The Undercloud server is a basic, single-node, OpenStack installation running on a single physical server used to deploy, test, manage, and update the overcloud servers. There is no HA configuration for the Undercloud.
Undercloud is a single node OpenStack installation deployed either in a Virtual machine or in bare metal server. The objective of undercloud is to install, test, manage and update its overcloud servers (Controller, Compute & Storage nodes). In Red Hat terminology undercloud is known as “Director“. Undercloud basically contains all the images that is required to deployed Overcloud Controller, Computes, Swift and Storage.
It contains a strictly limited sub-set of OpenStack, just enough to interact with the Overcloud. The services running on Undercloud are Nova, Neutron, Glance, Keystone, Ironic, Heat, Horizon & Ceilometer. This server also contains HP Helion content distribution catalog service, which provides a mechanism to download and install content and updates for the Overcloud.
The Undercloud also hosts images for various server types which will form the functional cloud environment – the Overcloud. These images are Overcloud Controller, Overcloud Compute, Overcloud Swift & Overcloud Compute Proxy (required for cloud which supports VMWare ESX as a hypervisor).
Overcloud is the actual functional cloud for any organization where all the Virtual Machines are created on compute nodes and handle all the workloads. In other words we can say overcloud provides the openstack services like Nova, Keystone , Horizon, Neutron, cinder, Glance, Heat & Ceilometer Services. The Overcloud is the functional cloud available to run guest virtual machines and workloads.
TripleO is the friendly name for “OpenStack on OpenStack”. It is an official OpenStack project with the goal of allowing you to deploy and manage a production cloud onto bare metal hardware using a subset of existing OpenStack components. With TripleO, you start by creating an “undercloud” (a deployment cloud) that will contain the necessary OpenStack components to deploy and manage an “overcloud” (a workload cloud). The overcloud is the deployed solution and can represent a cloud for any purpose (e.g. production, staging, test, etc). TripleO leverages several existing core components of OpenStack including Nova, Ironic, Neutron, Heat, Glance and Ceilometer to deploy OpenStack on baremetal hardware. Nova and Ironic are used in the undercloud to manage baremetal instances that comprise the infrastructure for the overcloud. Neutron is utilized to provide a networking environment in which to deploy the overcloud, machine images are stored in Glance, and Ceilometer collects metrics about your overcloud.