Load Balancing in a Cluster

 Oracle WebLogic Server clusters provide load balancing support for different types of objects. 

• Load Balancing for Servlets and JSPs
• Load Balancing for EJBs and RMI Objects
• Load Balancing for JMS

Load Balancing for Servlets and JSPs:

You can accomplish load balancing of servlets and JSPs with the built-in load balancing capabilities of a WebLogic proxy plug-in or with separate load balancing hardware.

Load Balancing with a Proxy Plug-in

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 on a round-robin basis. This load balancing method is described in Round-Robin Load Balancing.

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.

WebLogic Server supports the following Web servers and associated proxy plug-ins:

• WebLogic Server with the HttpClusterServlet
• Netscape Enterprise 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

Load Balancing HTTP Sessions with an External Load Balancer

Clusters that employ a hardware load balancing solution can use any load balancing algorithm supported by the hardware. These can include advanced load-based balancing strategies that monitor the utilization of individual machines.

Load Balancer Configuration 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.

Passive Cookie Persistence

Passive cookie persistence enables WebLogic Server to write a cookie containing session parameter information through the load balancer to the client.

Active Cookie Persistence

You can use certain active cookie persistence mechanisms with WebLogic Server clusters, provided the load balancer does not modify the WebLogic Server cookie. WebLogic Server clusters do not support active cookie persistence mechanisms that overwrite or modify the WebLogic HTTP session cookie. If the load balancer's active cookie persistence mechanism works by adding its own cookie to the client session, no additional configuration is required to use the load balancer with a WebLogic Server cluster.

SSL Persistence

When SSL persistence is used, the load balancer performs all encryption and decryption of data between clients and the WebLogic Server cluster. The load balancer then uses the plain text cookie that WebLogic Server inserts on the client to maintain an association between the client and a particular server in the cluster.

Load Balancers and the WebLogic Session Cookie

A load balancer that uses passive cookie persistence can use a string in the WebLogic session cookie to associate a client with the server hosting its primary HTTP session state. The string uniquely identifies a server instance in the cluster. You must configure the load balancer with the offset and length of the string constant. The correct values for the offset and length depend on the format of the session cookie.

The format of a session cookie is:

Copysessionid!primary_server_id!secondary_server_id

where:

sessionid is a randomly generated identifier of the HTTP session. The length of the value is configured by the IDLength parameter in the <session-descriptor> element in the weblogic.xml file for an application. By default, the sessionid length is 52 bytes.

primary_server_id and secondary_server_id are 10 character identifiers of the primary and secondary hosts for the session.

Note:For sessions using non-replicated memory, cookie, or file-based session persistence, the secondary_server_id is not present. For sessions that use in-memory replication, if the secondary session does not exist, the secondary_server_id is "NONE".

Load Balancing for EJBs and RMI Objects

WebLogic Server cluster use load balancing algorithms for EJBs and RMI objects. The load balancing algorithm for an object is maintained in the replica-aware stub obtained for a clustered object.

Following are different types of load balancing algorithms available:

• Round-Robin Load Balancing
• Weight-Based Load Balancing
• Random Load Balancing

By default, WebLogic Server Cluster use round-robin load balancing algorithm. You can configure a different default load balancing method for a cluster by using WebLogic Admin Console to set weblogic.cluster.defaultLoadAlogorithm. You can also specify the load balancing algorithm for a specific RMI objects using the -loadAlgorithm option in rmic, or with the home-load-algorithm or stateless-bean-load-algorithm in an EJB's deployment descriptor. A load balancing algorithm that you configure for an object overrides the default load balancing algorithm for the cluster. 

Round Robin Load Balancing

WebLogic Server uses the round-robin algorithm as the default load balancing strategy for clustered object stubs when no algorithm is specified. This algorithm is supported for RMI objects and EJBs. It is also the method used by WebLogic proxy plug-ins.

The round-robin algorithm cycles through a list of WebLogic Server instances in order. For clustered objects, the server list consists of WebLogic Server instances that host the clustered object. For proxy plug-ins, the list consists of all WebLogic Server instances that host the clustered servlet or JSP.

The advantages of the round-robin algorithm are that it is simple, cheap and very predictable. The primary disadvantage is that there is some chance of convoying. Convoying occurs when one server is significantly slower than the others. Because replica-aware stubs or proxy plug-ins access the servers in the same order, a slow server can cause requests to "synchronize" on the server, then follow other servers in order for future requests.

Weight based Load Balancing

This algorithm applies only to EJB and RMI object clustering.

Weight-based load balancing improves on the round-robin algorithm by taking into account a pre-assigned weight for each server. You can use the Server > Configuration > Cluster page in the WebLogic Server Administration Console to assign each server in the cluster a numerical weight between 1 and 100, in the Cluster Weight field. This value determines what proportion of the load the server will bear relative to other servers. If all servers have the same weight, they will each bear an equal proportion of the load. If one server has weight 50 and all other servers have weight 100, the 50-weight server will bear half as much as any other server. This algorithm makes it possible to apply the advantages of the round-robin algorithm to clusters that are not homogeneous.

If you use the weight-based algorithm, carefully determine the relative weights to assign to each server instance. Factors to consider include:

• The processing capacity of the server's hardware in relationship to other servers (for example, the number and performance of CPUs dedicated to WebLogic Server).
• The number of non-clustered ("pinned") objects each server hosts.

Random Load Balancing

The random method of load balancing applies only to EJB and RMI object clustering.

In random load balancing, requests are routed to servers at random. Random load balancing is recommended only for homogeneous cluster deployments, where each server instance runs on a similarly configured machine. A random allocation of requests does not allow for differences in processing power among the machines upon which server instances run. If a machine hosting servers in a cluster has significantly less processing power than other machines in the cluster, random load balancing will give the less powerful machine as many requests as it gives more powerful machines.

Random load balancing distributes requests evenly across server instances in the cluster, increasingly so as the cumulative number of requests increases. Over a small number of requests the load may not be balanced exactly evenly.

Disadvantages of random load balancing include the slight processing overhead incurred by generating a random number for each request, and the possibility that the load may not be evenly balanced over a small number of requests.

Server Affinity Load Balancing Algorithms

WebLogic Server provides three load balancing algorithms for RMI objects that provide server affinity. Server affinity turns off load balancing for external client connections; instead, the client considers its existing connections to WebLogic Server instances when choosing the server instance on which to access an object. If an object is configured for server affinity, the client-side stub attempts to choose a server instance to which it is already connected, and continues to use the same server instance for method calls. All stubs on that client attempt to use that server instance. If the server instance becomes unavailable, the stubs fail over, if possible, to a server instance to which the client is already connected.

The purpose of server affinity is to minimize the number IP sockets opened between external Java clients and server instances in a cluster. WebLogic Server accomplishes this by causing method calls on objects to "stick" to an existing connection, instead of being load balanced among the available server instances. With server affinity algorithms, the less costly server-to-server connections are still load-balanced according to the configured load balancing algorithm—load balancing is disabled only for external client connections.

Server affinity is used in combination with one of the standard load balancing methods: round-robin, weight-based, or random:

Load Balancing for JMS

WebLogic Server JMS supports server affinity for distributed JMS destinations and client connections.

By default, a WebLogic Server cluster uses the round-robin method to load balance objects. To use a load balancing algorithm that provides server affinity for JMS objects, you must configure the desired method for the cluster as a whole. You can configure the load balancing algorithm by using the WebLogic Server Administration Console to set weblogic.cluster.defaultLoadAlgorithm.

Server Affinity for Distributed JMS Destinations

Server affinity is supported for JMS applications that use the distributed destination feature; this feature is not supported for standalone destinations. If you configure server affinity for JMS connection factories, a server instance that is load balancing consumers or producers across multiple members of a distributed destination will first attempt to load balance across any destination members that are also running on the same server instance.

Initial Context Affinity and Server Affinity for Client Connections

A system administrator can establish load balancing of JMS destinations across multiple servers in a cluster by configuring multiple JMS servers and using targets to assign them to the defined WebLogic Servers. Each JMS server is deployed on exactly one WebLogic Server and handles requests for a set of destinations. During the configuration phase, the system administrator enables load balancing by specifying targets for JMS servers. 

A system administrator can establish cluster-wide, transparent access to destinations from any server in the cluster by configuring multiple connection factories and using targets to assign them to WebLogic Servers. Each connection factory can be deployed on multiple WebLogic Servers. 

The application uses the Java Naming and Directory Interface (JNDI) to look up a connection factory and create a connection to establish communication with a JMS server. Each JMS server handles requests for a set of destinations. Requests for destinations not handled by a JMS server are forwarded to the appropriate server.

WebLogic Server provides server affinity for client connections. If an application has a connection to a given server instance, JMS will attempt to establish new JMS connections to the same server instance.

When creating a connection, JMS will try first to achieve initial context affinity. It will attempt to connect to the same server or servers to which a client connected for its initial context, assuming that the server instance is configured for that connection factory. For example, if the connection factory is configured for servers A and B, but the client has an Initial Context on server C, then the connection factory will not establish the new connection with A, but will choose between servers B and C.

If a connection factory cannot achieve initial context affinity, it will try to provide affinity to a server to which the client is already connected. For instance, assume the client has an Initial Context on server A and some other type of connection to server B. If the client then uses a connection factory configured for servers B and C it will not achieve initial context affinity. The connection factory will instead attempt to achieve server affinity by trying to create a connection to server B, to which it already has a connection, rather than server C.

If a connection factory cannot provide either initial context affinity or server affinity, then the connection factory is free to make a connection wherever possible. For instance, assume a client has an initial context on server A, no other connections and a connection factory configured for servers B and C. The connection factory is unable to provide any affinity and is free to attempt new connections to either server B or C.