- The Motivations for Clustered Infrastructure Solutions: Scalability and High Availability
- Understanding WebLogic Clusters
- Understanding How WebLogic Servers Communicate within a Cluster
- Designing the Architecture of a WebLogic Cluster
- Creating and Configuring a WebLogic Cluster
- Implementing a Load-Balancing Mechanism for Your Clustered Servlets and JSPs
- Implementing a Failover Mechanism for Your Clustered Servlets and JSPs
- Clustering Enterprise JavaBeans
Designing the Architecture of a WebLogic Cluster
To implement a WebLogic cluster that meets the high-availability and scalability requirements of your J2EE application, you must understand the elements that constitute a WebLogic clustering environment as well as the benefits of one type of clustering design over another. The following section addresses these two topics and brings clarity to how you can architecturally design your WebLogic cluster.
Understanding a Typical Clustering Environment
If you are new to the concept of clustering, it is imperative for you to understand what constitutes a typical WebLogic clustering environment before you even begin to design an architecture for your WebLogic cluster.
As illustrated in Figure 25.4, a typical WebLogic cluster environment can consist of the following logical software tiers and services, which do not necessarily imply a division in hardware in the overall clustering architecture.Figure 25.4 The typical software tiers and services that can constitute a WebLogic cluster environment.
The Load Balancer
A load balancer is a software or hardware mechanism that can distribute client connection requests to the servers in a WebLogic cluster. The objective of a load balancer is to ensure all clustered servers service incoming requests in the most optimal manner, which typically implies they are all operating at near full-processing capacity.
The load balancer serves as the single point of entry for a WebLogic cluster.
The distribution criteria of a load balancer are based on algorithms applied to the inbound network traffic. The following are some examples:
Round-robinSends each subsequent client request to the next server in the cluster
Least number of usersRedirects client requests to the server that has the least number of users
Least amount of trafficRedirects client requests to the server with the least amount of network traffic
The exact available algorithm depends on the capabilities of your load balancer. For example, the WebLogic Server HttpClusterServlet proxy supports only a round-robin algorithm for distributing requests, whereas dedicated load-balancing appliances offer more sophisticated load-balancing algorithms.
The Web Proxy Server
If you have an existing Web tier composed of Netscape, Microsoft, or Apache Web servers, you can leverage this software and hardware layer with a proxy plug-in supplied with WebLogic Server to serve only the static HTTP content of your J2EE application. Dynamic content requests to servlets or JSPs are delegated via the Web Server's proxy plug-in to the servers in the WebLogic cluster that form the Presentation tier. You can also use WebLogic Server as a Web Proxy server by deploying the HttpClusterServlet.
Using a Web Proxyserver does have its advantages because you can leverage the existing infrastructure as well as any firewall policies for the Web tier, which prevents direct connections to the WebLogic cluster. However, having a separate Web tier does involve extra setup and administration activities, and the load-balancing capabilities are constrained to a round-robin type strategy. Also, if clients are primarily accessing dynamic content, having a Web tier will increase the number of network connections required to access the WebLogic cluster, which will affect the latency of a client's response.
As illustrated in Figure 25.4, WebLogic Server can be divided into two logical tiers, as follows:
Presentation tierThis tier is, in essence, the Web container of WebLogic Server and is responsible for serving the dynamic content of a J2EE application via servlets and JavaServer Pages.
Object tierThis tier is, in essence, the EJB container of WebLogic Server, which provides an execution environment for the business logic in a J2EE application via Enterprise JavaBeans.
If you do not use a Web Proxy server, the Presentation tier also includes the Web tier because it will serve static and dynamic content.
The DeMilitarized Zone (DMZ)
The DeMilitarized Zone, or DMZ as it is known in the network community, is a perimeter network that is used to protect a specific trusted network environment from direct exposure to an untrusted (external network) environment. To define a DMZ, you first must identify the network environment you need to protect and then identify all the entrance points (front and back doors) to that network. In most cases, the entrance point is a Web server that is connected to the Internet or an untrusted intranet. In such a scenario, the Web server exists in the DMZ.
As you can see in Figure 25.4, you can secure a DMZ by using two firewalls. The one in front of the DMZ should be between the external network (Internet) and your DMZ, which is typically a group of Web servers. The second firewall should be between the DMZ and the internal network.
A firewall is typically a hardware appliance that is used to institute a secure access policy for crossing a network's perimeterfor example, allowing only authorized traffic to pass from an untrusted network environment to a DMZ.
Recommended WebLogic Clustering Architectures
Generally, whenever an architectural discussion on how to design a WebLogic cluster occurs, the primary topics that influence the design directly relate to the way the cluster can guarantee the hosted application is very scalable and highly available. However, a WebLogic cluster in conjunction with a load-balancing mechanism, by default, provides a very scalable solution; WebLogic Servers can be added and removed from the cluster transparently to the clients.
Even though clustering is synonymous with both high availability and scalability, your architectural thoughts should be primarily aligned with ensuring a performance-oriented, highly available clustering solution. The following sections discuss two such clustering architectures you can consider: the combined-tier and multi-tier architectures, which differ based on the physical location of the presentation and object tiers.
The Combined-Tier Architecture
The combined-tier architecture, as illustrated in Figure 25.5, combines the Presentation and Object tiers of a J2EE application into a single WebLogic cluster. If a Web Proxy server is not used, the Web tier is also introduced into the cluster.Figure 25.5 The combined-tier WebLogic cluster architecture.
This clustering architecture meets the high-availability and scalability requirements for most J2EE applications, with the addition of the following advantages:
Ease of administration of the WebLogic cluster because a J2EE application needs to be deployed/undeployed only to and from the same WebLogic cluster using the Administration Console.
Optimized local method calls between the Presentation and Object tier. WebLogic Server uses a collocation strategy to optimize method calls to clustered objects (EJB or RMI classes), which prioritizes method calls to objects that are hosted on the same WebLogic Server instance as their replica-aware stub and avoids incurring any additional network overhead. Because all application objects are available locally to each WebLogic Server in the same cluster, the combined-tier architecture presents the best performance for J2EE applications where there is a high frequency of method invocations in the Object tier by the Presentation tier.
The limitation of the combined-tier architecture is that it does not provide an opportunity for introducing load balancing between the Presentation and Object tiers, which may be necessary if the method calls to the Object tier become unbalanced between the servers in the cluster. In such a scenario, the appropriate approach would be to split the Presentation and Object tiers of the J2EE application onto separate physical clusters as in the multi-tier architecture.
The Multi-Tier Architecture
The primary purpose of the multi-tier architecture is to introduce load balancing between the Presentation and Object tiers of a clustered J2EE application. As illustrated in Figure 25.6, this load balancing is achieved by deploying the Presentation and Objects tiers of a J2EE application onto two different clustered servers.
By hosting the Object tier on a dedicated clustered server, WebLogic Servers in the Presentation tier cannot use the collocation optimization strategy for accessing clustered objects.
As depicted in Figure 25.6, however, the Presentation tier does gain the capability to load-balance each method call to a clustered object. In the multi-tier architecture, a servlet acts as the client to the clustered objects. When requests are made to invoke an EJB, the servlet performs a JNDI lookup for the EJB and obtains a replica-aware stub for that bean, which contains the addresses of all servers that host the bean, as well as the load-balancing logic for accessing bean replicas.
EJB replica-aware stubs and EJB home load algorithms are specified using elements of the EJB deployment descriptor.
The following are additional benefits for using the multi-tier architecture:
You have more options for load balancing the entire WebLogic cluster without having to scale the Presentation and Object tiers in tandem. For example, if you have a higher frequency of client access to the Presentation tier than the Object tier, you can increase (scale) the number of WebLogic Servers hosting the Presentation tier to a smaller number of Object tier WebLogic Servers.
Two clusters present a higher availability option for a J2EE application than just one, as in the case of the combined-tier architecture.
Although the multi-tier clustering architecture provides greater scalability and high-availability options, it does impose the following disadvantages when compared to the combined-tier architecture:
Whenever you have to deploy and manage an application over two clusters, it requires twice as much effort as just using one cluster.
There is a network overhead for all method calls to clustered objects. You also utilize more IP sockets in the Presentation tier to replicate sessions and connect to WebLogic Servers that host the clustered objects.
If you create a DMZ for the Presentation tier by placing a firewall between the Presentation and Object tiers, you will need to bind all WebLogic Servers in the Object tier to public DNS names as opposed to IP addresses because IP address translation problems can occur between the two tiers.
You can use the Administration Console to set the ExternalDNSName attribute for a WebLogic Server instance, which defines its external DNS name.