RogerBW's Blog

Migrating a Discourse server 09 March 2021

I recently moved discussion.tekeli.li to a new host. Here's how I did it (with the false starts removed).

You will need control of DNS as well as root access to both servers.

First of all, warn your users of when the change will happen; there will be a temporary unavailability of the site. (I'd hoped to minimise this with incremental transfers, but it didn't work.)

Set the DNS TTL down to a suitably low value (e.g. 30 seconds) so that when you point it to the new server people will see it straight away. (How far in advance you should do this depends on what you have TTL set to normally – half a day, 43200 seconds, is quite usual.) At the same time set up a temporary server name pointing to the new server's IP address. I'll assume below that SERVER is the main name you want to use, and TEMP the temporary one.

On the old Discourse installation, log in as an admin user, and go to Admin, Backups. Set "Enable read-only". Then Settings, Backups, and make sure that "enable backups", "backup with uploads", and "include thumbnails in backups" are all ticked. (The last is not essential but trades off a larger backup for a shorter setup time on the new server.)

Then back to Admin, Backups, and select Backup – to generate one last backup on the old server. This takes a while, particularly the essentially-pointless process of compression.

While this is going on, set up Discourse on the new server, using the name TEMP, but otherwise the same settings as on the old server. (Email is likely to be configured differently.) Proceed through the web-based setup – the details don't really matter – to the point that you can log in (on TEMP) as an admin user.

On the old server, the backup can be found under /var/discourse/shared/standalone/backups/default. Copy it across to the same directory on the new server.

In the web interface of the new server, Settings, Backups, allow restore. Then Admin, Backups, and Restore the backup; then, if it's set, turn off "enable read-only". Also if you get a banner saying "All outoing email has been globally disabled by an administrator", you can fix that with Settings, Email, "disable emails" = "no".

Back to the root prompt on the new server. Edit /var/discourse/containers/app.yml to change TEMP to SERVER (and perhaps compare it with the same file on the old server, to make sure any plugins etc. are set up), then in /var/discourse run ./launcher rebuild app.

When that finishes, change the DNS to point SERVER to the new machine's IP address (and remove TEMP), and everything should be working. Once you're sure it is, set the DNS TTL back up to where it was before.

Tags: computing

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