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Oracle TopLink Developer's Guide 10g Release 3 (10.1.3) B13593-01 |
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# Understanding Cache Coordination

As Figure 90-2 shows, cache coordination is a session feature that allows multiple, possibly distributed, instances of a session to broadcast object changes among each other so that each session's cache is either kept up-to-date or notified that the cache must update an object from the data source the next time it is read.

Note: You cannot use isolated client sessions (see "Isolated Client Sessions") with cache coordination.

Figure 90-2 Cache Coordination

Description of "Figure 90-2 Cache Coordination"

When sessions are distributed, that is, when an application contains multiple sessions (in the same JVM, in multiple JVMs, possibly on different servers), as long as the servers hosting the sessions are interconnected on the network, sessions can participate in cache coordination. Coordinated cache types that require discovery services also require the servers to support User Datagram Protocol (UDP) communication and multicast configuration (for more information, see "Coordinated Cache Architecture").

This section describes the following:

- When to use Cache Coordination

- Coordinated Cache Architecture

- Coordinated Cache Types

- Custom Coordinated Cache

For more information, see "Configuring a Coordinated Cache".

## When to use Cache Coordination

Cache coordination can enhance performance and reduce the likelihood of stale data for applications that have the following characteristics:

- Changes are all being performed by the same Java application operating with multiple, distributed sessions

- Primarily read-based

- Regularly requests and updates the same objects

To maximize performance, avoid cache coordination for applications that do not have these characteristics. For more information about alternatives to cache coordination, see "Cache Optimization".

Cache coordination enhances performance mainly by avoiding data source access.

Cache coordination reduces the occurrence of stale data by increasing the likelihood that distributed caches are kept up-to-date with changes and are notified when one of the distributed caches must update an object from the data source the next time it is read.

Cache coordination reduces the number of optimistic lock exceptions encountered in a distributed architecture, and decreases the number of failed or repeated transactions in an application. However, cache coordination in no way eliminates the need for an effective locking policy. To effectively ensure working with up-to-date data, cache coordination must be used with optimistic or pessimistic locking. Oracle recommends that you use cache coordination with an optimistic locking policy (see "Configuring Locking Policy").

For other options to reduce the likelihood of stale data, see "Handling Stale Data".

## Coordinated Cache Architecture

TopLink provides coordinated cache implementations that perform discovery and message transport services using various technologies including the following:

- Java Message Service (JMS)–See "JMS Coordinated Cache"

- Remote Method Invocation (RMI)–See "RMI Coordinated Cache"

- Common Object Request Broker Architecture (CORBA)–See "CORBA Coordinated Cache"

Regardless of the type of discovery and message transport you choose to use, the following are the principal objects that provide coordinated cache functionality:

- Session

- Descriptor

- Unit of Work

### Session

When you enable a session for change propagation, the session provides discovery and message transport services using either JMS, RMI, or CORBA.

Discovery services ensure that sessions announce themselves to other sessions participating in cache coordination. Discovery services use UDP communication and multicast configuration to monitor sessions as they join and leave the coordinated cache. All coordinated cache types (except JMS) require discovery services.

Message transport services allow the session to broadcast object change notifications to other sessions participating in cache coordination when a unit of work from this session commits a change.

### Descriptor

You can configure how object changes are broadcast on a descriptor-by-descriptor basis. This lets you fine-tune the type of notification to make.

For example, for an object with few attributes, you can configure its descriptor to send object changes. For an object with many attributes, it may be more efficient to configure its descriptor so that the object is flagged as invalid (so that other sessions will know to update the object from the data source the next time it is read).

### Unit of Work

Only changes committed by a unit of work are subject to propagation when cache coordination is enabled. The unit of work computes the appropriate change set based on the descriptor configuration of affected objects.

## Coordinated Cache Types

You can create the following types of coordinated cache:

- JMS Coordinated Cache

- RMI Coordinated Cache

- CORBA Coordinated Cache

### JMS Coordinated Cache

For a JMS coordinated cache, when a particular session's coordinated cache starts up, it uses its JNDI naming service information to locate and create a connection to the JMS server. The coordinated cache is ready when all participating sessions are connected to the same topic on the same JMS server. At this point, sessions can start sending and receiving object change messages. You can then configure all sessions that are participating in the same coordinated cache with the same JMS and JNDI naming service information.

Because you must supply the necessary information to connect to the JMS Topic, a JMS coordinated cache does not use a discovery service.

If you do use cache coordination, Oracle recommends that you use JMS cache coordination: JMS is robust, easy to configure, and provides efficient support for asynchronous change propagation.

For more information, see Chapter 92, "Configuring a JMS Coordinated Cache".

### RMI Coordinated Cache

For an RMI coordinated cache, when a particular session's coordinated cache starts up, the session binds its connection in its naming service (either an RMI registry or JNDI), creates an announcement message (that includes its own naming service information), and broadcasts the announcement to its multicast group (see "Configuring a Multicast Group Address" and "Configuring a Multicast Port"). When a session that belongs to the same multicast group receives this announcement, it uses the naming service information in the announcement message to establish bidirectional connections with the newly announced session's coordinated cache. The coordinated cache is ready when all participating sessions are interconnected in this way, at which point, sessions can start sending and receiving object change messages. You can then configure each session with naming information that identifies the host on which the session is deployed.

If you do use cache coordination, Oracle recommends that you use RMI cache coordination only if you require synchronous change propagation (see "Configuring the Synchronous Change Propagation Mode").

TopLink also supports cache coordination using RMI over the Internet Inter-ORB Protocol (IIOP). An RMI/IIOP coordinated cache uses RMI (and a JNDI naming service) for discovery and message transport services.

Note: If you use an RMI coordinated cache, Oracle recommends that you use RMI/IIOP only if absolutely necessary.

For more information, see Chapter 93, "Configuring an RMI Coordinated Cache".

### CORBA Coordinated Cache

For a CORBA coordinated cache, when a particular session's coordinated cache starts up, the session binds its connection in JNDI, creates an announcement message (that includes its own JNDI naming service information), and broadcasts the announcement to its multicast group (see "Configuring a Multicast Group Address" and "Configuring a Multicast Port"). When a session that belongs to the same multicast group receives this announcement, it uses the naming service information in the announcement message to establish bidirectional connections with the newly announced session's coordinated cache. The coordinated cache is ready when all participating sessions are interconnected in this way, at which point, sessions can start sending and receiving object change messages. You can then configure each session with naming information that identifies the host on which the session is deployed.

Currently, TopLink provides support for the Sun Object Request Broker.

For more information on configuring a CORBA coordinated cache, see Chapter 94, "Configuring a CORBA Coordinated Cache".

## Custom Coordinated Cache

Using the classes in `oracle.toplink.remotecommand` package, you can define your own coordinated cache for custom solutions. For more information, contact your TopLink support representative.

Once you have created the required cache coordination classes, for more information on configuring a user-defined coordinated cache, see Chapter 95, "Configuring a Custom Coordinated Cache".