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.