WebLogic Server instances in a cluster communicate with one another using two basic network technologies:
IP unicast or multicast, which server instances use to broadcast availability of services and heartbeats that indicate continued availability. See Considerations for Choosing Unicast or Multicast for information on selecting unicast or multicast.
IP sockets, which are the conduits for peer-to-peer communication between clustered server instances.
This chapter includes the following sections:
WebLogic Server supports two cluster messaging protocols:
Multicast: This protocol relies on UDP multicast and has been supported in WebLogic Server clusters since WebLogic Server 4.0.
Unicast: This protocol relies on point-to-point TCP/IP sockets and was added in WebLogic Server 10.0.
This section includes the following topics:
Multicast is a simple broadcast technology that enables multiple applications to "subscribe" to a given IP address and port number and listen for messages.
Note:A multicast address is an IP address in the range from 220.127.116.11 to 18.104.22.168. The default multicast value used by WebLogic Server is 22.214.171.124. You should not use any multicast address within the range x.0.0.1. Multicast ports have the normal UDP port ranges (0 to 65535), however certain UDP ports are reserved for specific purposes and should generally be avoided.
Multicast broadcasts messages to applications, but it does not guarantee that messages are actually received. If an application's local multicast buffer is full, new multicast messages cannot be written to the buffer and the application is not notified when messages are "dropped." Because of this limitation, WebLogic Server instances allow for the possibility that they may occasionally miss messages that were broadcast over multicast.
The WebLogic Server multicast implementation uses standard UDP multicast to broadcast the cluster messages to a group that is explicitly listening on the multicast address and port over which the message is sent. Since UDP is not a reliable protocol, WebLogic Server builds its own reliable messaging protocol into the messages it sends to detect and retransmit lost messages.
Most operating systems and switches support UDP multicast by default between machines in the same subnet. However, most routers do not support the propagation of UDP multicast messages between subnets by default. In environments that do support UDP multicast message propagation, UDP multicast has a time-to-live (TTL) mechanism built into the protocol. Each time the message reaches a router, the TTL is decremented by 1 before it routes the message. When the TTL reaches zero, the message will no longer be propagated between networks, making it an effective control for the range of a UDP multicast message. By default, WebLogic Server sets the TTL for its multicast cluster messages to 1, which restricts the message to the current subnet.
When using multicast, the cluster heartbeat mechanism will remove a server instance from the cluster if it misses three heartbeat messages in a row to account for the fact that UDP is not considered a reliable protocol. Since the default heartbeat frequency is one heartbeat every 10 seconds, this means it can take up to 30 seconds to detect that a server instance has left the cluster. Socket death detection or failed connection attempts can also accelerate this detection.
In summary, WebLogic Server multicast cluster messaging protocol:
Uses a very efficient and scalable peer-to-peer model where a server instance sends each message directly to the network once and the network makes sure that each cluster member receives the message directly from the network.
Works out of the box in most environments where the cluster members are in a single subnet.
Requires additional configuration in the router and WebLogic Server (for example multicast TTL) if the cluster members span more than one subnet.
Uses three consecutive missed heartbeats to remove a server instance from another server's cluster membership list.
To test an environment for its ability to support the WebLogic Server multicast messaging protocol, WebLogic Server provides a Java command-line utility known as MulticastTest.
WebLogic Server uses multicast for all one-to-many communications among server instances in a cluster. This communication includes:
Cluster-wide JNDI updates—Each WebLogic Server instance in a cluster uses multicast to announce the availability of clustered objects that are deployed or removed locally. Each server instance in the cluster monitors these announcements and updates its local JNDI tree to reflect current deployments of clustered objects. For more details, see Cluster-Wide JNDI Naming Service.
Cluster heartbeats—Each WebLogic Server instance in a cluster uses multicast to broadcast regular "heartbeat" messages that advertise its availability. By monitoring heartbeat messages, server instances in a cluster determine when a server instance has failed. (Clustered server instances also monitor IP sockets as a more immediate method of determining when a server instance has failed.)
Clusters with many nodes—Multicast communication is the option of choice for clusters with many nodes.
Because multicast communications control critical functions related to detecting failures and maintaining the cluster-wide JNDI tree (described in Cluster-Wide JNDI Naming Service) it is important that neither the cluster configuration nor the network topology interfere with multicast communications. The sections that follow provide guidelines for avoiding problems with multicast communication in a cluster.
In many deployments, clustered server instances reside within a single subnet, ensuring multicast messages are reliably transmitted. However, you may want to distribute a WebLogic Server cluster across multiple subnets in a Wide Area Network (WAN) to increase redundancy, or to distribute clustered server instances over a larger geographical area.
If you choose to distribute a cluster over a WAN (or across multiple subnets), plan and configure your network topology to ensure that multicast messages are reliably transmitted to all server instances in the cluster. Specifically, your network must meet the following requirements:
Full support of IP multicast packet propagation. In other words, all routers and other tunneling technologies must be configured to propagate multicast messages to clustered server instances.
Network latency low enough to ensure that most multicast messages reach their final destination in approximately 10 milliseconds.
Multicast Time-To-Live (TTL) value for the cluster high enough to ensure that routers do not discard multicast packets before they reach their final destination. For instructions on setting the Multicast TTL parameter, see Configure Multicast Time-To-Live (TTL).
Note:Distributing a WebLogic Server cluster over a WAN may require network facilities in addition to the multicast requirements described above. For example, you may want to configure load balancing hardware to ensure that client requests are directed to server instances in the most efficient manner (to avoid unnecessary network hops).
Although it may be possible to tunnel multicast traffic through a firewall, this practice is not recommended for WebLogic Server clusters. Treat each WebLogic Server cluster as a logical unit that provides one or more distinct services to clients of a Web application. Do not split this logical unit between different security zones. Furthermore, any technologies that potentially delay or interrupt IP traffic can disrupt a WebLogic Server cluster by generating false failures due to missed heartbeats.
Although multiple WebLogic Server clusters can share a single IP multicast address and port, other applications should not broadcast or subscribe to the multicast address and port used by your cluster or clusters. That is, if the machine or machines that host your cluster also host other applications that use multicast communications, make sure that those applications use a different multicast address and port than the cluster does.
Sharing the cluster multicast address with other applications forces clustered server instances to process unnecessary messages, introducing overhead. Sharing a multicast address may also overload the IP multicast buffer and delay transmission of WebLogic Server heartbeat messages. Such delays can result in a WebLogic Server instance being marked as failed, simply because its heartbeat messages were not received in a timely manner.
For these reasons, assign a dedicated multicast address for use by WebLogic Server clusters, and ensure that the address can support the broadcast traffic of all clusters that use the address.
If server instances in a cluster do not process incoming messages on a timely basis, increased network traffic, including negative acknowledgement (NAK) messages and heartbeat re-transmissions, can result. The repeated transmission of multicast packets on a network is referred to as a multicast storm, and can stress the network and attached stations, potentially causing end-stations to hang or fail. Increasing the size of the multicast buffers can improve the rate at which announcements are transmitted and received, and prevent multicast storms. See Configure Multicast Buffer Size.
WebLogic Server provides an alternative to using multicast to handle cluster messaging and communications. Unicast configuration is much easier because it does not require the cross network configuration that multicast requires. Additionally, it reduces potential network errors that can occur from multicast address conflicts.
The WebLogic Server unicast protocol uses standard TCP/IP sockets to send messages between cluster members. Since all networks and network devices support TCP/IP sockets, unicast simplifies out of the box cluster configuration. It typically requires no additional configuration, regardless of the network topology between the cluster members. Unicast is the default cluster protocol in WebLogic Server.
Since TCP/IP sockets are a point-to-point mechanism, WebLogic Server's unicast implementation uses a group leader strategy to limit the growth in the number of sockets required as the cluster size grows. The cluster is split into one or more groups and each group has a group leader. Group members communicate with the group leader and the group leaders also communicate with other group leaders in the cluster. If a group leader fails, the group elects another group leader.
For small clusters of 10 Managed Servers or less, the cluster contains a single group and therefore a single group leader. The other server instances in the group make a TCP/IP socket connection to the group leader that they use to send and receive cluster messages. When the group leader receives a cluster message from one server instance, it retransmits that message to all other members of the group. The group leader acts as a message relay to propagate the messages across the cluster.
For larger clusters, the cluster splits into multiple groups of 10 Managed Servers. For example, a cluster of 16 Managed Servers will have two groups, one with 10 members and one with six. In these clusters with multiple groups, the group leaders are connected directly to one another. When a group leader receives a cluster message, it not only retransmits that message to other members of its group but also to every other group leader. This allows the entire cluster to receive every cluster message.
When using unicast, the cluster heartbeat mechanism will remove a server instance from the cluster if it misses a single heartbeat message since TCP/IP is a reliable protocol. Unicast will check every 15 seconds to see if it has missed a heartbeat. This extra five seconds is to allow sufficient time for the message to travel up to three hops, from the remote group's member to the remote group's leader to the local group's leader, and finally to the local group's member. Since the default heartbeat frequency is one heartbeat every 10 seconds, this means it should take no more than 15 seconds to detect that a server instance has left the cluster. Socket death detection or failed connection attempts can also accelerate this detection.
To understand how to expand or shrink running clusters that use unicast messaging, it is important to understand how Managed Servers are organized internally into groups. Each of the Managed Servers in a WebLogic Server cluster has a name. For unicast clusters, WebLogic Server reads these Managed Server names and then sorts them into an ordered list by alphanumeric name. The first 10 Managed Servers in the list (up to 10 Managed Servers) become the first unicast clustering group. The second set of 10 Managed Servers (if applicable) becomes the second group, and so on until all Managed Servers in the cluster are organized into groups of 10 Managed Servers or less. The first Managed Server for each group becomes the group leader for the other (up to) nine Managed Servers in the group.
When adding Managed Servers to an existing, running unicast cluster, Oracle recommends that you add these Managed Servers using Managed Server names that will be sorted into the bottom of the Managed Server list. If you add Managed Server names that will be sorted to the top of the list, this causes renegotiation of cluster group membership within the cluster, creating performance degradation or service interruptions. To avoid this scenario, when adding new Managed Servers to running unicast clusters, specify the new Managed Server names such that they will be sorted to the bottom of the list of Managed Server names. Using a simple naming convention, such as
serverN will typically avoid the issues described above. This restriction regarding Managed Server naming conventions does not apply to dynamic cluster configurations (see Chapter 11, "Creating Dynamic Clusters"), to clusters that are shut down and restarted when Managed Servers are added, or to multicast clusters.
In addition, when adding Managed Servers to running unicast clusters, if you add enough Managed Servers such that another unicast group and group leader will be created (for instance, you add an 11th or 21st Managed Server to the cluster configuration), this will also cause a change to the group and group leader membership, causing intracluster communication problems and potentially creating performance degradation or service interruptions. Oracle recommends that you avoid adding this many Managed Servers to running unicast clusters, including configured unicast clusters and dynamic unicast clusters (see Chapter 11, "Creating Dynamic Clusters"). This restriction does not apply to clusters that are shut down and restarted when Managed Servers are added, or to multicast clusters.
In summary, the unicast cluster messaging protocol:
Uses a group leader model where a server instance sends each message directly to the group leader. The group leader is responsible for retransmitting the message to every other group member and other group leaders, if applicable.
Works out of the box in virtually any environment.
Requires no additional configuration, regardless of the network topology.
Uses a single missed heartbeat to remove a server instance from another server's cluster membership list.
Has limitations regarding how many servers can be dynamically added to running unicast clusters.
Unicast is configured using
ClusterMBean.isUnicastBasedClusterMessagingEnabled(). The default value of this parameter is false. Changes made to this MBean are not dynamic. You must restart your cluster for changes to take effect.
To define a specific channel for unicast communications, you can use the
setNetworkChannelForUnicastMessaging(String NetworkChannelName). When unicast is enabled, servers will attempt to use the value defined in this MBean for communications between clusters. If the unicast channel is not explicitly defined, the default network channel is used.
When configuring WebLogic Server clusters for unicast communications, if the servers are running on different machines, you must explicitly specify their listen addresses or DNS names.
The following considerations apply when using unicast to handle cluster communications:
All members of a cluster must use the same message type. Mixing between multicast and unicast messaging is not allowed.
You must use multicast if you need to support previous versions of WebLogic Server within your cluster.
Individual cluster members cannot override the cluster messaging type.
The entire cluster must be shutdown and restarted to message modes.
JMS topics configured for multicasting can access WebLogic clusters configured for unicast because a JMS topic publishes messages on its own multicast address that is independent of the cluster address. However, the following considerations apply:
The router hardware configurations that allow unicast clusters may not allow JMS multicast subscribers to work.
JMS multicast subscribers need to be in a network hardware configuration that allows multicast accessibility.
For more details, see "Using Multicasting with WebLogic JMS" in Developing JMS Applications for Oracle WebLogic Server.
Unicast is the default protocol because it simplifies out of the box cluster configuration and because it is likely to meet the majority of user requirements. However, Oracle fully supports both protocols equally. Both protocols require that the cluster members get sufficient processing time to send and receive cluster messages in a timely fashion. This prevents unnecessary cluster membership changes and the inherent resynchronization costs associated with leaving and rejoining the cluster. It is recommended that you eliminate unnecessary cluster membership changes due to over-utilization of available resources.
When using unicast in particular, make sure that the group leaders are not resource constrained since they act as the message relay to deliver a cluster message to the rest of the cluster. Any slowness on their part can impact multiple cluster members and even result in the group electing a new group leader.
Contrast this with multicast, where a slow member can only really impact its own membership to the cluster. Multicast clusters are generally more efficient in terms of cluster message propagation, and therefore tend to be more resilient to oversubscription of resources. For these reasons, multicast may be a better option for clusters with high throughput requirements, for larger clusters (more than 10 Managed Servers), or for clusters with complex WebLogic Server configuration requirements, provided the network environment supports WebLogic Server cluster UDP requirements.
Each protocol has its own benefits.Table 3-1 highlights some of the differences between multicast and unicast.
Uses UDP multicast
Requires additional configuration to routers, TTL when clustering across multiple subnets
Requires no additional configuration to account for network topology
Requires configuring the multicast listen address and listen port. May need to specify the network interface to use on machines with multiple NICs
Only requires specifying the listen address. Supports using the default channel or a custom network channel for cluster communications
Each message delivered directly to and received directly from the network
Each message is delivered to a group leader, which retransmits the message to other group members (N - 1) and any other group leaders (M - 1), if they exist. The other group leaders then retransmit the message to their group members resulting in up to NxM network messages for every cluster message. Message delivery to each cluster member takes between one and three network hops.
Every server sees every other server
Group leaders act as a message relay point to retransmit messages to its group members and other group leaders
Cluster membership changes require three consecutive missed heartbeat messages to remove a member from the cluster list
Cluster membership changes require only a single missed heartbeat message to remove a member from the cluster
IP sockets provide a simple, high-performance mechanism for transferring messages and data between two applications. Clustered WebLogic Server instances use IP sockets for:
Accessing non-clustered objects deployed to another clustered server instance on a different machine.
Replicating HTTP session states and stateful session EJB states between a primary and secondary server instance.
Accessing clustered objects that reside on a remote server instance. (This generally occurs only in a multi-tier cluster architecture, such as the one described in Recommended Multi-Tier Architecture.)
Note:The use of IP sockets in WebLogic Server extends beyond the cluster scenario—all RMI communication takes place using sockets, for example, when a remote Java client application accesses a remote object.
Proper socket configuration is crucial to the performance of a WebLogic Server cluster. Two factors determine the efficiency of socket communications in WebLogic Server:
Whether the server instance host system uses a native or a pure-Java socket reader implementation.
For systems that use pure-Java socket readers, whether the server instance is configured to use enough socket reader threads.
Although the pure-Java implementation of socket reader threads is a reliable and portable method of peer-to-peer communication, it does not provide the optimal performance for heavy-duty socket usage in a WebLogic Server cluster. With pure-Java socket readers, threads must actively poll all opened sockets to determine if they contain data to read. In other words, socket reader threads are always "busy" polling sockets, even if the sockets have no data to read. This unnecessary overhead can reduce performance.
The performance issue is magnified when a server instance has more open sockets than it has socket reader threads—each reader thread must poll more than one open socket. When the socket reader encounters an inactive socket, it waits for a timeout before servicing another. During this timeout period, an active socket may go unread while the socket reader polls inactive sockets, as shown in Figure 3-1.
For optimal socket performance, configure the WebLogic Server host machine to use the native socket reader implementation for your operating system, rather than the pure-Java implementation. Native socket readers use far more efficient techniques to determine if there is data to read on a socket. With a native socket reader implementation, reader threads do not need to poll inactive sockets—they service only active sockets, and they are immediately notified (via an interrupt) when a given socket becomes active.
Note:Applets cannot use native socket reader implementations, and therefore have limited efficiency in socket communication.
For instructions on how to configure the WebLogic Server host machine to use the native socket reader implementation for your operating system, see Configure Native IP Sockets Readers on Machines that Host Server Instances.
If you do use the pure-Java socket reader implementation, you can still improve the performance of socket communication by configuring the proper number of socket reader threads for each server instance. For optimal performance, the number of socket reader threads in WebLogic Server should equal the potential maximum number of opened sockets. This configuration avoids the situation in which a reader thread must service multiple sockets, and ensures that socket data is read immediately.
To determine the proper number of reader threads for server instances in your cluster, see the following section, Determining Potential Socket Usage.
For instructions on how to configure socket reader threads, see Set the Number of Reader Threads on Machines that Host Server Instances.
Each WebLogic Server instance can potentially open a socket for every other server instance in the cluster. However, the actual maximum number of sockets used at a given time depends on the configuration of your cluster. In practice, clustered systems generally do not open a socket for every other server instance, because objects are deployed homogeneously—to each server instance in the cluster.
If your cluster uses in-memory HTTP session state replication, and you deploy objects homogeneously, each server instance potentially opens a maximum of only two sockets, as shown in Figure 3-2.
The two sockets in this example are used to replicate HTTP session states between primary and secondary server instances. Sockets are not required for accessing clustered objects, due to the collocation optimizations that WebLogic Server uses to access those objects. (These optimizations are described in Optimization for Collocated Objects.) In this configuration, the default socket reader thread configuration is sufficient.
Deployment of "pinned" services—services that are active on only one server instance at a time—can increase socket usage, because server instances may need to open additional sockets to access the pinned object. (This potential can only be released if a remote server instance actually accesses the pinned object.) Figure 3-3 shows the potential effect of deploying a non-clustered RMI object to Server A.
In this example, each server instance can potentially open a maximum of three sockets at a given time, to accommodate HTTP session state replication and to access the pinned RMI object on Server A.
Note:Additional sockets may also be required for servlet clusters in a multi-tier cluster architecture, as described in Configuration Notes for Multi-Tier Architecture.
Clients of a cluster use the Java implementation of socket reader threads.
WebLogic Server allows you to configure server affinity load balancing algorithms that reduce the number of IP sockets opened by a Java client application. A client accessing multiple objects on a server instance will use a single socket. If an object fails, the client will failover to a server instance to which it already has an open socket, if possible. In older version of WebLogic Server, under some circumstances, a client might open a socket to each server instance in a cluster.
For optimal performance, configure enough socket reader threads in the Java Virtual Machine (JVM) that runs the client. For instructions, see Set the Number of Reader Threads on Client Machines.
Clients of a non-clustered WebLogic Server server instance access objects and services by using a JNDI-compliant naming service. The JNDI naming service contains a list of the public services that the server instance offers, organized in a tree structure. A WebLogic Server instance offers a new service by binding into the JNDI tree a name that represents the service. Clients obtain the service by connecting to the server instance and looking up the bound name of the service.
Server instances in a cluster utilize a cluster-wide JNDI tree. A cluster-wide JNDI tree is similar to a single server instance JNDI tree, insofar as the tree contains a list of available services. In addition to storing the names of local services, however, the cluster-wide JNDI tree stores the services offered by clustered objects (EJBs and RMI classes) from other server instances in the cluster.
Each WebLogic Server instance in a cluster creates and maintains a local copy of the logical cluster-wide JNDI tree. The follow sections describe how the cluster-wide JNDI tree is maintained, and how to avoid naming conflicts that can occur in a clustered environment.
Caution:Do not use the cluster-wide JNDI tree as a persistence or caching mechanism for application data. Although WebLogic Server replicates a clustered server instance's JNDI entries to other server instances in the cluster, those entries are removed from the cluster if the original instance fails. Also, storing large objects within the JNDI tree can overload multicast or unicast traffic and interfere with the normal operation of a cluster.
Each WebLogic Server in a cluster builds and maintains its own local copy of the cluster-wide JNDI tree, which lists the services offered by all members of the cluster. Creation of a cluster-wide JNDI tree begins with the local JNDI tree bindings of each server instance. As a server instance boots (or as new services are dynamically deployed to a running server instance), the server instance first binds the implementations of those services to the local JNDI tree. The implementation is bound into the JNDI tree only if no other service of the same name exists.
Note:When you start a Managed Server in a cluster, the server instance identifies other running server instances in the cluster by listening for heartbeats, after a warm-up period specified by the
ClusterMBean. The default warm-up period is 30 seconds.
Once the server instance successfully binds a service into the local JNDI tree, additional steps are performed for clustered objects that use replica-aware stubs. After binding the clustered object's implementation into the local JNDI tree, the server instance sends the object's stub to other members of the cluster. Other members of the cluster monitor the multicast or unicast address to detect when remote server instances offer new services.
Figure 3-4 shows a snapshot of the JNDI binding process.
In the previous figure, Server A has successfully bound an implementation of clustered Object X into its local JNDI tree. Because Object X is clustered, it offers this service to all other members of the cluster. Server C is still in the process of binding an implementation of Object X.
Other server instances in the cluster listening to the multicast or unicast address note that Server A offers a new service for clustered object, X. These server instances update their local JNDI trees to include the new service.
Updating the local JNDI bindings occurs in one of two ways:
If the clustered service is not yet bound in the local JNDI tree, the server instance binds a new replica-aware stub into the local tree that indicates the availability of Object X on Server A. Servers B and D would update their local JNDI trees in this manner, because the clustered object is not yet deployed on those server instances.
If the server instance already has a binding for the cluster-aware service, it updates its local JNDI tree to indicate that a replica of the service is also available on Server A. Server C would update its JNDI tree in this manner, because it will already have a binding for the clustered Object X.
In this manner, each server instance in the cluster creates its own copy of a cluster-wide JNDI tree. The same process would be used when Server C announces that Object X has been bound into its local JNDI tree. After all broadcast messages are received, each server instance in the cluster would have identical local JNDI trees that indicate the availability of the object on Servers A and C, as shown in Figure 3-5.
Note:In an actual cluster, Object X would be deployed homogeneously, and an implementation which can invoke the object would be available on all four server instances.
Simple JNDI naming conflicts occur when a server instance attempts to bind a non-clustered service that uses the same name as a non-clustered service already bound in the JNDI tree. Cluster-level JNDI conflicts occur when a server instance attempts to bind a clustered object that uses the name of a non-clustered object already bound in the JNDI tree.
WebLogic Server detects simple naming conflicts (of non-clustered services) when those services are bound to the local JNDI tree. Cluster-level JNDI conflicts may occur when new services are advertised over multicast or unicast. For example, if you deploy a pinned RMI object on one server instance in the cluster, you cannot deploy a replica-aware version of the same object on another server instance.
If two server instances in a cluster attempt to bind different clustered objects using the same name, both will succeed in binding the object locally. However, each server instance will refuse to bind the other server instance's replica-aware stub in to the JNDI tree, due to the JNDI naming conflict. A conflict of this type would remain until one of the two server instances was shut down, or until one of the server instances undeployed the clustered object. This same conflict could also occur if both server instances attempt to deploy a pinned object with the same name.
To avoid cluster-level JNDI conflicts, you must homogeneously deploy all replica-aware objects to all WebLogic Server instances in a cluster. Having unbalanced deployments across WebLogic Server instances increases the chance of JNDI naming conflicts during startup or redeployment. It can also lead to unbalanced processing loads in the cluster.
If you must pin specific RMI objects or EJBs to individual server instances, do not replicate the object's bindings across the cluster.
When a clustered object is removed (undeployed from a server instance), updates to the JNDI tree are handled similarly to the updates performed when new services are added. The server instance on which the service was undeployed broadcasts a message indicating that it no longer provides the service. Again, other server instances in the cluster that observe the multicast or unicast message update their local copies of the JNDI tree to indicate that the service is no longer available on the server instance that undeployed the object.
Once the client has obtained a replica-aware stub, the server instances in the cluster may continue adding and removing host servers for the clustered objects. As the information in the JNDI tree changes, the client's stub may also be updated. Subsequent RMI requests contain update information as necessary to ensure that the client stub remains up-to-date.
Clients that connect to a WebLogic Server cluster and look up a clustered object obtain a replica-aware stub for the object. This stub contains the list of available server instances that host implementations of the object. The stub also contains the load balancing logic for distributing the load among its host servers.
For more information about replica-aware stubs for EJBs and RMI classes, see Replication and Failover for EJBs and RMIs.
For a more detailed discussion of how WebLogic JNDI is implemented in a clustered environment and how to make your own objects available to JNDI clients, see "Using WebLogic JNDI in a Clustered Environment" in Developing JNDI Applications for Oracle WebLogic Server