# License: (CC 3.0) BY-NC-SA

Chapter 1. Getting Started with OpenStack

OpenStack Compute, which offers computing power through virtual machine and network management, and OpenStack Object Storage which is software for redundant, scalable object storage capacity.

Components of OpenStack

OpenStack Compute is a cloud fabric controller, used to start up virtual instances for either a user or a group. It’s also used to configure networking for each instance or project that contains multiple instances for a particular project.

OpenStack Object Storage is a system to store objects in a massively scalable large capacity system with built-in redundancy and failover. Object Storage has a variety of applications, such as backing up or archiving data, serving graphics or videos (streaming data to a user’s browser), storing secondary or tertiary static data, developing new applications with data storage integration, storing data when predicting storage capacity is difficult, and creating the elasticity and flexibility of cloud-based storage for your web applications.

OpenStack Image Service is a lookup and retrieval system for virtual machine images. It can be configured in three ways: using OpenStack Object Store to store images; using Amazon’s Simple Storage Solution (S3) storage directly; or using S3 storage with Object Store as the intermediate for S3 access.

OpenStack Project Architecture Overview

Dashboard

Horizon is a modular Django web application that provides an end user and administrator interface to OpenStack services.

o Horizon is usually deployed via mod_wsgi in Apache. The code itself is separated into a reusable python module with most of the logic (interactions with various OpenStack APIs) and presentation (to make it easily customizable for different sites). o A database (configurable to which one). As it relies mostly on the other services for data, it stores very little data of it’s own.

Compute

nova-api accepts and responds to end user compute and volume API calls. It supports OpenStack API, Amazon’s EC2 API and a special Admin API (for privileged users to perform administrative actions). It also initiates most of the orchestration activities (such as running an instance) as well as enforces some policy (mostly quota checks). In the Essex release, nova-api has been modularized, allowing for implementers to run only specific APIs.

The nova-compute process is primarily a worker daemon that creates and terminates virtual machine instances via hypervisor’s APIs (XenAPI for XenServer/XCP, libvirt for KVM or QEMU, VMwareAPI for VMware, etc.). The process by which it does so is fairly complex but the basics are simple: accept actions from the queue and then perform a series of system commands (like launching a KVM instance) to carry them out while updating state in the database.

nova-volume manages the creation, attaching and detaching of persistent volumes to compute instances (similar functionality to Amazon’s Elastic Block Storage). It can use volumes from a variety of providers such as iSCSI or Rados Block Device in Ceph.

The nova-network worker daemon is very similar to nova-compute and nova-volume. It accepts networking tasks from the queue and then performs tasks to manipulate the network (such as setting up bridging interfaces or changing iptables rules).

The nova-schedule process is conceptually the simplest piece of code in OpenStack Nova: take a virtual machine instance request from the queue and determines where it should run (specifically, which compute server host it should run on).

The queue provides a central hub for passing messages between daemons. This is usually implemented with RabbitMQ today, but could be any AMPQ message queue (such as Apache Qpid).

The SQL database stores most of the build-time and run-time state for a cloud infrastructure. This includes the instance types that are available for use, instances in use, networks available and projects. Theoretically, OpenStack Nova can support any database supported by SQL-Alchemy but the only databases currently being widely used are sqlite3 (only appropriate for test and development work), MySQL and PostgreSQL.

Object Store

The swift architecture is very distributed to prevent any single point of failure as well as to scale horizontally. It includes the following components:

Proxy server accepts incoming requests via the OpenStack Object API or just raw HTTP. It accepts files to upload, modifications to metadata or container creation. In addition, it will also serve files or container listing to web browsers. The proxy server may utilize an optional cache (usually deployed with memcache) to improve performance.

Account servers manage accounts defined with the object storage service.

Container servers manage a mapping of containers (i.e folders) within the object store service.

Object servers manage actual objects (i.e. files) on the storage nodes.

There are also a number of periodic process which run to perform housekeeping tasks on the large data store. The most important of these is the replication services, which ensures consistency and availability through the cluster. Other periodic processes include auditors, updaters and reapers.

Image Store

The Glance architecture has stayed relatively stable since the Cactus release. The biggest architectural change has been the addition of authentication, which was added in the Diablo release. Just as a quick reminder, Glance has four main parts to it:

glance-api accepts Image API calls for image discovery, image retrieval and image storage

glance-registry stores, processes and retrieves metadata about images (size, type, etc.)

A database to store the image metadata. Like Nova, you can choose your database depending on your preference (but most people use MySQL or SQlite).

A storage repository for the actual image files. In the diagram, I have shown the most likely configuration (using Swift as the image repository), but this is configurable. In addition to Swift, Glance supports normal filesystems, RADOS block devices, Amazon S3 and HTTP. Be aware that some of these choices are limited to read-only usage.

There are also a number of periodic process which run on Glance to support caching. The most important of these is the replication services, which ensures consistency and availability through the cluster. Other periodic processes include auditors, updaters and reapers.