Nowadays, in a lot of companies, the IT environment is way more important than it was a few years ago. The IT environment is one of the primary processes. For example, the order system or the e-mails need to be fully functional 24 hours a day, 7 days a week. This makes it hard for the IT department to perform maintenance or migrations, for example.

Because of this, companies need to invest more money in the IT environment to make sure that it is fully functional at any time, for example, with data centers at different locations. This data center could be on the other side of the building or at a separate location. This separated location could be a colocation provider, for example.

When implementing a second or a third data center, we also need to be aware of the changes that need to be made to the environment. All data centers need to be connected with each other through a good connection with a high capacity to transfer data between the data center locations.

Also, we need to think about whether the secondary data center will be in an active/active configuration or an active/passive configuration. In an active/active configuration, it's possible to let users connect to both data centers. In active/passive mode, only one data center will be available for the user; if the current active data center stops running, the IT department can switch over to the passive data center.

Depending on the needs and the available finances, we need to consider whether the IT environment should be in active/active or active/passive mode. Also, between the active/passive functionality we can make a difference. This is because it's possible to have the passive data center start up automatically when the primary data center stops running.

Normally, the storage will always be in an active/active configuration. This is because if the active data center stops running, we don't like to use old data when running on the passive data center.

When we have an active/active data center or an active/passive data center with automatic failover, we need to think about whether the users have a connection to the environment. In the case of Citrix, all users are connected through Citrix StoreFront or the Citrix NetScaler Gateway functionality. The user connects through the Citrix Receiver or through a web browser to the environment. So basically, the user is connecting through HTTP or HTTPs traffic to a Fully Qualified Domain Name (FQDN). This FQDN will be translated by the DNS server into an IP address. In the case of an active/active data center configuration, or when the environment is configured with different IP addresses, we need at least two IP addresses available for Citrix StoreFront and Citrix NetScaler Gateway in every data center.

Because of these different IP addresses, users don't know how to connect should there be a problem. The GSLB functionality can help us with this issue. With GSLB, we can use different IP addresses; if there is a problem, the GSLB functionality recognizes this and disables one of the IP addresses.

In the next diagram, we can find an active/active Citrix environment configuration based on the Citrix NetScaler Enterprise or Platinum license. In this configuration, we're using Citrix NetScaler to determine whether the user is logging in.

Global server load balancing is configured on the fastest response time, which means that the fastest data center will respond to the user.

In every data center, we're using Citrix NetScaler to load balance Citrix StoreFront, the Citrix Data Collectors, and the Active Directory with failover to the other data center. These failover virtual servers will be used only if the servers in their own data center stop running. This ensures that there is no single point of failure.

As discussed in Chapter 1, Configuring the Standard Features of NetScaler ^®, we need to perform a callback from the Citrix StoreFront server to Citrix NetScaler. When using GSLB, it could be possible that the callback functionality is sent to the other data center.

To arrange this, we're installing a separate Citrix NetScaler Gateway virtual server; it will be used only for the callback. This ensures that always the correct Citrix NetScaler Gateway will be contacted. This Citrix NetScaler Gateway virtual server for callback needs a separate URL, for example, dc1-citrix.company.com.

The Citrix StoreFront environment will be configured as a separated server group per data center. We're creating separated server groups per data center because we need to make some adjustments to each data center to ensure some settings. As server farms in Citrix StoreFront, we will create two farms, one per data center. Since the same applications/desktops are running in both data centers, we will see all the appliances double; this is because we're searching in two server farms. With a little adjustment from Citrix StoreFront, we will display only the primary server farm (the Citrix environment) in its own data center. This customization needs to be done on all the Citrix StoreFront servers manually.

The following configuration items will be used:

<resourcesWingConfigurations>
  <resourcesWingConfiguration name="Default" wingName="Default" />
</resourcesWingConfigurations>

<resourcesWingConfigurations>
  <resourcesWingConfiguration name="Default" wingName="Default">
    <userFarmMappings>
      <clear />
      <userFarmMapping name="user_mapping">
        <groups>
          <group name="domainusergroup" sid="securityidentifier" />
          <group ... />
          ...
        </groups>
        <equivalentFarmSets>
          <equivalentFarmSet name="setname" loadBalanceMode="{LoadBalanced | Failover}"
           aggregationGroup="aggregationgroupname">
            <primaryFarmRefs>
              <farm name="primaryfarmname" />
            </primaryFarmRefs>
            <backupFarmRefs>
              <farm name="backupfarmname" />
            </backupFarmRefs>
          </equivalentFarmSet>
        </equivalentFarmSets>
      </userFarmMapping>

    </userFarmMappings>
  </resourcesWingConfiguration>
</resourcesWingConfigurations>

<optimalGatewayForFarmsCollection />

<optimalGatewayForFarmsCollection>
  <optimalGatewayForFarms enabledOnDirectAccess="{true | false}">
    <farms>
      <farm name="farmname" />
    </farms>
    <optimalGateway key="_" name="deploymentname" stasUseLoadBalancing="{true | false}"
     stasBypassDuration="hh:mm:ss" enableSessionReliability="{true | false}"
     useTwoTickets="{true | false}">
      <hostnames>
        <add hostname="appliancefqdn:port" />
      </hostnames>
      <staUrls>
        <add staUrl="https://stapath/scripts/ctxsta.dll" />
      </staUrls>
    </optimalGateway>
  </optimalGatewayForFarms>
  <optimalGatewayForFarms>
    ...
  </optimalGatewayForFarms>
</optimalGatewayForFarmsCollection>