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## 2.2. Features

Here are the current features and benefits of PojoCache:

- Fine-grained replication. The replication modes supported are the same as that of Core Cache: `LOCAL`, `REPL_SYNC`, `REPL_ASYNC`, `INVALIDATION_SYNC`, and `INVALIDATION_ASYNC` (see the main JBoss Cache reference documentation for details). The replication level is fine-grained and is performed automatically once the POJO is mapped into the internal cache store. When a POJO field is updated, a replication request will be sent out only to the key corresponding to that modified attribute (instead of the whole object). This can have a potential performance boost during the replication process; e.g., updating a single key in a big HashMap will only replicate the single field instead of the whole map!

- Transactions. All attached objects participate in a user transaction context. If a rollback occurs, the previous internal field state of the object will be restored:

      POJO p = new POJO();
      p.setName("old value");
      pojoCache.attach("id", p);
      tx.begin(); // start a user transaction
      p.setName("some pojo");
      tx.rollback(); // this will cause the rollback
      p.getName(); // is "old value"

  In addition, operations under a transaction is batched. That is, the update is not performed until the `commit` phase. Further, if replication is enabled, other nodes will not see the changes until the transaction has completed successfully.

- Passivation. POJO Cache supports the same passivation provided by Core Cache. When a node mapped by POJO Cache has reached a configured threshold, it is evicted from memory and stored using a cache loader. When the node is accessed again, it will be retrieved from the cache loader and put into memory. The configuration parameters are the same as those of the Cache counterpart. To configure the passivation, you will need to configure both the eviction policy and cache loader.

- Object cache by reachability, i.e., recursive object mapping into the cache store. On attach, `POJO Cache` will attach all referenced objects as well. This feature is explained in more detail later.

- Natural Object Relationships. Java references are preserved as they were written. That is, a user does not need to declare any object relationship (e.g., one-to-one, or one-to-many) to use the cache.

- Object Identity. Object identity is preserved. Not only can a cached object be compared using `equals()`, but the comparison operator, `==`, can be used as well. For example, an object such as `Address` may be multiple referenced by two `Person`s (e.g., `joe` and `mary`). The objects retrieved from `joe.getAddress()` and `mary.getAddress()` should be identicali, when when retrieved from a different node in the cluster then that which attached them.

- Inheritance. POJO Cache preserves the inheritance hierarchy of any object in the cache. For example, if a `Student` class inherits from a Person class, once a `Student` object is mapped to POJO Cache (e.g., `attach` call), the fields in the base class `Person` are mapped as well.

- Collections. Java Collection types (e.g. List, Set, and Map) are transparently mapped using Java proxies. Details are described later.

- Annotation based. Starting from release 2.0, JDK 5 annotations are used to indicate that an object should be instrumented for use under POJO Cache (once attached).

- Transparent. Once a POJO is attached to the cache, subsequent object model changes are transparently handled. No further API calls are required.

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