Backing up Discourse with RDS

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We are launching a community site with Discourse at CircleCI and I ran into some issues with performing backups.

Since we want to be #webscale, rather than installing Discourse using the easy Docker method, we opted to split up the services on AWS.

We are using EC2, RDS, ElasticCache, ELB and S3 for backups. When I initially launched PostgreSQL on RDS I just picked the latest version (9.4 at time of writing) and didn’t put much thought into it. The problem is that the version of pg_dump inside the Docker container provided by Discourse is 9.3. Apparently you are not able to do a pg_dump when your client and server are mismatched.

When you try to do this, you get a sad error that states:

aborting because of server version mismatch

In order to fix this, we need to upgrade the client version on the Docker container (or alternatively avoid this all together by using the same version of PostgreSQL on RDS). Since it is easier to update the client than downgrade RDS, I opted to upgrade the client.

To do this:

  1. Log into the EC2 Instance that is running Discourse
  2. Go to the Discourse install location (mine is in /var/docker)
  3. Enter the container with:
     sudo ./launcher enter app
    
  4. Upgrade the version of PostgreSQL client
     apt-get install postgresql-9.4
    
  5. Swap out the pg_dump symbolic link for the newer version
     ln -s /usr/lib/postgresql/9.4/bin/pg_dump /usr/bin/pg_dump
    
  6. Perform your backup

When you update the Docker container for the next release you will likely have to repeat these steps until Discourse gets to the same version of PostgreSQL as the one you chose on RDS.


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