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July 22, 2013 by Bas van Kaam 3 Comments XenDesktop 7, and some of the earlier XD editions as well, is based on the FlexCast Management Architecture or FMA in short.
Unlike XenApp servers, Delivery Agents only communicate with the Site Delivery Controller(s) and do not need to access the Site configuration database or license server directly.
Note that although Delivery Controllers are comparable to XenApp Data Collectors there are some distinct differences. If you have two or more Controllers as part of your Site infrastructure they only communicate with the central Site configuration database and license server, not with each other as apposed to XenApp Data Collectors mentioned above.
In an FMA, or XenDesktop architecture Delivery Controllers don’t have a LHC so if they need to authenticate a user or enumerate applications for example they will (always) need to contact the central Site configuration database for information. Since all static configuration information like Site policies, Machine Catalogs, Delivery Groups and published applications and or (hosted) desktops is stored in the central database, and all live dynamic runtime data like who is connected to which resource, on which server, server load and connection statuses used for load balance decision making as well, this database has become and is very important. It’s recommended to backup your database on a regular basis so it can be restored if necessary when the database server fails (or the database itself) In addition there are several high availability solutions to consider. Using one of the above methods, in combination with regular (daily) back-ups, will ensure that your central Site configuration database will always (well…) be online, or will at least narrow your chances of running into any issues. Although it’s hard to imagine this ever happening it’s good to know what your options are :-) When enabled, if the communication with all Delivery Controllers fails high availability mode is initiated after a pre set period of time, which is configurable.
As soon as a Controller becomes available the VA will try and register itself without any interruptions to the user. As per Citrix: High availability mode is suitable only for use with dedicated desktops, where the mapping between the user and the DA is known. Have a look here it will lead you to the E-Docs article explaining how to set and create the appropriate registry key’s and how to create an ICA launch file. Be aware that unless proper action is taken your central SQL configuration database might be a single point of failure, it seems obvious, but still… Be sure to spent some time understanding how this works, get to know the FMA architecture, it can save you a lot of trouble.
Yesterday I had the opportunity to present a TED-like talk at the GeekSpeak Tonight Session at Citrix Synergy Conference. The session was mostly vendor agnostic, where I discussed high level concepts about distributed file systems and web-scale technologies. As we move to a software centric and VM centric world, we start being able to play with different properties that until then were not easily available via simple API queries.
In another scenario, call center desktops may not require such high availability and a Bronze SLA is selected. All this configuration should happen in the background without any special requirement, hardware configuration and major team work. The screenshot below demonstrate a fully integrate Service Level Automation engine built into Citrix XenDesktop Studio**.

I’m not going into to much detail here, just think of the IMA and Site configuration database as one central location where all configuration information regarding the Farm or Site gets stored and pulled from when needed. Having said that, XenApp workers (session host only servers) offer the same sort of benefit. Sure, the both handle user authentication and load balancing for example but in very different ways. If you need or want to separate them like we did with zones, for geographical purposes perhaps, you will need to create two separate Sites and apply load balance policies on Site level instead of zone preference or load balance policies on zone level, not a necessity but often preferable. Mirroring the Database ensures that, should you lose the active database server, the automatic failover process happens in a matter of seconds, so that users are generally unaffected. However, setting up this solution is more complicated, and the automatic failover process is typically slower than with alternatives such as SQL Mirroring # AlwaysOn Availability Groups… a high-availability and disaster recovery solution introduced in SQL Server 2012 to enable you to maximize availability for one or more user databases. Have a closer look at the above solutions and decide for yourself which one works best for you. If a user device is already connected to the desktop, users are unable to connect from a different user device # Delivery Controller-originated policies. The talk was about hyper-convergence and web-scale technologies and how they redefine the way VDI is architected and deployed. In last 3 slides, I talk about the work that Nutanix is doing with Citrix to enable a full featured service level automation and service level agreement for VDI.
Nutanix will soon make an official announcement  on the XenDesktop Studio integration with Nutanix to drive SLAs.
In the case of IMA (XenApp) all servers have a Local Host Cache in which a copy of the IMA data store information gets cached.
As they only host user sessions and will (or can) never be ‘elected’ as an Data Collector for their zone they won’t get all the IMA store (database) information pushed into their LHC enhancing overall performance.
Data Collectors are part of zones with each zone having its own (there can be only one per zone) Data Collector. In XenDesktop there’s no need for this since the Delivery Controllers don’t have a LHC and get all their information directly from the central Site configuration database, live runtime data included.
If for whatever reason this database becomes unreachable, running sessions will keep working but new sessions cannot be established and configuration changes aren’t possible. AlwaysOn Availability Groups requires that the SQL Server instances reside on Windows Server Failover Clustering (WSFC) nodes.
So your database is fine but your Controller(s) aren’t hmm… You can configure your Virtual Agents (VA) to operate in high availability mode, this way users can continue to use their desktops and installed applications. If during these 30 days the VA isn’t able to register itself with one of the Controllers the desktop(s) will stop listening for connections and will be no longer available.

Policies originating on the Controller, such as those governing client drive mapping and access to the clipboard, will not function as there is no connection to the Controller. This is a very important topic, because we are affectively driving Software Defined Datacenter concepts and being able to apply properties and policies without relying on special hardware configuration, or other groups in the IT organization*.
We are now able to drive Service Level Automations that can be easily triggered by administrators, and in the future users.
However, these workers still consist of the same bits and bytes as installed on a Data Collector (the full XenApp installation) compared to ‘just’ a Delivery Agent which are lighter weight, as Citrix puts it.
Data Collectors also hold and collect dynamic live runtime data used for making load balance decisions. Although the above differences do raises some questions as far as Farm vs Site designs go, XenDesktop has been doing it this way for a few years now so we should be ok. This solution is less expensive than mirroring as it uses your existing Host software and you can also use SQL Express. In high availability mode the VA will accept direct ICA connections from users instead of connections brokered by a Delivery Controller. During this time the VA will attempt to register itself with the, or one of the controllers while your users will continue to use their desktop and or installed applications.
You have to create an ICA file for each user who requires this feature; Citrix does not create or distribute ICA files for this purpose.
XenApp Data Collectors also cache live runtime data used for load balancing, with XenDesktop all this is stored in the central Site configuration database, read on. However, the automatic failover process is slower, as it can take time for a new machine to start for the database, which may interrupt the service to users. Note that policies from a previous registration persist and are applied, so outdated policies might take affect # Power management. When the desktop powers up, it attempts to register, fails and, after the timeout, enters high availability mode # NetScaler Gateway and Remote Access. Controllers are part of your Site and you can have multiple spreading the load, no problem.

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