Archive for October, 2014

6 October, 2014

RDS: Three weeks in

by gorthx

I’ve spent the past few weeks learning my way around Amazon’s RDS offering (specifically Postgres, and a bit of elasticache). It’s a mixed bag so far; for every feature I think “Hey, this is neat!” I find at least one or two others that are not so thrilling.

One of the things that may annoy you if you’re used to running your own Pg server is not having “real” superuser access to your own cluster. There’s an rdsadmin database which you won’t have access to, and a couple of rds users as well. This is part of Amazon’s security implementation to protect all their users from destroying each other.

You need to exclude the rdsadmin database out of any management queries you run, like so:
SELECT datname, pg_size_pretty(pg_database_size(datname))
FROM pg_database
WHERE datname NOT IN ('postgres','template0','template1', 'rdsadmin')
ORDER BY datname;

Otherwise you’ll get a permissions error.

Today, I had this bit of fun:
pg_dumpall --globals-only
ERROR: permission denied for relation pg_authid

So no dumping your roles. (No pg_dumpall, period.) I don’t have high hopes for an alternative.

I also haven’t found a place to report bugs – the “Feedback” button only allows you to contact tech support if you have a contract. So far I’m relying on the “I know someone who knows someone” method and the RDS user forums; AWS team members respond fairly quickly to posts there.

Aside from all that, I’m primarily interested in automating my instance management, so I’ve focused on the CLI tools.

My first thought was “Egads, I can’t get away from Java, can I”, but installing the toolkit turned out to be the easiest part. I set my $JAVA_HOME as outlined here to avoid that annoying “Unable to find $JAVA_HOME” error.

The CLI support is much more extensive than I expected – you can manage pretty much everything, very easily, once you find your way around and learn the appropriate incantation. Which is my biggest complaint: the docs on the web and the cli help don’t always match each other, and sometimes neither are correct. It cost me a significant amount of startup time flipping back and forth between the cli help, the web help, and just flat-out experimenting and cursing that mysterious “Malformed input” error message. (I probably have unrealistically high standards after working with the Pg docs for so many years.)

Fun stuff you can do:
RDS offers “event subscriptions” to help you keep tabs on your instance health (failover, storage, etc)1. They’re pretty easy to configure from the web console, but once you’ve done so, there’s no way to view or edit them except from the CLI. (At least, not that I can find.)

You can grab your Cloudwatch metrics, if you need to “roll your own” and integrate them into an existing monitoring system. (Yes, I briefly considered this!)

Log files are also accessible through the CLI for watching or downloading. It’s a huge improvement on the tiny green-text-on-black background on the web console. There’s no glob expression matching, so you have to request them one at a time. If you request a log file that doesn’t exist, you don’t get an error.

log_line_prefix is one of the unconfigurable GUCs on RDS, so if you are planning to use pgbadger 2, specify Amazon’s format as outlined at the bottom of this page.



1 – Be warned that restoring a snapshot will generate an “instance deleted” alert for your newly-restored instance. Your on-call person may not appreciate this.

2 – Alternatively, the pg_stat_statements extension is available on RDS, so you could get query stats this way.

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