AWS Elastic Beanstalk - why would I use leader_only for a command?

16
votes

I am writing a django app which I plan on deploying to AWS via Elastic Beanstalk. I am trying to understand why I would need to specify 'leader_only' for a container command I want to run for my app. More details about this can be found here.

It says:

Additionally, you can use leader_only. One instance is chosen to be the leader in an Auto Scaling group. If the leader_only value is set to true, the command runs only on the instance that is marked as the leader.

If I have several instances running my app because I want to scale it, wouldn't using 'leader_only' run the command on only one instance, and not affect the rest? I am probably misunderstanding the purpose of it, but that seems non-ideal because the environment in the leader may differ from the other instances, and the end user may get different results depending on which instance they happen to connect to.

1
amazon-web-servicesamazon-elastic-beanstalk
As stated below, but to be concise, it's used when you only need to run something once against the stack. Example: running database migrations. – Ryan Fisher

1 Answers

27
votes

From a technical point of view, elastic beanstalk is autoscaling group and when you deploy something you need to assume that potentially your commands can be executed simultaneously on several ec2 instances.

Main goal of the leader_only option is to make sure that your commands will be executed on only one ec2 instance. It is useful for use cases such as execution of the db migration scripts, creation of db, etc., that should be executed just once on one ec2. So leader_only is just a marker that some commands will be executed on this instance only.

However, you need to keep in mind, the leader attribute is set once on creation of your environment and in case if leader died and was replaced by new instance possible situation when you don't have any leaders in autoscaling group.