Open source cloud technologies at a glance

Cloud Quartet


OpenNebula version 3.8 was released December 2012 and can point to a large number of installations in its nearly five years of history. Prominent supporters include IBM, Dell, and SAP. The OpenNebula developers also put great emphasis on interoperability and open standards. The supported hypervisors, include Xen, KVM, and VMware, and Hyper-V will be added in the near future.

OpenNebula is just as versatile when it comes to cloud APIs: It supports EC2, EBS, OFG, OCCI, and vCloud. The complete architecture is very flexible, making it easy to operate completely different hardware and software combinations in the data center. As of version 3.6, developers can offer prebuilt appliances via the OpenNebula Marketplace. The most prominent user of OpenNebula is CERN.

CloudStack and Citrix

The four open source IaaS solutions can draw on a number of commercial public/private cloud offerings: AWS, vSphere/vCloud, Windows Azure, Oracle VM, Red Hat, Rackspace, and the CloudStack-based Citrix Cloud Platform. It is interesting to note that open source solutions are leading the way in cloud computing. However, on closer inspection, this is not surprising because no other type of software depends to the same extent on interoperability and open standards. An interesting classification of cloud-based computing models can be found on the OpenNebula blog [5] (see Figure 1).

Figure 1: An attempt to classify current cloud platforms with an open source license. (From

If you believe the statements of the respective communities, CloudStack is the fastest growing cloud management platform. The approximately 30,000 community members to date, is a clear message. Citrix also cites several thousand certified apps and several hundred productive clouds.

According to Citrix, more than 50 well-known companies and organizations rely on CloudStack or the Citrix Cloud Platform, including GoDaddy, Nokia, Intel, Zynga, RightScale, PuppetLabs, Juniper Networks, Enstratus, TrendMicro, and Equinix. Brand name users such as ServiceMesh, Engine Yard, and RightScale already welcomed the change to the Apache License about a year ago.

According to Citrix, the change from OpenStack to CloudStack mainly came about because of CloudStack's compatibility with Amazon, in that the Cloud API was a must-have. This is also why Citrix abandoned its own OpenStack-based cloud distribution, "Olympus," and committed itself to CloudStack.

What's New in Version 4.0 and 4.0.1

Version 4.0 of CloudStack from last November contained numerous innovations that justify its promotion to an Apache top-level project, including inter-VLAN routing (Figure 2), site-to-site VPN, and tags.

Figure 2: One of the new features in CloudStack 4.0: Inter-VLAN routing. (From Apache CloudStack wiki)

Inter-VLAN routing makes it possible to control the network traffic between VLANs – a requirement for building private clouds. The site-to-site VPN function helps circumvent the establishment of VPN connections between individual virtual machines and establish a secure connection between the corporate network and a cloud infrastructure. The new tags feature lets users save metadata for cloud resources to facilitate categorization. CloudStack's biggest advantage over previous versions is that it includes the new version of Amazon's API, CloudBridge [6], whereas the component previously needed to be installed separately. The current 4.0.1 release from February of this year, however, is purely a bug fix release.

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