A dynamic cluster is a server cluster that uses weights
and workload management to balance the workloads of its cluster members
dynamically, based on performance information that is collected from
the cluster members. Dynamic clusters enable application server virtualization.
A dynamic cluster is an application deployment target that
can expand and contract depending on the workload in your environment.
Dynamic clusters work with autonomic managers, including the application
placement controller and the dynamic workload manager to maximize
the use of your computing resources. Dynamic clusters are required
for many of the WebSphere® Virtual
Enterprise autonomic
functions, including high availability and service policies.
Dynamic clusters are similar to the server clusters that you can
create with WebSphere Application
Server Network Deployment, but
key differences exist that make dynamic clusters much more robust. For complete life-cycle management servers, the
product controls the creation and deletion of server instances and
can start and stop servers. For assisted life-cycle management servers,
the product can control the state of servers by stopping and starting
servers from a pool of predefined server instances.
Dynamic cluster membership
In WebSphere Virtual
Enterprise Version
6.1 and later, two options exist for adding members to a dynamic cluster:
automatically define cluster members with rules or manually define
cluster members.
Note: Dynamic
cluster membership definitions have changed from Version 6.0. In Version
6.0, you define a node group that contains the nodes on which you
want the dynamic cluster members to run. In Version 6.1, you define
a membership policy that specifies the nodes on which the dynamic
cluster members run.
Automatically
define cluster members with rules
By
automatically defining cluster members with rules, you can create
a subexpression that automatically selects nodes to host dynamic cluster
members based on different node properties. This subexpression is
called a membership policy. After you create the membership
policy, you can preview the node membership before you finish creating
the dynamic cluster.
Automatically defining
cluster members with rules is available only for servers that have
complete life-cycle management. After you create the dynamic cluster
with a membership policy, dynamic cluster instances can start on any
of the selected nodes. If nodes become available that meet the criteria
of your membership policy, dynamic cluster instances can also start
on these nodes.
Manually define cluster
members
When you manually define cluster
members, you statically define which servers are cluster members by
selecting servers to add to the cluster. You use this option instead
of the membership policy for the following reasons:
- You have an existing static cluster that you want to convert to
a dynamic cluster.
- You are using assisted life-cycle management servers. Assisted
life-cycle management servers cannot be created from the administrative
console. With this option, you create representations of the servers
as cluster members. These members must homogeneous, that is, be all
of the same server type, for example, a group of BEA WebLogic servers.
The same version of the middleware software must be installed on all
the nodes in the dynamic cluster, and you must deploy the same applications
to these servers before you create the dynamic cluster.
Server templates
A server template is
a copy of a server configuration that can be used as a starting point
when a server is added to the dynamic cluster. Predefined templates
exist for different versions of WebSphere Virtual
Enterprise and
the different middleware server types. You
can also define your own server templates.
Dynamic cluster server templates
After you
create a dynamic cluster, the
dynamic cluster server template defines
the properties for all of the members in the dynamic cluster.
Note: When you make a change to a dynamic cluster server
template, this will overwrite any changes you made that are unique
to an individual member of that cluster.
Cluster instances
You can control the creation
and management of cluster instances for your dynamic cluster. These
options include:
- Creating a minimum and maximum number of cluster instances.
- Stopping cluster instances when other dynamic clusters need resources.
- Allowing more than one cluster instance to start on the same node,
also known as vertical stacking. With vertical stacking, you
can improve bottleneck conditions within an application. The stacking
number defines how many cluster instances can start on a single
node.
- Specifying if cluster instances from other
dynamic clusters can start on the same node, also known as dynamic
cluster isolation.
Operating modes
Dynamic clusters act differently
depending on the operating mode. Choose one of the following options
for mode of operation:
- Manual. In manual mode, the dynamic cluster is no different
from the standard application server environments with static clusters.
Manual mode does not support application placement, or runtime task
suggestions. The autonomic request flow manager and dynamic workload
management (DWLM) can work with the cluster.
- Supervised. In supervised mode, the environment provides
information about required corrective actions by generating runtime
tasks. You can accept or deny the recommendations of the autonomic
managers in the task management panel in the administrative console.
To manage runtime tasks, click .
- Automatic. In automatic mode, the environment takes corrective
actions automatically.
Important: To use dynamic application placement,
click automatic or supervised as
the mode of operation.
If you are using manual mode
for the autonomic request flow manager (ARFM) on either the cell level
or deployment target level, then you also must put your dynamic clusters
in manual mode. You can also use static clusters when ARFM is in manual
mode for the cell or deployment target. If ARFM is in automatic mode,
you can use any of the operating modes for the cluster.