The "immutable servers" concept is a new approach to deployment. Instead of upgrading your servers, replace them with new ones. Likewise, instead of updating the code running on those servers, deploy it from scratch on new servers, and throw the old ones away.

Sounds extreme? In a world of physical machines, probably. But when your servers are virtual machines that can be deployed in minutes, or containers that can be deployed in seconds, this pattern becomes possible, and even relatively easy to implement.

What's the point? Immutable servers give us:

• Fast, reliable rollbacks, since you can always revert a deployment by restarting the old servers;
• Easy A/B testing, since you can run old and new versions side by side easily, and compare their behavior;
• Security, since each deployment will use an up-to-date base system.

In this talk, we will explain in detail the advantages of immutable servers, then explain how to implement them with containers in general, and Docker in particular.