Appendix A: Availability Considerations : vCloud Director Cell Load Balancing
vCloud Director Cell Load Balancing
A load balanced, multicell vCloud Director architecture provides the following benefits:
*Scalability, by distributing session load across cells.
*Improved availability by monitoring cell server health and adding or removing cells from service based on status.
*Non-disruptive operating system patching and maintenance of the cell servers.
*Reduced impact to vCloud Director application upgrades.
Load balancing improves scalability in the following areas:
*Number of concurrent operations.
*Number of active and concurrent console sessions via the console proxy service.
*Number of concurrent users.
*Number of vCenter Server operations (in the case that multiple vCenter servers are attached to the vCloud Director instance).
vCloud Networking and Security Edge can be used to load balance vCloud Director cells, in addition to third-party external hardware or virtual appliances as load balancers.
The following table lists the design considerations for load balancing of vCloud Director cells.
Table 20. Load Balancer Considerations
A front-end firewall is typically deployed in front of the load balancer. In some environments, additional firewalls can be located between vCloud Director cells and the resource tiers managed by vCenter.
Load balancers can also provide NAT/SNAT (source network address translation) for the clustered cells.
VMware recommends securing access between cells and the other management and resource group components. Refer to the vCloud Director Installation and Configuration Guide for ports that must be opened.
Single vCloud Director site and scope
This architecture covers load balancing of a single vCloud Director site or instance. It does not cover client application load balancing or global load balancing.
Sizing recommendations for number of cells
VMware recommends the number of vCloud Director cell instances = n + 1, where n is the number of vCenter Server instances providing compute resources for vCloud consumption. Based on the service definition requirements, two vCloud Director cell instances are sufficient to increase availability and upgradability (first upgrade one vCloud Director cell, then the other).
Requirements for multicell configurations
Multiple vCloud Director cells require NTP (Network Time Protocol), which is a design guideline for all elements of the vCloud infrastructure.
See the white paper, Timekeeping in VMware Virtual Machines ( for more information on how to set up NTP.
Load balancer availability
Use at least two load balancers in a HA configuration to reduce single points of failure. There are multiple strategies for this depending on vendor or software used.
Proxy configuration
Each load-balanced vCloud Director cell requires setting a proxy console IP address that is typically provided by the load balancer.
Rest API URL configuration
Map the vCloud service URL to the address that the load balancer provides. This is configured in the vCloud Director administrator GUI and in the load balancer configuration. Use this address to check the health status of the vCloud Director cell.
Awareness of multicell roles
Some vCloud Director cell tasks (such as image transfer) can consume significant resources. All cells can perform the same set of tasks, but it is possible to set policies that affect which ones are used. See the advanced configuration settings.
Load balancer session persistence
Sessions are generally provided in secure methods and are terminated at the cells. Because of this, session persistence should be enabled using SSL.
Load balancing algorithm
Least connections or round robin is generally acceptable.
vCloud Director cell status health checks
*Configure the load balancer service to check the health of individual vCloud Director cells. Because each cell responds by way of HTTPS, this can be configured through the IP and API end point URL. Load balancers might support other types of health checks. Example UI URL
In the second example, the versions supported by this endpoint are returned as XML.
Check services periodically based on load. A good starting point is to check every five seconds.
Public IP/port
Specify the service IP appropriately before adding cells to the service group. Typically, port 443 (standard HTTPS) is the only port exposed.
Web Application Firewall
Can be used to apply URL restrictions on vCloud Director access to admin or organization portals based on source address. Requires SSL sessions to be terminated on the load balancer.
SSL Initiation
Used when SSL is terminated on the load balancer to initiate an SSL session to the vCloud Director cells (which only accept HTTPS).
Advanced configurations
Load balancers can also provide Layer 7 content switching or direction, which can allow a vCloud Director configuration to send certain types of client traffic to dedicated cells. Although each cell can perform any function, it is possible to separate functions by directing certain types of requests to specific cells.
Connection mapping
When a cell joins an existing vCloud Director server group, it might try and load balance sessions. This can affect connection mapping through the load balancer as it is unaware of the balancing that occurring within the server group.