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Oracle® Coherence Developer's Guide
Release 3.7

Part Number E18677-01
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5 Introduction to Coherence Clusters

The following sections are included in this chapter:

5.1 Cluster Overview

A Coherence cluster is a collection of JVM processes. At run time, JVM processes that run Coherence automatically join and cluster. JVMs that join a cluster are called cluster members or cluster nodes. Cluster members communicate using Tangosol Cluster Management Protocol (TCMP). Cluster members use TCMP for both multicast communication (broadcast) and unicast communication (point-to-point communication).

A cluster contains services that are shared by all cluster members. The services include connectivity services (such as the Cluster service), cache services (such as the Distributed Cache service), and processing services (such as the invocation service). Each cluster member can provide and consume such services. The first cluster member is referred to as the senior member and typically starts the core services that are required to create the cluster. If the senior member of the cluster is shutdown, another cluster member assumes the senior member role.

5.2 Understanding TCMP

TCMP is an IP-based protocol that is used to discover cluster members, manage the cluster, provision services, and transmit data. TCMP uses a combination of UDP/IP multicast, UDP/IP unicast and TCP/IP as follows:

Protocol Reliability

The TCMP protocol provides fully reliable, in-order delivery of all messages. Since the underlying UDP/IP protocol does not provide for either reliable or in-order delivery, TCMP uses a queued, fully asynchronous ACK- and NACK-based mechanism for reliable delivery of messages, with unique integral identity for guaranteed ordering of messages.

Protocol Resource Utilization

The TCMP protocol requires only three UDP/IP sockets (one multicast, two unicast) and six threads per JVM, regardless of the cluster size. This is a key element in the scalability of Coherence, in that regardless of the number of servers, each node in the cluster can still communicate either point-to-point or with collections of cluster members without requiring additional network connections.

The optional TCP/IP ring uses a few additional TCP/IP sockets, and a total of one additional thread.

Protocol Tunability

The TCMP protocol is very tunable to take advantage of specific network topologies, or to add tolerance for low-bandwidth and high-latency segments in a geographically distributed cluster. Coherence comes with a pre-set configuration. Some TCMP attributes are dynamically self-configuring at run time, but can also be overridden and locked down for deployment purposes.

5.3 Understanding Cluster Services

Coherence functionality is based on the concept of cluster services. Each cluster node can participate in (which implies both the ability to provide and to consume) any number of named services. These named services may exist, which is to say that they may be running on one or more other cluster nodes, or a cluster node can register new named services. Each named service has a service name that uniquely identifies the service within the cluster, and a service type, which defines what the service can do. There may be multiple named instances of each service type (other than the root Cluster service). By way of analogy, a service instance corresponds roughly to a database schema, and for data services, a hosted named cache corresponds roughly to a database table. While services can be added, many applications only require the default set of services shipped with Coherence. There are several service types that are supported by Coherence.

Connectivity Services

Processing Services

Data Services

Regarding resources, a clustered service typically uses one daemon thread, and optionally has a thread pool that can be configured to provide the service with additional processing bandwidth. For example, the invocation service and the distributed cache service both fully support thread pooling to accelerate database load operations, parallel distributed queries, and agent invocations.

It is important to note that these are only the basic clustered services, and not the full set of types of caches provided by Coherence. By combining clustered services with cache features such as backing maps and overflow maps, Coherence can provide an extremely flexible, configurable, and powerful set of options for clustered applications. For example, the Near Cache functionality uses a Distributed Cache as one of its components.

Within a cache service, there exists any number of named caches. A named cache provides the standard JCache API, which is based on the Java collections API for key-value pairs, known as java.util.Map. The Map interface is the same API that is implemented by the Java Hashtable class, for example.