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Leading-Edge Java
Dynamic Clustering with Jini Technology
by Frank Sommers
January 31, 2006

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Application Clustering

Service interfaces on a cluster can define not only lower-level services, such as a filesystem, but also more specialized services in support of an application domain. An example is Boomi's cluster architecture that defines a series of services supporting dynamic system integration. A fundamental problem of system integration is the need to access various data sources, accept input from those data sources, translate that input into another data format, and then present the results to employees or systems that can act on those results.

Boomi's cluster architecture breaks up system integration processes into discrete tasks, such as those responsible for data input, rules execution, data transformation, or providing output. Services representing those tasks can then be distributed on cluster nodes and executed in parallel.

Such distribution yields improved reliability. For example, implementations of a service that transform an incoming purchase order into an email and inserts that email in a mail queue, can be deployed on multiple cluster nodes. Since instances of that data transformation service are interchangeable, any of those service instances can handle an incoming request. Because that data transformation service is offered by a collection of several redundant transformation service instances, the failure of a single cluster node does not also cause the data transformation service on the cluster to fail.

As well, cluster-based distribution of redundant services provides load balancing, since incoming messages, such as purchase orders, could be handled by a service instance on the least utilized node. Finally, cluster-based distribution of system integration tasks provides almost linear scalability: adding twice as many cluster nodes results in twice the throughput, not considering added communication overhead.

Distributing such services on a cluster, however, introduces the problem of locating instances of a service running on certain cluster nodes and accessing those services from other nodes. If such services would have to be managed on the level of individual nodes, the scalability and reliability benefits of the cluster could be obtained only at the cost of increased system administration. In addition, if dealing with node failure required redistribution of workload by a system administrator, operating the cluster would prove a costly enterprise.

To automate such cluster management chores, Boomi turned to Jini technology. As a dynamic networking tool, Jini technology aids cluster management by its ability to expose implementations of Java interfaces as services on a cluster network, and by enabling clients to locate a service based on the service's type.

Type-Based Service Lookup with Jini Technology

Jini technology makes type-based service definition and lookup explicit by requiring that Java interface type define a network service: A Jini service implements one or more Java interfaces, and the Jini service lookup infrastructure enables network clients to find services based on what Java interfaces a service implements. Jini technology's dynamic service lookup capability reduces cluster administration chores. Instead of looking in configuration files to locate services on the network, clients can use Jini lookup services to locate services dynamically.

To appreciate that benefit, consider the find command present on most Unix systems. find takes a directory path and a file pattern as arguments, and finds files with names matching that pattern on the specified directory tree. If a file system specified to find was not mounted in advance on the node where find is executed, the command will, of course, not search for files on that unmounted system.

But a possible Jini version of find would not have to contend with that limitation. Jini technology's service lookup capability would allow a Jini version of find to dynamically locate file system services on the network, invoke those services, and attempt to search those remote file systems as well.

Jini lookup services serve as central rendezvous points for Jini service lookup, and are themselves Jini services. Clients use the Java interface type, in addition to optional service attribute values, to find Jini services registered in lookup services on a cluster. Implementations of those interfaces, in turn, must register a proxy object with lookup services.

A Jini service's proxy object plays a crucial role in how a client invokes that service. Such proxies are serialized Java objects that implement the service's Java interface. Thus, when a client locates a matching service registration in a Jini lookup service, the client retrieves that service's proxy object, and then invokes service methods on the proxy. The proxy may implement the functionality completely in the proxy, or interact with a remote service implementation via any communication protocol possible between proxy and remote server. As a result, the client does not have to care about what communication protocol to use with a service, because such protocols are hidden away into the service's proxy.

Figure 2: The role of the Jini service proxy

A fundamental assumption of a Jini system is that the network is not reliable. One way a Jini system deals with an unreliable network is by leasing references to remote network resources to clients. A lease places a time limit on a client's use of a network's resources, such as a Jini service proxy, or the Jini lookup service. Lookup service registrations are also time-limited: A service must renew the lease of its proxy in a lookup service, or the lookup service will purge that proxy.

The lease-based access to resources defined in the Jini architecture fits well with the dynamic nature of a cluster. Since cluster nodes can fail at any time, a failed node's service registrations will be purged by lookup services. As well, introducing a new cluster node involves merely installing and starting new service implementations on the new node: Those services will register their proxies with the cluster's lookup services, making the new node available to clients without further administrative work.

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