#418 - Document optimistic locking using @Version.

Author: mp911de

src/main/asciidoc/reference/r2dbc-repositories.adoc

@@ -317,6 +317,7 @@ The following table describes the strategies that Spring Data R2DBC offers for d
 If the identifier property is `null`, then the entity is assumed to be new. Otherwise, it is assumed exist in the datbase.
 |Implementing `Persistable`             |If an entity implements `Persistable`, Spring Data R2DBC delegates the new detection to the `isNew(…)` method of the entity.
 See the link:$$https://docs.spring.io/spring-data/data-commons/docs/current/api/index.html?org/springframework/data/domain/Persistable.html$$[Javadoc] for details.
+|Optimistic Locking through `@Version`  | If an entity uses Optimistic Locking by (version property annotated with `@Version`), Spring Data R2DBC checks if the entity is new by inspecting the version property whether its value corresponds with Java's default initialization value. That is `0` for primitive types and `null` for wrapper types.
 |Implementing `EntityInformation`       |You can customize the `EntityInformation` abstraction used in `SimpleR2dbcRepository`  by creating a subclass of `R2dbcRepositoryFactory` and overriding `getEntityInformation(…)`.
 You then have to register the custom implementation of `R2dbcRepositoryFactory` as a Spring bean.
 Note that this should rarely be necessary. See the link:{spring-data-r2dbc-javadoc}/org/springframework/data/r2dbc/repository/support/R2dbcRepositoryFactory.html[Javadoc] for details.
@@ -334,6 +335,44 @@ One important constraint is that, after saving an entity, the entity must not be
 Note that whether an entity is new is part of the entity's state.
 With auto-increment columns, this happens automatically, because the ID gets set by Spring Data with the value from the ID column.
 
+[[r2dbc.optimistic-locking]]
+=== Optimistic Locking
+
+The `@Version` annotation provides syntax similar to that of JPA in the context of R2DBC and makes sure updates are only applied to documents with a matching version.
+Therefore, the actual value of the version property is added to the update query in such a way that the update does not have any effect if another operation altered the document in the meantime. In that case, an `OptimisticLockingFailureException` is thrown.
+The following example shows these features:
+
+====
+[source,java]
+----
+@Table
+class Person {
+
+  @Id Long id;
+  String firstname;
+  String lastname;
+  @Version Long version;
+}
+
+R2dbcEntityTemplate template = …;
+
+Mono<Person> daenerys = template.insert(new Person("Daenerys"));                      <1>
+
+Person other = template.select(Person.class)
+				.matching(query(where("id").is(daenerys.getId())))
+				.first().block();                                                     <2>
+
+daenerys.setLastname("Targaryen");
+template.save(daenerys);                                                              <3>
+
+template.save(other).subscribe(); // emits OptimisticLockingFailureException          <4>
+----
+<1> Initially insert row. `version` is set to `0`.
+<2> Load the just inserted row. `version` is still `0`.
+<3> Update the row with `version = 0`. Set the `lastname` and bump `version` to `1`.
+<4> Try to update the previously loaded document that still has `version = 0`. The operation fails with an `OptimisticLockingFailureException`, as the current `version` is `1`.
+====
+
 :projection-collection: Flux
 include::../{spring-data-commons-docs}/repository-projections.adoc[leveloffset=+2]