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Using WebLogic Server Clusters

# Failover and Replication in a Cluster

In order for a cluster to provide high availability it must be able to recover from service failures. The following sections describe how WebLogic Server detect failures in a cluster, and provides an overview of how failover is accomplished for different types of objects:

- How WebLogic Server Detects Failures
- Replication and Failover for Servlets and JSPs
- Replication and Failover for EJBs and RMIs
- Migration for Pinned Services
- Failover and JDBC Connections

# How WebLogic Server Detects Failures

WebLogic Server instances in a cluster detect failures of their peer server instances by monitoring:

- Socket connections to a peer server
- Regular server heartbeat messages

## Failure Detection Using IP Sockets

WebLogic Server instances monitor the use of IP sockets between peer server instances as an immediate method of detecting failures. If a server connects to one of its peers in a cluster and begins transmitting data over a socket, an unexpected closure of that socket causes the peer server to be marked as "failed," and its associated services are removed from the JNDI naming tree.

## The WebLogic Server "Heartbeat"

If clustered server instances do not have opened sockets for peer-to-peer communication, failed servers may also be detected via the WebLogic Server heartbeat. All server instances in a cluster use multicast to broadcast regular server heartbeat messages to other members of the cluster. Each heartbeat message contains data that uniquely identifies the server that sends the message. Servers broadcast their heartbeat messages at regular intervals of 10 seconds. In turn, each server in a cluster monitors the multicast address to ensure that all peer servers' heartbeat messages are being sent.

If a server monitoring the multicast address misses three heartbeats from a peer server (i.e., if it does not receive a heartbeat from the server for 30 seconds or longer), the monitoring server marks the peer server as "failed." It then updates its local JNDI tree, if necessary, to retract the services that were hosted on the failed server.

In this way, servers can detect failures even if they have no sockets open for peer-to-peer communication.

Note: For more information about how WebLogic Server uses IP sockets and multicast communications see WebLogic Server Communication in a Cluster.

# Replication and Failover for Servlets and JSPs

In clusters that utilize Web servers with WebLogic proxy plug-ins, the proxy plug-in handles failover transparently to the client. If a server fails, the plug-in locates the replicated HTTP session state on a secondary server and redirects the client's request accordingly.

For clusters that use a supported hardware load balancing solution, the load balancing hardware simply redirects client requests to any available server in the WebLogic Server cluster. The cluster itself obtains the replica of the client's HTTP session state from a secondary server in the cluster.

## HTTP Session State Replication

To support automatic failover for servlet and JSP HTTP session states, WebLogic Server replicates the session state in memory. WebLogic Server creates a primary session state on the server to which the client first connects, and a secondary replica on another WebLogic Server instance in the cluster. The replica is kept up-to-date so that it may be used if the server that hosts the servlet fails. The process of copying a session state from one server instance to another is called in-memory replication.

Note: WebLogic Server can also maintain the HTTP session state of a servlet or JSP using file-based or JDBC-based persistence. For more information on these persistence mechanisms, see Configuring Session Persistence in Programming WebLogic HTTP Servlets.

### Requirements for HTTP Session State Replication

To utilize in-memory replication for HTTP session states, you must access the WebLogic Server cluster using either a collection of Web servers with identically configured WebLogic proxy plug-ins, or load balancing hardware.

#### Supported Server and Proxy Software

The WebLogic proxy plug-in maintains a list of WebLogic Server instances that host a clustered servlet or JSP, and forwards HTTP requests to those instances using a round-robin strategy. The plug-in also provides the logic necessary to locate the replica of a client's HTTP session state if a WebLogic Server instance should fail.

In-memory replication for HTTP session states is supported by the following Web server and proxy software:

- WebLogic Server with the `HttpClusterServlet`
- Sun One Web Server with the Netscape (proxy) plug-in
- Apache with the Apache Server (proxy) plug-in
- Microsoft Internet Information Server with the Microsoft-IIS (proxy) plug-in

For instructions on setting up proxy plug-ins, see Configure Proxy Plug-Ins.

#### Load Balancer Requirements

If you choose to use load balancing hardware instead of a proxy plug-in, it must support a compatible passive or active cookie persistence mechanism, and SSL persistence. For details on these requirements, see Load Balancer Configuration Requirements. For instructions on setting up a load balancer, see Configuring Load Balancers that Support Passive Cookie Persistence.

#### Programming Considerations for Clustered Servlets and JSPs

This section highlights key programming constraints and recommendations for servlets and JSPs that you will deploy in a clustered environment.

- Session Data Must Be Serializable

To support in-memory replication of HTTP session states, all servlet and JSP session data must be serializable.

Note: Serialization is the process of converting a complex data structure, such as a parallel arrangement of data (in which a number of bits are transmitted at a time along parallel channels) into a serial form (in which one bit at a time is transmitted); a serial interface provides this conversion to enable data transmission.

Every field in an object must be serializable or transient in order for the object to be considered serializable. If the servlet or JSP uses a combination of serializable and non-serializable objects, WebLogic Server does not replicate the session state of the non-serializable objects.

- Use setAttribute to Change Session State

In an HTTP servlet that implements `javax.servlet.http.HttpSession`, use `HttpSession.setAttribute` (which replaces the deprecated `putValue`) to change attributes in a session object. If you set attributes in a session object with `setAttribute`, the object and its attributes are replicated in a cluster using in-memory replication. If you use other set methods to change objects within a session, WebLogic Server does not replicate those changes. Every time a change is made to an object that is in the session, `setAttribute()` should be called to update that object across the cluster.

Likewise, use `removeAttribute` (which, in turn, replaces the deprecated `removeValue`) to remove an attribute from a session object.

Note: Use of the deprecated `putValue` and `removeValue` methods will also cause session attributes to be replicated.

- Consider Serialization Overhead

Serializing session data introduces some overhead for replicating the session state. The overhead increases as the size of serialized objects grows. If you plan to create very large objects in the session, test the performance of your servlets to ensure that performance is acceptable.

- Control Frame Access to Session Data

If you are designing a Web application that utilizes multiple frames, keep in mind that there is no synchronization of requests made by frames in a given frameset. For example, it is possible for multiple frames in a frameset to create multiple sessions on behalf of the client application, even though the client should logically create only a single session.

In a clustered environment, poor coordination of frame requests can cause unexpected application behavior. For example, multiple frame requests can "reset" the application's association with a clustered instance, because the proxy plug-in treats each request independently. It is also possible for an application to corrupt session data by modifying the same session attribute via multiple frames in a frameset.

To avoid unexpected application behavior, carefully plan how you access session data with frames. You can apply one of the following general rules to avoid common problems:

- In a given frameset, ensure that only one frame creates and modifies session data.
- Always create the session in a frame of the first frameset your application uses (for example, create the session in the first HTML page that is visited). After the session has been created, access the session data only in framesets other than the first frameset.

### Using Replication Groups

By default, WebLogic Server attempts to create session state replicas on a different machine than the one that hosts the primary session state. You can further control where secondary states are placed using replication groups. A replication group is a preferred list of clustered servers to be used for storing session state replicas.

Using the WebLogic Server Console, you can define unique machine names that will host individual server instances. These machine names can be associated with new WebLogic Server instances to identify where the servers reside in your system.

Machine names are generally used to indicate servers that run on the same machine. For example, you would assign the same machine name to all server instances that run on the same machine, or the same server hardware.

If you do not run multiple WebLogic Server instances on a single machine, you do not need to specify WebLogic Server machine names. Servers without a machine name are treated as though they reside on separate machines. For detailed instructions on setting machine names, see Configure Machine Names.

When you configure a clustered server instance, you can assign the server to a replication group, and a preferred secondary replication group for hosting replicas of the primary HTTP session states created on the server.

When a client attaches to a server in the cluster and creates a primary session state, the server hosting the primary state ranks other servers in the cluster to determine which server should host the secondary. Server ranks are assigned using a combination of the server's location (whether or not it resides on the same machine as the primary server) and its participation in the primary server's preferred replication group. The following table shows the relative ranking of servers in a cluster.

Table 5-1 Ranking Server Instances for Session Replication

Server Rank | Server Resides on a Different Machine | Server is a Member of Preferred Replication Group
--- | --- | ---
1 | Yes | Yes
2 | Yes | No
3 | No | Yes
4 | No | No

Using these rules, the primary WebLogic Server ranks other members of the cluster and chooses the highest-ranked server to host the secondary session state. For example, the following figure shows replication groups configured for different geographic locations.

Figure 5-1 Replication Groups for Different Geographic Locations

In this example, Servers A, B, and C are members of the replication group "Headquarters" and use the preferred secondary replication group "Crosstown." Conversely, Servers X, Y, and Z are members of the "Crosstown" group and use the preferred secondary replication group "Headquarters." Servers A, B, and X reside on the same machine, "sardina."

If a client connects to Server A and creates an HTTP session state,

- Servers Y and Z are most likely to host the replica of this state, since they reside on separate machines and are members of Server A's preferred secondary group.
- Server X holds the next-highest ranking because it is also a member of the preferred replication group (even though it resides on the same machine as the primary.)
- Server C holds the third-highest ranking since it resides on a separate machine but is not a member of the preferred secondary group.
- Server B holds the lowest ranking, because it resides on the same machine as Server A (and could potentially fail along with A if there is a hardware failure) and it is not a member of the preferred secondary group.

To configure a server's membership in a replication group, or to assign a server's preferred secondary replication group, follow the instructions in Configure Replication Groups.

## Accessing Clustered Servlets and JSPs Using a Proxy

This section describes the connection and failover processes for requests that are proxied to clustered servlets and JSPs. For instructions on setting up proxy plug-ins, see Configure Proxy Plug-Ins.

The following figure depicts a client accessing a servlet hosted in a cluster. This example uses a single WebLogic Server to serve static HTTP requests only; all servlet requests are forwarded to the WebLogic Server cluster via the `HttpClusterServlet`.

Figure 5-2 Accessing Servlets and JSPs using a Proxy

Note: The discussion that follows also applies if you use a third-party Web server and WebLogic proxy plug-in, rather than WebLogic Server and the `HttpClusterServlet`.

### Proxy Connection Procedure

When the HTTP client requests the servlet, `HttpClusterServlet` proxies the request to the WebLogic Server cluster. `HttpClusterServlet` maintains the list of all servers in the cluster, and the load balancing logic to use when accessing the cluster. In the above example, `HttpClusterServlet` routes the client request to the servlet hosted on WebLogic Server A. WebLogic Server A becomes the primary server hosting the client's servlet session.

To provide failover services for the servlet, the primary server replicates the client's servlet session state to a secondary WebLogic Server in the cluster. This ensures that a replica of the session state exists even if the primary server fails (for example, due to a network failure). In the example above, Server B is selected as the secondary.

The servlet page is returned to the client through the `HttpClusterServlet`, and the client browser is instructed to write a cookie that lists the primary and secondary locations of the servlet session state. If the client browser does not support cookies, WebLogic Server can use URL rewriting instead.

#### Using URL Rewriting to Track Session Replicas

In its default configuration, WebLogic Server uses client-side cookies to keep track of the primary and secondary server that host the client's servlet session state. If client browsers have disabled cookie usage, WebLogic Server can also keep track of primary and secondary servers using URL rewriting. With URL rewriting, both locations of the client session state are embedded into the URLs passed between the client and proxy server. To support this feature, you must ensure that URL rewriting is enabled on the WebLogic Server cluster. For instructions on how to enable URL rewriting, see Using URL Rewriting, in Assembling and Configuring Web Applications.

### Proxy Failover Procedure

Should the primary server fail, `HttpClusterServlet` uses the client's cookie information to determine the location of the secondary WebLogic Server that hosts the replica of the session state. `HttpClusterServlet` automatically redirects the client's next HTTP request to the secondary server, and failover is transparent to the client.

After the failure, WebLogic Server B becomes the primary server hosting the servlet session state, and a new secondary is created (Server C in the previous example). In the HTTP response, the proxy updates the client's cookie to reflect the new primary and secondary servers, to account for the possibility of subsequent failovers.

In a two-server cluster, the client would transparently fail over to the server hosting the secondary session state. However, replication of the client's session state would not continue unless another WebLogic Server became available and joined the cluster. For example, if the original primary server was restarted or reconnected to the network, it would be used to host the secondary session state.

## Accessing Clustered Servlets and JSPs with Load Balancing Hardware

To support direct client access via load balancing hardware, the WebLogic Server replication system allows clients to use secondary session states regardless of the server to which the client fails over. WebLogic Server uses client-side cookies or URL rewriting to record primary and secondary server locations. However, this information is used only as a history of the servlet session state location; when accessing a cluster via load balancing hardware, clients do not use the cookie information to actively locate a server after a failure.

The following sections describe the connection and failover procedure when using HTTP session state replication with load balancing hardware.

### Connection with Load Balancing Hardware

The following figure illustrates the connection procedure for a client accessing a cluster through a load balancer.

Figure 5-3 Connection with Load Balancing Hardware

When the client of a Web application requests a servlet using a public IP address:

1. The load balancer routes the client's connection request to a WebLogic Server cluster in accordance with its configured policies. It directs the request to WebLogic Server A.
2. WebLogic Server A acts as the primary host of the client's servlet session state. It uses the ranking system described in Using Replication Groups to select a server to host the replica of the session state. In the example above, WebLogic Server B is selected to host the replica.
3. The client is instructed to record the location of WebLogic Server instances A and B in a local cookie. If the client does not allow cookies, the record of the primary and secondary servers can be recorded in the URL returned to the client via URL rewriting.

Note: You must enable WebLogic Server URL rewriting capabilities to support clients that disallow cookies, as described in Using URL Rewriting to Track Session Replicas.

1. As the client makes additional requests to the cluster, the load balancer uses an identifier in the client-side cookie to ensure that those requests continue to go to WebLogic Server A (rather than being load-balanced to another server in the cluster). This ensures that the client remains associated with the server hosting the primary session object for the life of the session.

### Failover with Load Balancing Hardware

Should Server A fail during the course of the client's session, the client's next connection request to Server A also fails, as illustrated in the following figure.

Figure 5-4 Failover with Load Balancing Hardware

In response to the connection failure:

1. The load balancing hardware uses its configured policies to direct the request to an available WebLogic Server in the cluster. In the above example, assume that the load balancer routes the client's request to WebLogic Server C after WebLogic Server A fails.
2. When the client connects to WebLogic Server C, the server uses the information in the client's cookie (or the information in the HTTP request if URL rewriting is used) to acquire the session state replica on WebLogic Server B. The failover process remains completely transparent to the client.

WebLogic Server C becomes the new host for the client's primary session state, and WebLogic Server B continues to host the session state replica. This new information about the primary and secondary host is again updated in the client's cookie, or via URL rewriting.

# Replication and Failover for EJBs and RMIs

For clustered EJBs and RMIs, failover is accomplished using the object's replica-aware stub. When a client makes a call through a replica-aware stub to a service that fails, the stub detects the failure and retries the call on another replica.

With clustered objects, automatic failover occurs only in cases where the object is idempotent. However, failover can happen even for non-idempotent methods.

An object is idempotent if any method can be called multiple times with no different effect than calling the method once. This is always true for methods that have no permanent side effects. Methods that do have side effects have to be written with idempotence in mind.

Consider a shopping cart service call `addItem()` that adds an item to a shopping cart. Suppose client C invokes this call on a replica on Server S1. After S1 receives the call, but before it successfully returns to C, S1 crashes. At this point the item has been added to the shopping cart, but the replica-aware stub has received an exception. If the stub were to retry the method on Server S2, the item would be added a second time to the shopping cart. Because of this, replica-aware stubs will not, by default, attempt to retry a method that fails after the request is sent but before it returns. This behavior can be overridden by marking a service idempotent.

WebLogic Server allows failover for non-idempotent methods only when any of the following exceptions is thrown by the underlying JVM:

- UnknownHostException
- ConnectException
- ConnectIOException
- NoSuchObjectException
- QueueThrottleException

## Clustering Objects with Replica-Aware Stubs

If an EJB or RMI object is clustered, instances of the object are deployed on all WebLogic Server instances in the cluster. The client has a choice about which instance of the object to call. Each instance of the object is referred to as a replica.

The key technology that supports object clustering objects in WebLogic Server is the replica-aware stub. When you compile an EJB that supports clustering (as defined in its deployment descriptor), `appc` passes the EJB's interfaces through the `rmic` compiler to generate replica-aware stubs for the bean. For RMI objects, you generate replica-aware stubs explicitly using command-line options to `rmic`, as described in WebLogic RMI Compiler, in Programming WebLogic RMI.

A replica-aware stub appears to the caller as a normal RMI stub. Instead of representing a single object, however, the stub represents a collection of replicas. The replica-aware stub contains the logic required to locate an EJB or RMI class on any WebLogic Server instance on which the object is deployed. When you deploy a cluster-aware EJB or RMI object, its implementation is bound into the JNDI tree. As described in Cluster-Wide JNDI Naming Service, clustered WebLogic Server instances have the capability to update the JNDI tree to list all server instances on which the object is available. When a client accesses a clustered object, the implementation is replaced by a replica-aware stub, which is sent to the client.

The stub contains the load balancing algorithm (or the call routing class) used to load balance method calls to the object. On each call, the stub can employ its load algorithm to choose which replica to call. This provides load balancing across the cluster in a way that is transparent to the caller. To understand the load balancing algorithms available for RMI objects and EJBs, see Load Balancing for EJBs and RMI Objects. If a failure occurs during the call, the stub intercepts the exception and retries the call on another replica. This provides a failover that is also transparent to the caller.

## Clustering Support for Different Types of EJBs

EJBs differ from plain RMI objects in that each EJB can potentially generate two different replica-aware stubs: one for the `EJBHome` interface and one for the `EJBObject` interface. This means that EJBs can potentially realize the benefits of load balancing and failover on two levels:

- When a client looks up an EJB object using the `EJBHome` stub
- When a client makes method calls against the EJB using the `EJBObject` stub

The following sections describe clustering support for different types of EJBs.

### Clustered EJBHomes

All bean homes interfaces—used to find or create bean instances—can be clustered, by specifying the determined by the home-is-clusterable element in `weblogic-ejb-jar.xml`.

Note: Stateless session beans, stateful session beans, and entity beans have home interfaces. Message-driven beans do not.

When a bean is deployed to a cluster, each server binds the bean's home interface to its cluster JNDI tree under the same name. When a client requests the bean's home from the cluster, the server instance that does the look-up returns a `EJBHome` stub that has a reference to the home on each server.

When the client issues a `create()` or `find()` call, the stub routes selects a server from the replica list in accordance with the load balancing algorithm, and routes the call to the home interface on that server. The selected home interface receives the call, and creates a bean instance on that server instance and executes the call, creating an instance of the bean.

Note: WebLogic Server 8.1 supports new load balancing algorithms that provide server affinity for EJB home interfaces. To understand server affinity and how it affects load balancing and failover, see Round-Robin Affinity, Weight-Based Affinity, and Random-Affinity.

### Clustered EJBObjects

An `EJBObject` stub tracks available replicas of an EJB in a cluster.

#### Stateless Session Beans

When a home creates a stateless bean, it returns a `EJBObject` stub that lists all of the servers in the cluster, to which the bean should be deployed. Because a stateless bean holds no state on behalf of the client, the stub is free to route any call to any server that hosts the bean. The stub can automatically fail over in the event of a failure. The stub does not automatically treat the bean as idempotent, so it will not recover automatically from all failures. If the bean has been written with idempotent methods, this can be noted in the deployment descriptor and automatic failover will be enabled in all cases.

Note: In WebLogic Server 8.1, new load balancing options provide server affinity for stateless EJB remote interfaces. To understand server affinity and how it affects load balancing and failover, see Round-Robin Affinity, Weight-Based Affinity, and Random-Affinity.

#### Stateful Session Beans

Method-level failover for a stateful service requires state replication. WebLogic Server satisfies this requirement by replicating the state of the primary bean instance to a secondary server instance, using a replication scheme similar to that used for HTTP session state.

When a home interface creates a stateless session bean instance, it selects a secondary instance to host the replicated state, using the same rules defined in Using Replication Groups. The home interface returns a `EJBObject` stub to the client that lists the location of the primary bean instance, and the location for the replicated bean state.

The following figure shows a client accessing a clustered stateful session EJB.

Figure 5-5 Client Accessing Stateful Session EJB

As the client makes changes to the state of the EJB, state differences are replicated to the secondary server instance. For EJBs that are involved in a transaction, replication occurs immediately after the transaction commits. For EJBs that are not involved in a transaction, replication occurs after each method invocation.

In both cases, only the actual changes to the EJB's state are replicated to the secondary server. This ensures that there is minimal overhead associated with the replication process.

Note: The actual state of a stateful EJB is non-transactional, as described in the EJB specification. Although it is unlikely, there is a possibility that the current state of the EJB can be lost. For example, if a client commits a transaction involving the EJB and there is a failure of the primary server before the state change is replicated, the client will fail over to the previously-stored state of the EJB. If it is critical to preserve the state of your EJB in all possible failover scenarios, use an entity EJB rather than a stateful session EJB.

#### Failover for Stateful Session EJBs

Should the primary server fail, the client's EJB stub automatically redirects further requests to the secondary WebLogic Server instance. At this point, the secondary server creates a new EJB instance using the replicated state data, and processing continues on the secondary server.

After a failover, WebLogic Server chooses a new secondary server to replicate EJB session states (if another server is available in the cluster). The location of the new primary and secondary server instances is automatically updated in the client's replica-aware stub on the next method invocation, as shown below.

Figure 5-6 Replica Aware Stubs are Updated after Failover

### Entity EJBs

There are two types of entity beans to consider: read-write entity beans and read-only entity beans.

- Read-Write Entities

When a home finds or creates a read-write entity bean, it obtains an instance on the local server and returns a stub pinned to that server. Load balancing and failover occur only at the home level. Because it is possible for multiple instances of the entity bean to exist in the cluster, each instance must read from the database before each transaction and write on each commit.

- Read-Only Entities

When a home finds or creates a read-only entity bean, it returns a replica-aware stub. This stub load balances on every call but does not automatically fail over in the event of a recoverable call failure. Read-only beans are also cached on every server to avoid database reads.

#### Failover for Entity Beans and EJB Handles

Failover for entity beans and EJB handles requires that the cluster address be specified as a DNS name that maps to all server instances in the cluster and only server instances in the cluster. The cluster DNS name should not map to a server instance that is not a member of the cluster.

## Clustering Support for RMI Objects

WebLogic RMI provides special extensions for building clustered remote objects. These are the extensions used to build the replica-aware stubs described in the EJB section. For more information about using RMI in clusters, see WebLogic RMI Features and Guidelines in Programming WebLogic RMI.

## Object Deployment Requirements

If you are programming EJBs to be used in a WebLogic Server cluster, read the instructions in this section to understand the capabilities of different EJB types in a cluster. Then ensure that you enable clustering in the EJB's deployment descriptor. weblogic-ejb-jar.xml Deployment Descriptor Reference in Programming WebLogic Enterprise JavaBeans describes the XML deployment elements relevant for clustering.

If you are developing either EJBs or custom RMI objects, also refer to "Using WebLogic JNDI in a Clustered Environment in Programming WebLogic JNDI to understand the implications of binding clustered objects in the JNDI tree.

### Other Failover Exceptions

Even if a clustered object is not idempotent, WebLogic Server performs automatic failover in the case of a `ConnectException` or `MarshalException`. Either of these exceptions indicates that the object could not have been modified, and therefore there is no danger of causing data inconsistency by failing over to another instance.

# Migration for Pinned Services

For HTTP session states and EJBs, a WebLogic Server cluster provides high availability and failover by duplicating the object or service on redundant servers in the cluster. However, certain services, such as JMS servers and the JTA transaction recovery service, are designed with the assumption that there is only one active instance of the service running in a cluster at any given time. These types of services are referred to as "pinned" services because they remain active on only one server instance at a time.

WebLogic Server allows the administrator to migrate pinned services from one server to another in the cluster, either in response to a server failure or as part of regularly-scheduled maintenance. This capability improves the availability of pinned services in a cluster, because those services can be quickly restarted on a redundant server should the host server fail.

In this release, migration is supported only for JMS servers and the JTA transaction recovery services. This document refers to these services as migratable services, because you can move them from one server to another within a cluster.

Caution: When migrating a JMS service that solely participates in a global transaction, for the migration to work and not leave any pending transactions, you must migrate both the JMS and JTA services, and the JMS service migration must occur before the JTA service migration.

In this release JMS also offers distributed destinations, which provides improved service continuity in the event of a single Weblogic Server failure by enabling you to configure multiple standalone destinations (queues and topics) as part of a single distributed destination set. For more information about distributed destinations, see Using Distributed Destinations in Programming WebLogic JMS.

However, WebLogic Server does not support automatic migration (failover) for pinned services in this release. For information about migrating pinned services manually, see Migrating a Pinned Service to a Target Server Instance.

## How Migration of Pinned Services Works

Clients access a migratable service in a cluster using a migration-aware RMI stub. The RMI stub keeps track of which server currently hosts the pinned service, and it directs client requests accordingly. For example, when a client first accesses a pinned service, the stub directs the client request to the server instance in the cluster that currently hosts the service. If the service migrates to a different WebLogic Server between subsequent client requests, the stub transparently redirects the request to the correct target server.

WebLogic Server implements a migration-aware RMI stub for JMS servers and the JTA transaction recovery service when those services reside in a cluster and are configured for migration.

## Migrating a Service When Currently Active Host is Unavailable

There are special considerations when you migrate a service from a server instance that has crashed or is unavailable to the Administration Server. If the Administration Server cannot reach the previously active host of the service at the time you perform the migration, that Managed Server's local configuration information will not be updated to reflect that it is no longer the active host for the service. In this situation, you must purge the unreachable Managed Server's local configuration cache before starting it again. This prevents the previous active host from re-activating at startup a service that has been migrated to another Managed Server. For more information see Migrating When the Currently Active Host is Unavailable.

## Defining Migratable Target Servers in a Cluster

By default, WebLogic Server can migrate the JTA transaction recovery service or a JMS server to any other server in the cluster. You can optionally configure a list of servers in the cluster that can potentially host a pinned service. This list of servers is referred to as a migratable target, and it controls the servers to which you can migrate a service. In the case of JMS, the migratable target also defines the list of servers to which you can deploy a JMS server.

For example, the following figure shows a cluster of four servers. Servers A and B are configured as the migratable target for a JMS server in the cluster.

Figure 5-7 Migratable Target in Cluster

In the above example, the migratable target allows the administrator to migrate the pinned JMS server only from Server A to Server B, or vice versa. Similarly, when deploying the JMS server to the cluster, the administrator selects either Server A or B as the deployment target to enable migration for the service. (If the administrator does not use a migratable target, the JMS server can be deployed or migrated to any available server in the cluster.)

WebLogic Server enables you to create separate migratable targets for the JTA transaction recovery service and JMS servers. This allows you to always keep each service running on a different server in the cluster, if necessary. Conversely, you can configure the same selection of servers as the migratable target for both JTA and JMS, to ensure that the services remain co-located on the same server in the cluster.

# Failover and JDBC Connections

JDBC is a highly stateful client-DBMS protocol, in which the DBMS connection and transactional state are tied directly to the socket between the DBMS process and the client (driver). For this reason, failover of a connection is not supported. If a WebLogic Server instance dies, any JDBC connections that it managed will die, and the DBMS(s) will roll back any transactions that were under way. Any applications affected will have to restart their current transactions from the beginning. All JDBC objects associated with dead connections will also be defunct. Clustered JDBC eases the reconnection process: the cluster-aware nature of WebLogic data sources in external client applications allow a client to request another connection from them if the server instance that was hosting the previous connection fails.

If you have replicated, synchronized database instances, you can use a JDBC multipool to support database failover. In such an environment, if a client cannot obtain a connection from one connection pool in the multipool because the pool doesn't exist or because database connectivity from the pool is down, WebLogic Server will attempt to obtain a connection from the next connection pool in the list of pools.

For instructions on clustering JDBC objects, see Configure Clustered JDBC.

Notes: If a client requests a connection for a pool in which all the connections are in use, an exception is generated, and WebLogic Server will not attempt to obtain a connection from another pool. You can address this problem by increasing the number of connections in the connection pool.

Any connection pool assigned to a multipool must be configured to test its connections at reserve time. This is the only way a pool can verify it has a good connection, and the only way a multipool can know when to fail over to the next pool on its list.