Functions of the Local Host Cache (LHC) in Citrix IMA

Information

This article contains information on functions, refreshing, and recreating of the Local Host Cache (LHC).

Functions of the Local Host Cache

Each XenApp server stores a subset of the data store in the 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 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).

  • 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 because of farm licensing information being kept on the data store. After the 96 hours grace period ended, the licensing subsystem would fail to verify licensing, and the server would stop accepting incoming connections.

    Because the LHC holds a copy of the published applications and Windows domain trust relationships, ICA Client application enumeration requests can be resolved locally by the LHC. This provides a faster response to the ICA Client for application enumerations as the local server does not have to contact other member servers or the zone data collector. The member server must still contact the zone data collector for load management resolutions.

    In some instances it might be necessary to either refresh or recreate the LHC. The following sections describe these situations.

    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 LHC by running dsmaint refreshlhc from a command prompt on the affected server. This action forces the LHC to read all changes immediately from the data store.

    A discrepancy in the LHC 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

    The following are some points because of which you might want to recreate the LHC:

    • If the IMA service does not start, the cause might 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, after the data store has been cleaned.

  • Steps to Recreate the Local Host Cache

    Important! The data store server must be available, for dsmaint recreatelhc command to work. If the data store is not available, the IMA service cannot start.

    Caution! Refer to the Disclaimer at the end of this article before using Registry Editor.

    Complete the following steps to recreate LHC:

    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.
  • 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 LHC database. When the IMA service is restarted, the LHC is recreated with the current data from the data store.

  • Restart the IMA service. This can be done using the command 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 and change value to 1.
    There is also a built-in utility to verify the LHC 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.

     

    Reference: Functions of the Local Host Cache