Set transaction aspects of servers for optimum availability
This page describes some considerations and actions that we can take to configure transaction-related aspects of application servers, to optimize the availability of those servers.
This helps the transactions to complete or recover more quickly. After changing transaction-related properties of an application server, restart the server.
To configure transaction-related aspects of appservers for optimum availability...
- Store the transaction log files on a fast disk in a highly-available file system, such as a RAID device.
The transaction log might have to be accessed by every global transaction and be used for transaction recovery after a crash. Therefore, the disk the log files are being written to should be on a highly-available file system, such as a RAID device.
The performance of the disk also directly affects the transaction performance. In general, a global transaction makes two disk writes, one after the prepare phase when the outcome of the transaction is known (this information is forced to disk) and a further disk write at transaction completion. Therefore, the transaction logs should be placed on the fastest disks available and not make use of network mounted devices.
- Mirror the transaction log files by using hardware disk mirroring or dual-ported disks.
If log files have been mirrored or can be recovered, they can be used when restarting a failed server or moved to an another machine and another server started there to perform recovery.
Hardware disk mirroring or dual-ported disks can be used by specifying the appropriate file system directory for the transaction logs using the WebSphere Administrative Console.
- Specify the optimum location of the transaction log directory for appservers.
By default, an application server places transaction log files in a subdirectory of the installed WAS, where the subdirectory name is the same as the server name. For example, the default directory for an appserver named server1 is
/IBM/WebSphere/AppServer/profiles/profile_name/tranlog/server1
We can define a specific location for the transaction log directory for an appserver by setting the Transaction Log Directory property for the server. If the directory for the transaction logs has not been created at appserver start up, the directory structure is created for you.
If we change the transaction log directory, you should apply the change and restart the appserver as soon as possible, to minimize the risk of problems occurring before the appserver is restarted. For example, if a problem causes the server to fail (with in-flight transactions), the server next starts with the new log directory and is unable to automatically resolve in-flight transactions that were recorded in the old log directory.
- Never allow more than one appserver to concurrently use the same set of log files.
Because the transaction logs record the state of global transactions within a server, if the logs become lost or corrupt, then transactions in the prepared state before failure can leave resources in an in-doubt state and prevent further updates or access to the resources by other users or servers. These transactions might have to be manually resolved by either committing or rolling back the transactions at the affected resource managers. The failed server can then be cold-started, which creates new empty transaction logs.
If log files have been mirrored or can be recovered, they can be used when restarting the failed server or moved to an alternate server or machine and another server restarted to perform recovery.
Never allow more than one appserver to concurrently use the same set of log files, because each server will destroy the information recorded by the other, resulting in corrupt log files that are unusable for future recovery purposes.
- Set appservers to always use the same listening port address at each startup.
If running distributed transactions between multiple appservers, the remote object references saved in the transaction log have to be redirected to the originating server on recovery.
On WAS ND, the node agents automatically redirect these remote object references to the appropriate appservers on recovery. However, if the distributed transaction is between application servers that are not on WAS ND, handle the redirection of remote object references so that transaction recovery can complete. For example, do this if an appserver is deployed on WAS (not the ND edition) and runs distributed transactions with non-WebSphere EJB or Corba servers.
In particular, the default restart action of an appserver not on Application Server ND is to use a different listening port address to the port when the server shuts down. This prevents transaction recovery from completing. To overcome this, you should always configure appservers to always use the same listening port address at each startup (see the ORB property com.ibm.CORBA.ListenerPort in ORB service settings that can be added to the admin console). We might have to make similar configuration changes to other appservers involved in transactions, to be able to access those servers during recovery.
Related concepts
Transactional high availability
Related tasks
Set transaction properties for an appserver
Moving a transaction log from one server to another
Restarting a server on a different host
Managing transaction logging for optimum server availability