Local Host Cache (LHC)

Each XenApp server stores a subset of the data store in the Local Host Cache (LHC). The LHC performs two primary functions:

·        Permits a server to function in the absence of a connection to the data store.

·        Improves performance by caching information used by ICA Clients for enumeration and application resolution.

The LHC is an Access database, Imalhc.mdb, stored, by default, in the <ProgramFiles>\Citrix\Independent Management Architecture folder.
The following information is contained in the local host cache:

·        All servers in the farm, and their basic information.

·        All applications published within the farm and their properties.

·        All Windows network domain trust relationships within the farm.

·        All information specific to itself. (Product code, SNMP settings, licensing information)

On the first startup of the member server, the LHC is populated with a subset of information from the data store. In future, the IMA service is responsible for keeping the LHC synchronized with the data store. The IMA service performs this task through change notifications and periodic polling of the data store.

If the data store is unreachable, the LHC contains enough information about the farm to allow normal operations for an indefinite period of time, if necessary. However, no new static information can be published, or added to the farm, until the farm data store is reachable and operational again.
Note: Prior to the release of Presentation Server 3.0, the LHC had a grace period of only 96 hours; this was due to farm licensing information being kept on the data store. Once the 96 hour grace period ended, the licensing subsystem would fail to verify licensing, and the server would stop accepting incoming connections.

When a change is made to the farm’s data store, a notification to update the local host cache is sent to all the servers in the farm. However, it is possible that some servers will miss an update because of network problems. Member servers periodically query the data store to determine if changes were made since the server’s local host cache was last updated. If changes were made, the server requests the changed information..

Refreshing the Local Host Cache

If the IMA service is currently running, but published applications do not appear correctly in ICA Client application browsing, force a manual refresh of the local host cache by executing dsmaint refreshlhc from a command prompt on the affected server. This action forces the local host cache to read all changes immediately from the data store.

A discrepancy in the local host cache occurs only if the IMA service on a server misses a change event and is not synchronized correctly with the data store.

Recreating the Local Host Cache

·        If the IMA service does not start, the cause may be a corrupt LHC.

·        If you have made extensive changes to the farm data store, such as publishing various applications, adding or removing servers from the farm, or creating new policies.

·        If you must clean the farm data store, using the DSCHECK utility, you should then rebuild the LHC on each of the servers in your farm, once the data store has been cleaned.

Steps to Recreate the Local Host Cache

1.     Stop the IMA service on the XenApp server, if it is started. This can be done using the command: net stop imaservice, or from services.

2.     Run dsmaint recreatelhc, which renames the existing LHC database, creates a new database, and modifies the following registry key HKEY_LOCAL_MACHINE\SOFTWARE\Citrix\IMA\Runtime\PSRequired key to 1.
Setting the value PSRequired to 1 forces the server to establish communication with the data store in order to populate the Local Host Cache database. When the IMA service is restarted, the LHC is recreated with the current data from the data store.

3.     Restart the IMA service. This can be done via the command line, net start imaservice, or from services.

Note: For XenApp 6 or later the registry key path is HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Citrix\IMA\ RUNTIME\PSRequired to 1.
There is also an available built-in utility to check the Local Host Cache called LHCTestACLsUtil.exe file located in C:\Program Files (x86)\Citrix\System32 of the XenApp server. To run this utility, you must have local administrator privileges.

Difference’s between ICA (HDX new name for ICA) and RDP (RDS)

ICA stands for Independent Computing Architecture. It is a proprietary protocol for an application server system, designed by Citrix Systems. ICA/HDX is a protocol that the Citrix platform uses for transmitting keystrokes, mouse movements, and screen updates between clients and the Citrix Presentation Server.

ICA/HDX can run on top of many operating systems including Windows, Linux, iOS. ICA/HDX is designed to run over industry-standard network protocols, such as TCP/IP, NetBEUI, IPX/SPX, and PPP and industry-standard transport protocols, such as async, ISDN, Frame Relay and ATM.

Within the ICA protocol, virtual channels are used to designate the various functionalities, such as client drive mappings, video, keyboard strokes, etc. Layer 3 (IP) and Layer 4 (TCP). You can have up to 32 virtual channels.

Citrix Xenapp ICA/HDX and Microsoft’s RDP are very similar, but there are definitely differences, especially when it comes to latency and unstable connections, which ICA is able to handle better, as well as many options on how to handle those types of connections. Based on the Remote Desktop Protocol (RDP), Terminal Services was first introduced in Windows NT 4.0 (Terminal Server Edition).

Main difference’s between ICA and RDP –

ICA/HDX – Independent Computing Architecture:

-        Application publishing is supported by ICA.

-        Low bandwidth is sufficient for ICA.

-        Encryption is possible in ICA.

-        Keyboard and Mouse inputs are present in ICA.

-        ICA ensures session reliability.

RDP – Remote Desktop Protocol:

-        RDP works only under TCP/IP.

-        RDP does not support applications to run in a browser.

-        Only 128-bit encryption, using the RC4 encryption algorithm, as of Version 6.

-        Session reliability is not ensured by RDP.

Performance Differences between RDP and ICA test result –

Like Oscar Pistorius, Microsoft might not always finish first – but you can bet on them to be a gold medalist and the inspiration for a lot of the competition.

10 interview question you are likely to be asked in Citrix XenApp Interview-1

Question 1 – Which network port does the ICA protocol use by default?
Ans- 1494 for Connections without session reliability or 2598 if session reliability is used.

Question 2– What is the purpose of the Data Collector?
Ans- The zone data collector is simply a Presentation Server which is performing the data collector role in a particular zone. The Presentation Server maintaining the data collector role is responsible for directing all requests from ICA clients launching published applications

Question 3– Is the information kept in the XenApp Data Store best described as static or dynamic?

Ans- Static

Question 4– What happens to existing, active user sessions in XenApp when the IMA service is restarted?

Ans- Nothing, if the service is restarted active sessions remain connected. (In earlier versions i.e Presentation Server 4.0 user sessions get disconnected when IMA is restarted because that also restarted the dependent Citrix XTE Service, which is used for Session Reliability.)

Question 5 – Can you name the feature that allows users to disconnect from their session and resume using it later from a different endpoint device?

Ans- Workspace control

Question 6– Which Citrix service is used for communication between XenApp servers?

Ans- Citrix XML service

Question 7– How to modify the Citrix License Port Number and Vendor Daemon Port Number ?

Ans- By default in License Server Version 11.5 Citrix License Manager Daemon communicates over port 27000 and the Citrix Vendor Daemon runs in 7279. You should not change this port number unless you have a firewall that uses another port number, or if this number is already in use

Question 8– What are different load evaluators are available in Citrix XenApp ?

Ans-

1. Default Load Evaluators

2. Advance Load evaluators.

3. Custom load evaluators.

Question 9– How do you repair the IMA local host cache?

Ans- dsmaint recreatelhc from cmd prompt.

Question 10– What is ICA and what are the advantage of ICA?

Ans- ICA - Independent Computing architecture.

it provides better compression and transmits High-level windows display information using low bandwidth.

More coming soon----

Citrix XenApp Product Editions and Licensing

Product Editions:-

• Fundamentals – Aimed at the small & medium business looking for secure, remote access.
• Advanced – A “solid foundation” for on-demand application delivery
• Enterprise – “Enterprise Class” on-demand application delivery optimized for larger environments
• Platinum – The “most robust, secure & scalable” for delivery applications on-demand.

Licensing Models:-

User Licensing – Provides access to named users:

• One User
• One Device
• Unlimited connections to virtual applications

Concurrent Licensing – Licenses are assigned to each connection from a central pool

  

Licensing Microsoft within a Citrix environment:-

The first thing to note is:

“All users/devices accessing the Citrix server need to be covered with a Remote Desktop Services (RDS) CAL”

To ensure you purchase the requisite number of CALs to go along with your Citrix use.

Next :

“Each machine accessing an application on the Citrix server must have an equivalent copy installed on the local machine.”

For example, if you have Office Pro 2010 on you Citrix server, devices licensed with:

• Office Std 2010
• Office Pro 2007

may NOT access that central server.

Another key point that can make a big difference to software purchase choices, particularly for smaller organizations is this:

“Only Volume License copies of applications may be used in a Citrix (or RDS) environment”,

so no:

• OEM (Original Equipment Manufacturer)
• FPP (Fully Packaged Product)

or even

• Office 365

If Office is to be used in a Citrix/RDS environment, it must be purchased via:

• Open
• Open Value
• Select
• Select Plus
• Enterprise Agreement (EA).

Citrix & RDS environments add an extra layer of complexity to Microsoft licensing and the management of such assets, so it’s important to understand the rules and what you can do to keep control of your software licensing. A key point is this:

“Any device capable of accessing the Citrix/RDS server must be appropriately licensed for the application/s on that server.”

Microsoft’s application licensing is done on a “per-device” basis so things such as Group Policy are not sufficient for restricting access to applications, as they apply at a user – rather than a device – level. So if you “publish” the apps to 10 people via Group Policy, those 10 people could access it from any one of your 300 machines…meaning you’d need 300 device licenses to be compliant.

Click here for details about citrix products licensing.