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WebLogic Frequently Asked Questions: Clustering

- Cluster-aware stubs

- Cluster failover services

- Adding servers to a cluster?

- Client performance during failover

- WebLogic Servers on multi-CPU machines

Cluster-aware stubs

How do stubs work in a WebLogic Server cluster?

Initially, a client does a DNS lookup to determine what server to connect to for the JNDI lookup. After it obtains an object reference, the cluster-aware stub for the object contains a list of available servers in the cluster.

Cluster failover services

What happens when a failure occurs and the stub cannot connect to a WebLogic Server instance?

When the failure occurs, the stub removes the failed server instance from its list. If there are no servers left in its list, it uses DNS again to find a running server and obtain a current list of running instances. Also, the stub periodically refreshes its list of available server instances in the cluster; this allows the stub to take advantage of new servers as they are added to the cluster.

How does a server know when another server is unavailable?

WebLogic Server uses two mechanisms to determine if a given server instance is unavailable. First, it uses a heartbeat mechanism with an internal timeout to monitor the status of instances in a cluster. The heartbeat mechanism is based on the cluster's multicast address. Once a given server instance receives a set number of heartbeats, it is dropped from the cluster.
Second, WebLogic Server monitors socket errors to determine when server instances become unavailable. For example, if server instance A has an open socket to server instance B, and the socket is unexpectedly closes, server A assumes that server B is offline.

Adding servers to a cluster?

How are notifications made when a server is added to a cluster?

The WebLogic Server cluster broadcasts the availability of a new server instance each time a new instance joins the cluster. Cluster-aware stubs also periodically update their list of available server instances.

How do clients learn about new WebLogic Server instances?

Once a client has done a JNDI lookup and begins using an object reference, it finds out about new server instances only after the cluster-aware stub has updated its list of available servers.

Client performance during failover

After a server fails, won't DNS continue to send requests to the unavailable machine? Doesn't this waste bandwidth? How can I address this problem from client-side applications?

For a Java client application, this problem occurs only during startup. WebLogic Server caches the DNS entries and removes the unavailable ones, to prevent the client from accessing a failed server twice.

Failed servers can be more of a problem for browser -based clients, because they always use DNS. To avoid unnecessary DNS requests with browser-based clients, use a third-party load-balancer such as Resonate, BigIP, and LocalDirector. These products mask multiple DNS addresses as a single address. They also provide more sophisticated load-balancing options than round-robin, and they keep track of failed servers to avoid routing unnecessary requests.

WebLogic Servers on multi-CPU machines

How many WebLogic Servers can I have on a multi-cpu machine?

There are many possible configurations and each has its own advantages and disadvantages. For performance reasons, the number of WebLogic Servers should always be equal to or less than the number of processors (CPUs) on the machine.

For example, if the machine has four processors, the maximum number of WebLogic Servers on that machine is four. Running more servers than the number of processors on a single-machine could lead to software failure.

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