Without continuous access to the application data, your application
servers cannot process client requests. WebSphere Application Server supports
various methods of maintaining data availability.
If your data resides on i5/OS servers, you can use one of the following
options to achieve higher availability for your data:
Data replication
To ensure the availability of
all components of your application server environment, you can use data replication
to create a backup copy of your application data. Data replication uses clustering,
remote journaling, and third-party data replication software to maintain two
physically separate copies of application data. Data replication can provide
disaster recovery capabilities for the databases because the systems in the
cluster can be geographically separated.
Clustering provides the basis
for communication between two or more servers. This communication is necessary
to back up your data on a physically separate machine.
Remote journaling
creates a copy of your application data to assure hot backup. There are two
types of remote journaling:
- Synchronous remote journaling, where data is written to both the primary
and backup databases simultaneously. This type of journaling ensures that
no entries are lost in the event of a system failure. However, it can negatively
impact performance, because it requires the application to wait while data
is written to both databases.
- Asynchronous remote journaling, where data is written to the primary database
directly from the application. This type of journaling, enables the application
to continue processing client requests while data is copied to the backup
system. It does not impact application performance, but the most recent entries
might be lost if a failure occurs.
Switchable disk
Switchable disk uses clustering,
local journaling, and Independent Auxiliary Storage Pool (IASP) to ensure
the availability of application data. The application data is stored in an
IASP, and if a failure occurs, the IASP can be switched to another node.
The
advantage of switchable disk is that no data replication occurs. Therefore,
you do not have to worry about data synchronization. However, the database
becomes a single point of failure. In addition, the backup node must be physically
close to the primary node. Therefore, this configuration does not provide
disaster recovery.
Switchable disk uses the following components and
services:
- A server cluster. You must have the High Availability Switchable
Resources function (5722-SS1 Option 41) installed on all of the nodes in your
cluster.
- Clustering. When you use switchable disk, clustering provides:
- The services required to switch the IASP.
- Wellness monitoring of backend database partitions.
- Local journaling. Switchable disk uses local journaling to preserve
database transaction boundaries.
Switchable disk is best utilized in a multiple cell topology.
that includes these features:
- All of the cells process client requests as described in multiple cell
topology.
- One of the cells hosts the primary database system, which receives data
from application servers on the other cells.
- The primary database system connects to an IASP.
- One of the other cells hosts the secondary database system, which is inactive
unless there is a failure in the primary system. If the primary system fails,
application data is directed to the secondary database system in this other
cell, and the IASP connects to the secondary system.