Chapter 8. Topologies 209
8.3.5 Horizontal scaling topology
In a horizontal scaling topology, you create one logical unit of servers across multiple systems
or LPARs where each member of the unit serves each request. Horizontal scaling at the
application server tier does not require an IP sprayer. If you also want to scale at the web
server tier, you can use an IP sprayer.
This section introduces two topologies. One is without an IP sprayer, and the other has the IP
sprayer component. For more information, see 8.3.6, “Horizontal scaling topology with an IP
sprayer” on page 211.
Horizontal scaling topology without an IP sprayer
In the topology illustrated in Figure 8-7, a single application spans multiple systems, but
presents itself as a single logical image. In this example, the WebSphere Application Server
cluster spans Systems B and C, each with one application server. The deployment manager
is installed on a separate server, System D.
Figure 8-7 Horizontal scaling with cluster
The web server plug-in distributes requests to the cluster members on each server that runs
load balancing. It offers an initial failover in a similar manner as it does in the vertical
clustering topology. If any component (hardware or software) on System B fails, the
application server on System C can continue to serve requests and vice versa.
System B System C
Cluster
System D
System A
Web
server
Deployment
manager
Directory
and
security
services
Existing
applications
and data
Outside World DMZ Internal Network
Protocol firewall
Domain firewall
I
N
T
E
R
N
E
T
User
HTTP/HTTPS
Application
server 1
Application
server 2
Important: The illustration in Figure 8-7 is intended to show a horizontal scaling topology
of application servers but still contains a SPOF (namely, the web server). To avoid this
SPOF, you must enhance the topology as illustrated in 8.3.6, “Horizontal scaling topology
with an IP sprayer” on page 211.
210 WebSphere Application Server V8.5 Concepts, Planning, and Design Guide
Advantages
Using horizontal scaling with clusters has the following advantages:
Improved throughput
Because multiple systems service client requests simultaneously without competing for
resources, you can expect improved throughput from your installation.
Improved response times
By hosting cluster members on multiple systems, each cluster member can use the
processing resources of the system, avoiding bottlenecks and resource contention.
Therefore, response times improve in most scenarios.
Benefits from the workload management capabilities of WebSphere Application Server
Because horizontal scaling is implemented through clusters, it benefits from the workload
management capabilities of WebSphere Application Server.
Provides enhanced failover support
Because the cluster members are spread over multiple systems, this topology provides
hardware failover capabilities. Client requests can be redirected to cluster members on
other systems if a system goes offline. The outage of a system or an operating system
failure does not stop a service from working.
Considerations
Keep in mind the following considerations when using horizontal scaling with clusters:
Increased resource usage
Because multiple systems are required to implement this topology, hardware costs
increase. To implement horizontal scaling, you need WebSphere Application Server
Network Deployment. Therefore, you need additional application server processes, such as
the deployment manager and the node agent process, to manage this type of environment.
This method increases processing and the memory footprint of the installation.
More complex administration
The maintenance and administration of the environment are more complex because the
number of systems increases.
Setting up the topology
To set up a topology environment similar to the one illustrated in Figure 8-7 on page 209,
complete the steps in this section. These steps include the minimum software configuration
that you need for this topology.
Setting up System A
To set up System A, complete the following steps:
1. Install IBM Installation Manager.
2. Using Installation Manager, install the following applications:
Web server plug-ins for WebSphere Application Server
WebSphere Customization Toolbox
IBM HTTP Server
If you are not using IBM HTTP Server, install a supported web server.
3. Open the WebSphere Customization Toolbox, and start the Web Server Plug-ins
Configuration Tool.

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