Is it possible to do an aws in place code deploy with zero downtime?

Question

Is it possible to achieve zero downtime deployments with in place deployment? I know that in the in deployment group description (for in place deployment) we have this:

Updates the instances in the deployment group with the latest application revision. During a deployment, each instance will be briefly taken offline for its update.

My question is: supposed I have at least two instances connected to a load balancer. Is it possible to do in place deployment using this sequence:

• instance A is disconnected from the LB (Load Balancer)
• instance A installs latest updates and restarts
• instance A is upgraded to latest code revision
• the LB disconnects from instance B which still has the old code revision, and points exclusively to instance A
• instance B installs latest revision and restarts
• instance B is upgraded to latest code revision
• the LB connects to instance B as well

Is the above possible? and if so how do i set it up? I've experimented with deploy one at a time, all at once etc but what happened is that in all cases the deployment always happened to all instances at the same time, and so there was some downtime.

Answer 1

I think one option would be to create a revision for each deployment in a separate folder and eventually link it via symlink using afterInstall hook after deployment is complete.

eg.

 releases
    ├── release1
    └── release2

public_html -> (symlink) release2

Answer 2

Yes, you can do exactly what you describe. If you configure your deployment group with the deployment option WITH_TRAFFIC_CONTROL and configure a load balancer, CodeDeploy will remove your instances from the load balancer before it runs the usual set of Lifecycle hook scripts.

You can see a chart here that demonstrates the order that the hooks are executed (note that BlockTraffic and AllowTraffic are performing the binding and unbinding). The host selection logic will be the same - your minimum healthy host settings. If you set your deployment to one at a time, CodeDeploy will not deploy to multiple instances at the same time.