Postmortem of databases failure on 2019-03-01

Note to the reader

This article was written a long ago and thus: it isn’t up-to-date with my current policy for upgrades and migrations, it isn’t a good postmortem, you probably shouldn’t be reading this. I decided to publish it anyway because I think it can help newcomers to the world of self-hosting to see that everyone makes mistakes.

Sometime around 2019-03-01 02:22:11 UTC Mattermost stopped working because it couldn’t communicate with its database (PostgreSQL).

Some duck RDBM context

Before the incident, there were two databases installed at their default locations: MariaDB and PostgreSQL. An maintenance operation was to be performed on both of them, in order to move their data folder to an external disk connected to the VPS in order to facilitate backup and migration of data. During this migration, all services relying on those databases (MariaDB: PrivateBin, Polr, 42over2/CatCatch, Zabbix ; PostgreSQL: Mattermost) would be down as it required to shutdown MariaDB and PostgreSQL, but it wasn’t considered a problem as it was night time in France.

duck databases death

The process was simple: shutdown the RDBM, copy the data folder to the new location, move the original one to a backup location, change the configuration of the RDBM and restart the RDBM. The plan was to start with PostgreSQL as the most important service (Mattermost) is relying on it. Everything went smoothly until the last step. PostgreSQL wasn’t restarted and therefore let Mattermost without its data. The issue wasn’t noticed due to the way Mattermost manages its frontend and backend. It wasn’t obvious Mattermost wasn’t working. Therefore, the migration went on with MariaDB. This one failed at the configuration step, but was left dead because it wasn’t an important service.

The sysadmin then went away from his computer for his nocturnal activities and did not notice something was wrong until 2019-03-01 20:37:.. UTC. By the time the Mattermost logs were checked and PostgreSQL was restarted, it was 2019-03-01 20:47:33 UTC, which implies 18 hours and 25 minutes of downtime. No data loss was endured.

Then it was time to fix MariaDB as the migration did not succeed. The problem was with the MariaDB socket, which wasn’t in the default place anymore and could not be found by php-fpm. Re-configuring all of it took a while. Somehow the users permissions were messed up and had to be reconfigured too. This situation was resolved at 2019-03-01 21:52:13 UTC.

Impact on Mattermost

Mattermost web interface was accessible during the whole incident but nothing was usable. Not data dating prior 2019-03-01 02:22:11 UTC was lost. All data input in Mattermost between 2019-03-01 02:22:11 UTC and 2019-03-01 20:47:33 UTC was lost as it wasn’t recorded in the database.

Impact on other services

The other services impacted were those running on MariaDB:

  • PrivateBin: still accessible, however Send wasn’t working anymore.
  • Polr: not accessible
  • 42over2.ovh/CatCatch: not accessible
  • Zabbix: backend not logging anymore but sending alerts saying the database was down. Web interface wasn’t accessible anymore.

Why did this happen?

  • PostgreSQL: The root cause of this is clearly human error. The migration process went on smoothly. The problem isn’t that the error was made, but that it was not detected.
  • MariaDB: The migration was a complete mess. There was no real documentation on how to move the databases without data loss. Hopefully everything was recovered.

Why wasn’t it detected earlier?

The monitoring system in place (Zabbix) was not configured to monitor PostgreSQL, neither MariaDB. The MariaDB fail was discovered because Zabbix relied on it, but the PostgreSQL fail wasn’t.

How can we prevent this from happening again?

First, by putting a strong monitoring system in place. Netdata has been set up in order to keep track of everything that could go wrong with duck systems. Notifications in Mattermost have been set up. Email notifications are still a TODO. Then, we need an external monitor checking that the websites are still accessible. Also, considering migrating away from MariaDB to PostgreSQL, easier to maintain and backup.

In retrospect

This incident took down almost all systems running on duck. It lasted for more than 18 hours, but no data loss was endured. It showed a lack of monitoring of the services in place. For a first major incident for a young sysadmin, I would say it wasn’t that bad.

Finally, I want to apologize to the users of the services I provide. It only is a beginning for me in the sysadmin world and I still have a lot to learn, but providing a working system with minimal downtime is a priority and I failed. Sorry :/