6. Flushing

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Flushing is the process of synchronizing the state of the persistence context with the underlying database.
The `EntityManager` and the Hibernate `Session` expose a set of methods, through which the application developer can change the persistent state of an entity.

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The persistence context acts as a transactional write-behind cache, queuing any entity state change.
Like any write-behind cache, changes are first applied in-memory and synchronized with the database during flush time.
The flush operation takes every entity state change and translates it to an `INSERT`, `UPDATE` or `DELETE` statement.

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Because DML statements are grouped together, Hibernate can apply batching transparently.
See the [Batching chapter](#batch) for more information.

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The flushing strategy is given by the [`flushMode`](https://docs.jboss.org/hibernate/orm/5.2/javadocs/org/hibernate/Session.html#getFlushMode--) of the current running Hibernate `Session`.
Although JPA defines only two flushing strategies ([`AUTO`](https://docs.oracle.com/javaee/7/api/javax/persistence/FlushModeType.html#AUTO) and [`COMMIT`](https://docs.oracle.com/javaee/7/api/javax/persistence/FlushModeType.html#COMMIT)),
Hibernate has a much broader spectrum of flush types:

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<dt class="hdlist1">ALWAYS</dt>
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Flushes the `Session` before every query.

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<dt class="hdlist1">AUTO</dt>
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This is the default mode and it flushes the `Session` only if necessary.

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<dt class="hdlist1">COMMIT</dt>
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The `Session` tries to delay the flush until the current `Transaction` is committed, although it might flush prematurely too.

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<dt class="hdlist1">MANUAL</dt>
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The `Session` flushing is delegated to the application, which must call `Session.flush()` explicitly in order to apply the persistence context changes.

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