A cluster is a new way to organize and manage instances that share a specific function.
The new cluster model is currently only available for Riak. Other cluster types will be announced as they become available.
In the past, the term "cluster" was sometimes used to indicate a multi-instance environment. You might still see that usage in the docs and UI as we transition to the new model.
The new cluster
As part of the evolution of the Engine Yard Cloud platform, we have developed a new model, which takes many of the variables that were once stored at the environment level, and moves them down to the cluster level. One environment can have many clusters, and each cluster can run different cookbooks and be in different regions.
In the future, we'll be working to optimize AMIs by use cases at the cluster level to minimize boot times. Riak was built on the cluster model and is our first release.
We are very excited about the flexibility that this next step will bring to our product!