Introduction: Clusters

Clusters are groups of servers that are managed together and participate in workload management. A cluster can contain nodes or individual application servers. A node is usually a physical computer system with a distinct host IP address that is running one or more application servers. Clusters can be grouped under the configuration of a cell, which logically associates many servers and clusters with different configurations and applications with one another depending on the discretion of the administrator and what makes sense in their organizational environments.

Clusters are responsible for balancing workload among servers. Servers that are a part of a cluster are called cluster members. When you install an application on a cluster, the application is automatically installed on each cluster member. You can configure a cluster to provide workload balancing with service integration or with message driven beans in the application server.

[AIX Solaris HP-UX Linux Windows]Because each cluster member contains the same applications, you can distribute client tasks in distributed platforms according to the capacities of the different machines by assigning weights to each server.

[AIX Solaris HP-UX Linux Windows]In distributed platforms, assigning weights to the servers in a cluster improves performance and failover. Tasks are assigned to servers that have the capacity to perform the task operations. If one server is unavailable to perform the task, it is assigned to another cluster member. This reassignment capability has obvious advantages over running a single application server that can become overloaded if too many requests are made.

Cluster startup process options

Normal runtime processing automatically starts all server components during the server startup process. This processing applies to all servers, including servers that are part of a cluster. However, you can configure servers, including servers that are cluster members, such that not all of the server components start during the server startup process. This capability enables the server to consume resources as needed, thereby providing a smaller and more manageable footprint, and normally results in a performance improvement.

When you configure cluster members such that not all of the cluster member components start when the cluster or a specific cluster member is started, the cluster member components are dynamically started as they are needed. For example, if an application module starts that requires a specific server component, that component is dynamically started.

Clusters and node groups

Any application you install to a cluster must be able to execute on any application server that is a member of that cluster. Because a node group forms the boundaries for a cluster, all of the members of a cluster must be members of the same node group. Therefore, for the application you deploy to run successfully, all of the members of a cluster must be located on nodes that meet the requirements for that application.

In a cell that has many different server configurations, it might be difficult to determine which nodes have the capabilities to host your application. A node group can be used to define groups of nodes that have enough in common to host members of a given cluster. All cluster members in a cluster must be in the same node group.

All nodes are members of at least one node group. When you create a cluster, the first application server you add to the cluster defines the node group within which all of the other cluster members must reside. All other cluster members you add to the cluster can only be on nodes that are members of this same node group. When you create a new cluster member in the administrative console, you are allowed to create the application server on a node that is a member of the node group for that cluster only.

Nodes can be members of multiple node groups. If the first cluster member you add to a cluster has multiple node groups defined, the system automatically chooses the node group that bounds the cluster. You can change the node group by modifying the cluster settings. Use the Server cluster settings page to change the node group.

Clusters and core groups

In a high availability environment, a group of clusters can be defined as a core group. All of the application servers defined as a member of one of the clusters included in a core group are automatically members of that core group. Individual application servers that are not members of a cluster can also be defined as a member of a core group. The use of core groups enables WebSphere® Application Server to provide high availability for applications that must always be available to end users. You can also configure core groups to communicate with each other using the core group bridge. The core groups can communicate within the same cell or across cells.

Cluster members

You can improve system performance if you configure each cluster member, such that each of their components are dynamically started as they are needed instead of letting all of these components automatically start when the cluster member starts. Selecting this option can improve cluster startup time, and reduce the memory footprint of the cluster members. Starting components as they are needed is most effective if all of the applications that are deployed on the cluster are of the same type. For example, using this option works better if all of your applications are web applications that use servlets, and JavaServer Pages (JSP). This option works less effectively if your applications use servlets, JSPs and Enterprise JavaBeans (EJB).

避免困難 避免困難: 如果您有在某環境中執行的用戶端:
  • 含有 Java™ 小型用戶端的環境,
  • 在多個 Cell 間遞送要求,或
  • 在包含舊版產品之節點的單一 Cell 中遞送要求,
它們可能會突然遇到目標叢集之成員相關埠資訊已過期的狀況。

當所有叢集成員都有動態埠,而且在沒有傳送任何要求的期間重新啟動時,最常發生此狀況。在這個狀況下,用戶端程序最終會試圖遞送至節點代理程式,以接收叢集成員的新埠資料,然後利用新的埠資料,遞送回叢集成員。

如果發生導致用戶端無法與節點代理程式通訊,或新的埠資料無法在叢集成員與節點代理程式之間傳播的任何問題,則可能會發生用戶端要求失敗。 在某些情況下,這些失敗是暫時的。在其他情況下,您需要重新啟動一或多個處理程序才能解決失敗。

如果要避開這些情況下可能發生的用戶端遞送問題,您可以對叢集成員配置靜態埠。有了靜態埠,埠資料就不會因為用戶端程序取得叢集成員相關資訊而變更。即使重新啟動叢集成員,或是程序之間有通訊或資料傳送問題,用戶端的埠資料仍然有效。這項補救措施不一定可以解決基礎通訊或資料傳送問題,但可以移除非預期或不穩定用戶端遞送決策的症狀。

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時間戳記圖示 前次更新: July 9, 2016 11:21
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