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+# Platform-Install Internals
+
+## Prerequisites
+
+When platform-install starts, it is assumed that `gradelew fetch` has already been run on the cord repo, fetching the necessary subrepositories for CORD. This includes fetching the platform-install repository.
+
+For the purposes of this document, paths are relative to the root of the platform-install repo unless specified otherwise. When starting from the uber-cord repo, platform-install is usually located at `/cord/components/platform-install`.
+
+## Configuration
+
+Platform-install uses a configuration file, `config/default.yml`, that contains several variables that will be passed to Ansible playbooks. Notable variables include the IP address of the target machine and user account information for SSHing into the target machine. There's also an extra-variable, `on-cloudlab` that will trigger additional cloudlab-specific actions to run.
+
+Cloudlab nodes boot with small disk partitions setup, and most of the disk space unallocated. Setting the variable `on-cloudlab` in `config/default.yml` to true will cause actions to be run that will allocate this unallocated space. 
+
+## Gradle Scripts
+
+The main gradle script is located in `build.gradle`. 
+
+`build.gradle` includes two notable tasks, `deployPlatform` and `deploySingle`. These are for multi-node and single-node pod installs and end up executing the Ansible playbooks `cord-head-playbook.yml` and `cord-single-playbook.yml` respectively. 
+
+## Ansible Playbooks
+
+Platform-install makes extensive use of Ansible Roles, and the roles are selected via two playbooks: `cord-head-playbook.yml` and `cord-single-playbook.yml`. 
+
+They key differences are that:
+* The single-node playbook sets up a simulated fabric, whereas the multi-node install uses a real fabric.
+* The single-node playbook sets up a single compute node running in a VM, whereas the multi-node playbook uses maas to provision compute nodes.
+* The single-node playbook installs a DNS server. The multi-node playbook only installs a DNS Server when maas is not used. 
+
+## Ansible Roles and Variables
+
+Ansible roles are located in the `roles` directory.
+
+Ansible variables are located in the `vars` directory. 
+
+### DNS-server and Apt Cache
+
+The first step in bringing up the platform is to setup a DNS server. This is done for the single-node install, and for the multi-node install if maas is not used. An apt cache is setup to facilitate package installation in the many VMs that will be setup as part of the platform. Roles executed include:
+
+* dns-nsd
+* dns-unbound
+* apt-cacher-ng
+
+### Pointing to the DNS server
+
+Assuming a DNS server was setup in the previous step, then the next step is to point the head node to use that DNS server. Roles executed include:
+
+* dns-configure
+
+### Prep system
+
+The next step is to prepare the system. This includes such tasks as installing default packages (tmux, vim, etc), configuring editors, etc. Roles executed include:
+
+* common-prep
+
+### Configuring the head node and setting up VMs
+
+Next the head node is configured and VMs are created to host the OpenStack and XOS services. Roles executed include:
+
+* head-prep
+* config-virt
+* create-vms
+
+### Set up VMs, juju, simulate fabric
+
+Finally, we install the appropriate software in the VMs. This is a large, time consuming step since it includes launching the OpenStack services (using juju), launching ONOS, and launching XOS (using service-platform). Roles executed include:
+
+* xos-vm-install
+* onos-vm-install
+* test-client-install
+* juju-setup
+* docker-compose
+* simulate-fabric
+* onos-load-apps
+* xos-start
+
+## Starting XOS
+
+The final ansible role executed by platform-install is to start XOS. This uses the XOS `service-profile` repository to bring up a stack of CORD services. 
+
+For a discussion of how the XOS service-profile system works, please see [Dynamic On-boarding System and Service Profiles](https://wiki.opencord.org/display/CORD/Dynamic+On-boarding+System+and+Service+Profiles). 
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