What is a Stemcell?

A stemcell is a versioned Operating System image wrapped with IaaS specific packaging.

A typical stemcell contains a bare minimum OS skeleton with a few common utilities pre-installed, a BOSH Agent, and a few configuration files to securely configure the OS by default. For example: with vSphere, the official stemcell for Ubuntu Trusty is an approximately 500MB VMDK file. With AWS, official stemcells are published as AMIs that can be used in your AWS account.

Stemcells do not contain any specific information about any software that will be installed once that stemcell becomes a specialized machine in the cluster; nor do they contain any sensitive information which would make them unable to be shared with other BOSH users. This clear separation between base Operating System and later-installed software is what makes stemcells a powerful concept.

In addition to being generic, stemcells for one OS (e.g. all Ubuntu Trusty stemcells) are exactly the same for all infrastructures. This property of stemcells allows BOSH users to quickly and reliably switch between different infrastructures without worrying about the differences between OS images.

The Cloud Foundry BOSH team is responsible for producing and maintaining an official set of stemcells. See the stemcells section of bosh.io to see the infrastructures and operating systems that are currently supported.

Stemcells are distributed as tarballs.

By introducing the concept of a stemcell, the following problems have been solved:

  • Capturing a base Operating System image
  • Versioning changes to the Operating System image
  • Reusing base Operating System images across VMs of different types
  • Reusing base Operating System images across different IaaS