XML and LHC
The XML service handles all client communication. Client communication like user authentication to the published app list, resolving the Citrix Zone Preference Policies, user application requests, and user connections requests. The XML service also hosts the Secure Ticket Authority that is used for storing secure tickets for user session requests and session reliability in case of a network interuption. Citrix Web Interface is considered a web-based client component, that can also be used in conjunction with Program Neighborhood Agent.
The XML service should be running on all XenApp servers, and might be sharing a port with IIS. The default XML services port is port 80.
Local Host Cache
A subset of data store information, the local host cache, exists on each server in the farm, providing each member server with quick access to data store information. The local host cache also provides redundancy of the data store information, if for example, a server in the farm loses connectivity to the data store.
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
You can force a manual refresh of a server’s local host cache by executing dsmaint refreshlhc from a command prompt. This action forces the local host cache to read all changes immediately from the farm’s data store. Refreshing the local host cache is useful, for example, if the Citrix Independent Management Architecture (IMA) Service is running, but published applications do not appear correctly when users browse for application sets.
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
You can manually create the local host cache from the farm’s data store. If the IMA Service fails to start or you have a corrupt local host cache, you may need to recreate it.
To recreate the local host cache, stop the IMA Service and then run the command dsmaint recreatelhc. Running this command performs three actions:
- Sets the value of the registry key HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Citrix\IMA\ RUNTIME\PSRequired to 1.
- Deletes the existing local host cache (Imalhc.mdb)
- Creates an empty local host cache (Imalhc.mdb)
You must restart the IMA Service after running dsmaint recreatelhc. When the IMA Service starts, the local host cache is populated with fresh data from the data store.
The data store server must be available for dsmaint recreatelhc to work. If the data store is not available, the IMA Service fails to start.
Tuning Local Host Cache Synchronization
You can adjust the interval by which member servers query the farm’s data store for missed changes. The default interval is 30 minutes. In most cases, this default setting is sufficient.
Caution: Editing the Registry incorrectly can cause serious problems that may require you to reinstall your operating system. Citrix cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk. Be sure to back up the registry before you edit it.
You can configure the interval by creating the following registry key on each server you want to adjust, with the value expressed in hexadecimal notation:
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Citrix\IMA\ DCNChangePollingInterval (DWORD)
Value: 0x1B7740 (default 1,800,000 milliseconds)
You must restart the IMA Service for this setting to take effect.
Most changes made through the Citrix AppCenter are written to the data store. When you open one of these tools, it connects to a specified server. The Citrix Independent Management Architecture (IMA) Service running on this server performs all reads and write operations to the data store for the AppCenter.
If the data store is experiencing high CPU usage when few read or write operations to the data store are occurring, it is possible that the data store is not powerful enough to manage a query interval of 30 minutes. To determine whether or not the data store query interval is causing the high CPU usage on the data store, you can set the query interval to a very large number and test CPU usage. If the CPU usage returns to normal after you set a large query interval, the data store query interval is probably the cause of the high CPU usage. You can adjust the query interval based on performance testing.
To test the query interval, set the interval to 60 minutes and then restart all the servers in the farm. If the data store is still experiencing constant high CPU usage, increase the query interval further. If the CPU usage returns to normal, you can try a smaller value. Continue these adjustments until data store CPU usage is normal.
Important: Do not set the data store query interval higher than necessary. This interval serves as an important safeguard against lost updates. Setting the interval higher than necessary can cause delays in updating the local host cache of the farm’s member servers.
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