What Is Local Host Cache?

In previous post I wrote about what will happen if the Citrix XenApp farm lost connection with its datastore. Well, because of Local Host Cache, users will still be able to access the applications without too much issue. But, what is Local Host Cache (LHC)?

LHC is like a mini-DB of the farm, and created on all Citrix XenApp servers. Why I said mini-DB? It is because it contains a portion (subset) of Data Store information. 


There are 4 main information stored in the LHC :

• 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)

There are 2 primary functions of LHC :

• Redundancy - a Citrix XenApp still function as normal if connection to datastore losts.
• Performance - LHC cache information used by ICA clients for enumeration and application resolution. By having this, a faster response to ICA client request is provided, as the server does not need to contact other member servers about published applications.

LHC info is stored in Imalhc.mdb (an Access database). By default, it is stored in %ProgramFiles%\Citrix\Independent Management Architecture folder.

IMA service is heavily related to LHC. IMA service is responsible to get LHC synchronzied with the data store. Hence, restarting IMA service during DB server outage will cause the service can't be started.



Reference :

• http://careerride.com/view.aspx?id=2774
• http://support.citrix.com/article/CTX759510

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What Happen If My XenApp Farm Cant Connect to the Data Store?

This is one of my favorite interview question, I just don't know why. it seems simple, but some candidates failed to give a proper answer (poor them). I wish all of them better luck next time.

Okay, now back to the real business. From Citrix Administrators Point-of-View, losing a DB may cost chaos, depending on how severe the issue is. However, from user perspective, nothing is different. Users wont really impacted of this. They still be able to launch Citrix published applications and works as normal. Thanks to Local Host Cache (LHC), it eases the burden of Citrix Admins (oh really?)

However, although all Citrix XenApp servers have LHC, there are some conditions that Citrix Admins need to remember :

• There is no grace period for this (MPS 3.0 and above). Users still be able to connect to the Citrix farm (lucky!)
• Only static information  available to users. No new info can be added and current info cannot be changed (not really...)

In any circumstances, DO NOT :

• Restart / reboot the Citrix XenApp servers. it will cause the server to contact DB server. This will result IMA service won't started.
• Restart IMA service (same reason as above).
• Re-create LHC (in order to re-create LHC, we need to stop IMA service).

Reference :

• http://citware.blogspot.com/2012/05/if-you-loose-connectivity-to-data-store.html
• http://citrixsolutions.blogspot.com/2009/04/all-about-local-host-cache.html
• http://support.citrix.com/article/CTX105763

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