target

Targets

Target files end with “.target” and are used to group units in general. In our case, the target file for vacuumdb service is as simple as the following code.

[Unit]
Description=CR Sunday target
StopWhenUnneeded=yes

Targets might be called by other targets. Running systemctl start cr-sunday.target would eventually lead to running all the services wanted by that target.

As I already mentioned, I find systemd services easy to code and test. If any of them should fail, you’d find a message in syslog or via systemctl status pgsql-vacuumdb.