understand-az.html.md.erb

---
title: Understanding Availability Zones in VMware Installations
owner: Ops Manager
---

<strong><%= modified_date %></strong>

Pivotal defines an Availability Zone (AZ) as an operator-assigned, functionally independent segment of network infrastructure. In cases of partial infrastructure failure, [Pivotal Cloud Foundry&reg;](https://network.pivotal.io/products/pivotal-cf) (PCF) Elastic Runtime distributes and balances all instances of running applications across remaining AZs. Strategic use of Availability Zones contributes to the fault tolerance and high availability of a Elastic Runtime deployment.

## <a id="azs-vmware"></a>AZ Differences Between VMware Products ##

Elastic Runtime on VMware vSphere supports distributing deployments across multiple AZs. See the section on AZs in [Configuring Ops Manager Director for VMware vSphere](./vsphere-config.html#create-az). VMware vCloud and vCloud Air support only one AZ, which is the virtual data center you access with <a href="./pcf-vchs-vcloud.html#access-interface">vShield Edge Gateway Services interface</a>.

## <a id="example-case"></a>Balancing Across AZs During Failure: Example Scenario ##

An operator scales an application to four instances in an Elastic Runtime environment distributed across three availability zones: 
A1, A2, and A3. The environment allocates the instances according to the [Diego Auction](../concepts/diego/diego-auction.html).

  <%= image_tag("az_er_1.png") %>

If A1 experiences a power outage or hardware failure, the two application instances running in A1 terminate while the application instances in zones A2 and A3 continue to run:

  <%= image_tag("az_er_2.png") %>

If A1 remains unavailable, Elastic Runtime balances new instances of the application across the remaining availability zones:

  <%= image_tag("az_er_3.png") %>