11 Dynamic Clusters

Learn about dynamic clusters and how to create, configure, and use dynamic clusters in WebLogic Server.

What Are Dynamic Clusters?

Dynamic clusters consist of server instances that can be dynamically scaled up to meet the resource needs of your application. A dynamic cluster uses a single server template to define configuration for a specified number of generated (dynamic) server instances.

When you create a dynamic cluster, the dynamic servers are preconfigured and automatically generated for you, enabling you to easily scale up the number of server instances in your dynamic cluster when you need additional server capacity. You can simply start the dynamic servers without having to first manually configure and add them to the cluster.

If you need additional server instances on top of the number you originally specified, you can increase the maximum number of servers instances (dynamic) in the dynamic cluster configuration or manually add configured server instances to the dynamic cluster. A dynamic cluster that contains both dynamic and configured server instances is called a mixed cluster.

The following table defines the terminology associated with dynamic clusters:

Term Definition

dynamic cluster

A cluster that contains one or more generated (dynamic) server instances that are based on a single shared server template.

configured cluster

A cluster in which you manually configure and add each server instance.

dynamic server

A server instance that is generated by WebLogic Server when creating a dynamic cluster. Configuration is based on a shared server template.

configured server

A server instance for which you manually configure attributes.

mixed cluster

A cluster that contains both dynamic and configured server instances.

server template

A prototype server definition that contains common, non-default settings and attributes that can be assigned to a set of server instances, which then inherit the template configuration. For dynamic clusters, the server template is used to generate the dynamic servers. See Server Templates in Understanding Domain Configuration for Oracle WebLogic Server.

You cannot configure dynamic servers individually; there are no server instance definitions in the config.xml file when using a dynamic cluster. Therefore, you cannot override the server template with server-specific attributes or target applications to an individual dynamic server instance. Example 11-1 shows an example config.xml file that includes a dynamic cluster.

Example 11-1 Example config.xml File Using a Dynamic Cluster

<server-template> 
    <name>dynamic-cluster-server-template</name>
    <accept-backlog>2000</accept-backlog>
    <auto-restart>true</auto-restart>
    <restart-max>10</restart-max>
    <startup-timeout>600</startup-timeout>
</server-template>
 
<cluster>
    <name>dynamic-cluster</name>
    <dynamic-servers>
      <server-template>dynamic-cluster-server-template</server-template>
      <maximum-dynamic-server-count>10</maximum-dynamic-server-count>
      <calculated-machine-names>true</calculated-machine-names>
      <machine-name-match-expression>dyn-machine*</machine-name-match-expression>
      <server-name-prefix>dynamic-server-</server-name-prefix>
    </dynamic-servers>
</cluster>

Why Do You Use Dynamic Clusters?

With dynamic clusters, you can easily scale up your cluster when you need additional server capacity by simply starting one or more of the preconfigured dynamic server instances. You do not need to manually configure a new server instance and add it to the cluster or perform a system restart.

How Do Dynamic Clusters Work?

Learn how to create, configure, and use dynamic clusters.

Creating and Configuring Dynamic Clusters

When you create a dynamic cluster, you perform the following actions:

  • Specify the number of server instances you anticipate needing at peak load

  • Create or select the server template upon which you want to base server configuration

  • Define how WebLogic Server should calculate server-specific attributes

WebLogic Server then generates the specified number of dynamic server instances and applies the calculated attribute values to each dynamic server instance.

Note:

Ensure you have the performance capacity to handle the maximum number of server instances you specify in the dynamic cluster configuration. For information on design and deployment best practices when creating a cluster, see Clustering Best Practices.

You create dynamic clusters using WebLogic Scripting Tool (WLST), the WebLogic Server Administration Console, or the Fusion Middleware Control (FMWC) component of Enterprise Manager (EM).

Example 11-2 demonstrates using WLST. For information about using the WebLogic Server Administration Console, see Create dynamic clusters in the Oracle WebLogic Server Administration Console Online Help. For information about using FMWC, see Create a new dynamic cluster in Administering Oracle WebLogic Server with Fusion Middleware Control. When creating a dynamic cluster in either console, WebLogic Server creates the server template, dynamic cluster, and specified number of server instances for you. You do not have to specify them individually. You can configure dynamic clusters using any of the administration tools listed in Summary of System Administration Tools and APIs in Understanding Oracle WebLogic Server.

Using Server Templates

Server templates define common configuration attributes that a set of server instances share. Dynamic clusters use server templates for dynamic server configuration.

See Server Templates in Understanding Domain Configuration for Oracle WebLogic Server.

Calculating Server-Specific Attributes

You cannot configure individual dynamic server instances or override values in the server template at the dynamic server level when using a dynamic cluster. Server-specific attributes, such as server name, machines, and listen ports, must be calculated using the information provided when creating the dynamic cluster.

Note:

You must set a unique Listen Address value for the Managed Server instance that will host the JTA Transaction Recovery service. Otherwise, the migration will fail.

WebLogic Server calculates and applies the following server-specific attributes using the instance ID of the dynamic server:

  • Server name

  • (Optional) Listen ports (clear text and SSL)

  • (Optional) Network access point listen ports

  • (Optional) Machines or virtual machines

Calculating Server Names

The calculated server name is controlled by the ServerNamePrefix attribute. Server names are the specified prefix followed by the index number. For example, if the prefix is set to dyn-server-, then the dynamic servers will have the names dyn-server-1, dyn-server-2, and so on for the number of server instances you specified.

Calculating Listen Ports

The settings in the dynamic cluster configuration or server template determine the listen ports for the server instances in your dynamic cluster. If you do not calculate listen ports when creating your dynamic cluster, WebLogic Server uses the value in the server template. If you do not define listen ports in the dynamic cluster configuration or server template, WebLogic Server uses the default value. Listen port settings are controlled by the CalculatedListenPorts attribute. For more information about these settings, see Create dynamic clusters in the Oracle WebLogic Server Administration Console Online Help.

If you explicitly define a base listen port for your dynamic cluster in the server template or the dynamic cluster configuration, the listen port value for the first dynamic server instance is the base listen port incremented by one. For each additional dynamic server instance, the listen port value is incremented by one. If the default base listen port is used, WebLogic Server increments the "hundreds" digit by one and continues from there for each dynamic server instance.Table 11-1 shows examples of calculated listen port values.

Table 11-1 Calculating Listen Ports

Listen Port Type Configuration Setting in Server Template Dynamic Server Listen Port Values

Server listen port

Base listen port not set

dyn-server-1: 7101

dyn-server-2: 7102

...

Server listen port

Base listen port set to 7300

dyn-server-1: 7301

dyn-server-2: 7302

...

Server SSL listen port

SSL base listen port not set

dyn-server-1: 8101

dyn-server-2: 8102

...

Server SSL listen port

SSL base listen port set to 8200

dyn-server-1: 8201

dyn-server-2: 8202

...

Server network access point listen port

Network access point listen port not set

dyn-server-1: 9101

dyn-server-2: 9102

...

Server network access point listen port

Network access point listen port set to 9200

dyn-server-1: 9201

dyn-server-2: 9202

...

Server replication ports

Replication ports set to 8100

dyn-server-1: 8100

dyn-server-2: 8101

...

Server replication ports

Replication ports set to 8100-8104

dyn-server-1: 8100-8104

dyn-server-2: 8105-8109

dyn-server-3: 8110-8114

...

You can override listen ports at server startup by using system properties. For example:

To override the listen port:

-Dweblogic.ListenPort=7305

To override the SSL listen port:

-Dweblogic.ssl.ListenPort=7403

To override the listen port of the network access point named mynap:

-Dweblogic.networkaccesspoint.mynap.ListenPort=8201

Calculating Machine Names

The dynamic cluster attributes CalculatedMachineNames and MachineNameMatchExpression control how server instances in a dynamic cluster are assigned to a machine. If the CalculatedMachineNames attribute is set to false, then the dynamic servers will not be assigned to a machine. If the CalculatedMachineNames attribute is set to true, then the MachineNameMatchExpression attribute is used to select the set of machines used for the dynamic servers. If the MachineNameMatchExpression attribute is not set, then all of the machines in the domain are selected. Assignments are made using a round robin algorithm. Table 11-2 shows examples of machine assignments in a dynamic cluster.

Table 11-2 Calculating Machine Names

Machines in Domain MachineNameMatchExpression Configuration Dynamic Server Machine Assignments

M1, M2

Not set

dyn-server-1: M1

dyn-server-2: M2

dyn-server-3: M1

...

Ma1, Ma2, Mb1, Mb2

Ma1, Mb*

dyn-server-1: Ma1

dyn-server-2: Mb1

dyn-server-3: Mb2

dyn-server-4: Ma1

...

Starting and Stopping Servers in Dynamic Clusters

To easily manage server startup and shutdown in dynamic clusters, use the WLST scaleUp and scaleDown commands.

The scaleUp command increases the number of running servers for the specified dynamic cluster. The non-running server instance with the lowest server ID starts first, followed by the next highest non-running server ID, until the specified number of server instances is started.

You can start one, all, or any number of server instances in the dynamic cluster by specifying the desired number with the numServers argument in the scaleUp command. If all available server instances are already running, the scaleUp command increases the size of the cluster to the minimum number of requested server instances before starting the specified number of servers. For more information about expanding cluster size, see Expanding or Reducing Dynamic Clusters.

The scaleDown command gracefully shuts down the specified number of running servers. The server instance with the highest server ID shuts down first, followed by the next highest ID, until the specified number of server instances is shut down. For more information, see scaleUp and scaleDown in WLST Command Reference for WebLogic Server.

You can also start and stop server instances in dynamic clusters using the same methods you use to start and stop server instances in configured clusters: the WebLogic Server Administration Console, Fusion Middleware Control, WLST start and shutdown commands, Node Manager, and start scripts. Depending on which startup method you choose and the tasks you have already performed, you may have to follow several other procedures before you can start server instances. See Starting and Stopping Servers in Administering Server Startup and Shutdown for Oracle WebLogic Server.

Note:

Before you begin, ensure that WebLogic Server is installed on all hosts where you want to run your server instances. If you want to use Node Manager to start and stop your server instances, then you must also run Node Manager on these hosts.

Expanding or Reducing Dynamic Clusters

When you create a dynamic cluster, WebLogic Server generates the number of dynamic servers you specify. Before you decide upon the number of server instances, ensure you have the performance capacity to handle the desired number.

The dynamic server instances are based on the configuration you specified in the server template and calculated attributes.

  • To expand your cluster, start any number of the preconfigured dynamic servers.

  • To shrink your dynamic cluster, shut down the excess number of dynamic servers.

If you need additional server capacity on top of the number of server instances you originally specified, increase the maximum number of dynamic servers in the dynamic cluster configuration. To reduce the number of server instances in the dynamic cluster, decrease the value of the maximum number of dynamic servers attribute. Before lowering this value, shut down the server instances you plan to remove.

You can also use the WLST scaleUp and scaleDown commands to manage your dynamic cluster. To increase the number of dynamic servers in the dynamic cluster, use the scaleUp command and enable the updateConfiguration argument. WLST will increase the maximum size of the cluster by the specified number of servers and start the server instances.

To decrease the maximum size of the dynamic cluster, use the scaleDown command and enable the updateConfiguration argument. WLST will gracefully shut down the specified number of running server instances and remove them from the dynamic cluster. See scaleUp and scaleDown in WLST Command Reference for Oracle WebLogic Server.

Note:

You can only use the WLST scaleUp and scaleDown commands with dynamic server instances. In a mixed cluster, containing both manually configured and dynamic server instances, the scaleUp and scaleDown commands ignore the configured servers. You must manually start and stop configured server instances in a mixed cluster.

For example, a cluster contains two running dynamic servers and two non-running configured servers. If you use the scaleUp command, WLST adds one additional dynamic server instance to your cluster and starts the dynamic server.

The WLST scaleUp and scaleDown commands provide ways to manually scale your dynamic cluster. For automatic scaling, you can configure elasticity for your dynamic cluster. Elasticity enables a dynamic cluster to perform scaling and re-provisioning operations automatically in response to demand or on a calendar based schedule. WebLogic Server provides elasticity for dynamic clusters through the Policies and Actions system of the WebLogic Diagnostic Framework (WLDF). See Configuring Elasticity in Dynamic Clusters for Oracle WebLogic Server.

Using Whole Server Migration with Dynamic Clusters

WebLogic Server supports whole server migration with dynamic and mixed clusters. While configuration differs depending on the cluster type, whole server migration behavior is the same for all clusters. For information on how to enable whole server migration for dynamic clusters, see Whole Server Migration with Dynamic and Mixed Clusters.

Deploying Applications to Dynamic Clusters

When deploying applications to a dynamic cluster, you must target the application to the entire cluster. You cannot target an application to an individual server instance because dynamic clusters do not have individual dynamic server configuration. When you deploy an application to the dynamic cluster, all servers in the cluster, both dynamic and static, will deploy the application.

To deploy an application to a dynamic cluster, follow the same process as deploying to configured clusters. See Deploy Applications and Application Deployment for Clustered Configurations.

For a broad discussion of deployment topics, see Deploying Applications to Oracle WebLogic Server.

Using WebLogic Web Server Plug-Ins with Dynamic Clusters

Dynamic clusters provide the same WebLogic Web server plug-in support as configured clusters. By default, a Web server plug-in uses the DynamicServerList parameter to receive information about cluster changes, such as new server instances in a configured or dynamic cluster. Upon recognizing a cluster membership change, the plug-in automatically updates its server list.

For general information about using Web server plug-ins with WebLogic Server, see Using Oracle WebLogic Server Proxy Plug-Ins. For more information about the DynamicServerList parameter or the WebLogicCluster parameter (required when proxying to a WebLogic Server cluster), see General Parameters for Web Server Plug-Ins in Using Oracle WebLogic Server Proxy Plug-Ins.

Limitations and Considerations When Using Dynamic Clusters

When using dynamic clusters with WebLogic Server, be aware of certain limitations and considerations.

  • You cannot override values in the server template at the individual dynamic server level because there are no individual server definitions in the config.xml file when using dynamic clusters.

  • You need to ensure that the values are set on server template, such that each Dynamic server inherits unique values for required parameters. For example, set the value to LLR_DYN_SRV_${id} for LLR table under Server Template configuration. ${id} would be replaced with server number at runtime for each dynamic server.

    Note:

    See Servers: Configuration: General in the Oracle WebLogic Server Administration Console Online Help
  • You must ensure you have the performance capacity to handle the maximum number of server instances you specify in the dynamic cluster configuration.

  • Dynamic clusters do not support targeting to any individual dynamic server instance. Therefore, the following cannot be used with dynamic clusters:

    • Deployments that cannot target to a cluster, including migratable targets. Therefore, you cannot create a migratable target for a dynamic cluster.

    • Configuration attributes that refer to individual servers. This includes JTA migratable targets, constrained candidate servers, user preferred server, all candidate servers, and hosting server. Therefore, you cannot specify a dynamic server as the user preferred server for a migratable target.

    • Configuration attributes that are server specific. This includes replication groups, preferred secondary groups, and candidate machines (server level).

    • Constrained candidates for singleton services. You cannot limit a singleton service to dynamic servers.

  • For whole server migration with a dynamic cluster, you cannot limit the list of candidate machines that the dynamic cluster specifies, as the server template does not list candidate machines.

  • Dynamic clusters also have JMS limitations. See Simplified JMS Cluster Configuration in Administering JMS Resources for Oracle WebLogic Server.

  • A dynamic server that has multiple network channels configured cannot have those channels listen on different interfaces.

Dynamic Clusters Example

Examine the WLST commands to create a dynamic cluster.

Example 11-2 includes inline comments and describes how to:

  1. Create a server template and specify the desired server attributes in the server template.

  2. Create a dynamic cluster and specify the desired cluster attributes.

  3. Set the server template for the dynamic cluster.

  4. Set the maximum number of server instances you want in the dynamic cluster.

  5. Start and stop the server instances in the dynamic cluster.

Example 11-2 Creating Dynamic Clusters with WLST

#
# This example demonstrates the WLST commands needed to create a dynamic cluster
# (dynamic-cluster). The dynamic cluster uses a server template     
# dynamic-cluster-server-template. To keep this example simple, error handling
# was omitted.
#
connect()
edit()
startEdit()
#
# Create the server template for the dynamic servers and set the attributes for
# the dynamic servers. Setting the cluster is not required.
#
dynamicServerTemplate=cmo.createServerTemplate("dynamic-cluster-server-template")
dynamicServerTemplate.setAcceptBacklog(2000)
dynamicServerTemplate.setAutoRestart(true)
dynamicServerTemplate.setRestartMax(10)
dynamicServerTemplate.setStartupTimeout(600)
#
# Create the dynamic cluster, set the number of dynamic servers, and designate the server template.
#
dynCluster=cmo.createCluster("dynamic-cluster")
dynServers=dynCluster.getDynamicServers()
dynServers.setMaximumDynamicServerCount(10)
dynServers.setServerTemplate(dynamicServerTemplate)
#
# Dynamic server names will be dynamic-server-1, dynamic-server-2, ...,
# dynamic-server-10.
#
dynServers.setServerNamePrefix("dynamic-server-")
#
# Listen ports and machines assignments will be calculated. Using a round-robin
# algorithm, servers will be assigned to machines with names that start with
# dyn-machine.
#
dynServers.setCalculatedMachineNames(true)
dynServers.setMachineNameMatchExpression("dyn-machine*")
#
# activate the changes
#
activate()

The resulting config.xml file is:

<server-template> 
    <name>dynamic-cluster-server-template</name>
    <accept-backlog>2000</accept-backlog>
    <auto-restart>true</auto-restart>
    <restart-max>10</restart-max>
    <startup-timeout>600</startup-timeout>
</server-template>

<cluster>
    <name>dynamic-cluster</name>
    <dynamic-servers>
      <server-template>dynamic-cluster-server-template</server-template>
      <maximum-dynamic-server-count>10</maximum-dynamic-server-count>
      <calculated-machine-names>true</calculated-machine-names>
      <machine-name-match-expression>dyn-machine*</machine-name-match-expression>
      <server-name-prefix>dynamic-server-</server-name-prefix>
    </dynamic-servers>
</cluster>