Default Datacenter (Availability Zone)

2011/08/26

Today we wanted to shine some light on a small but useful feature of Scalarium: the default datacenter (or availability zone) of a cloud.

When you create a cloud you chose an AWS region that this cloud should live in. This defines which global EC2 installation you want to use, e.g. start your servers in Virginia (us-east-1) or in the EU (eu-west-1).

Within every AWS region there are at least two different availability zones, the actual datacenter where your servers and EBS volumes will live. When you add an instance to a cluster you can set the availability zone of the instance. Scalarium always allowed (and encouraged) you to place your instances in multiple availability zones.

The reason why you want to spread your servers across multiple availability zones is redundancy. Should one availability zone experience a problem, you still have unaffected instances. A typicall setup would be to have most of your instances in for example eu-west-1b for performance and cost reasons and then have one or two backup slaves in eu-west-1c or eu-west-1a.

Another setup would be to place entire clouds into a dedicated availability zone, e.g. the production cloud in us-east-1a and the backup cloud in us-east-1b.

When managing such setups you have to remenber where a new instance should be placed. And this is where the default availability zone setting of a cloud comes into play. It allows you to specify exactly that: the default availability for new instances in this cloud.

You can set the default availability zone of a cloud when editing a cloud via the "Edit Cloud" link in the actions menu.