Add Docs for R2DBC Channel adapters

src/reference/asciidoc/r2dbc.adoc

@@ -26,9 +26,95 @@ compile "org.springframework.integration:spring-integration-r2dbc:{project-versi
 [[r2dbc-inbound-channel-adapter]]
 === R2DBC Inbound Channel Adapter
 
-TBD
+The `R2dbcMessageSource` is a pollable `MessageSource` implementation based on the `R2dbcEntityOperations` and produces messages with a `Flux` or `Mono` as a payload for data fetched from database according an `expectSingleResult` option.
+The query to `SELECT` can be statically provided or based on a SpEL expression which is evaluated on every `receive()` call.
+The `R2dbcMessageSource.SelectCreator` is present as a root object for evaluation context to allow to use a `StatementMapper.SelectSpec` fluent API.
+By default this channel adapter maps records from the select into a `LinkedCaseInsensitiveMap` instances.
+It can be customized providing a `payloadType` options which is used underneath by the `EntityRowMapper` based on the `this.r2dbcEntityOperations.getConverter()`.
+The `updateSql` is optional and used to mark read records in the databased for skipping from the subsequent polls.
+The `UPDATE` operation can be supplied with a `BiFunction<DatabaseClient.GenericExecuteSpec, ?, DatabaseClient.GenericExecuteSpec>` to bind values into an `UPDATE` based on records in the `SELECT` result.
+
+A typical configuration for this channel adapter might look like this:
+
+====
+[source, java]
+----
+@Bean
+@InboundChannelAdapter("fromR2dbcChannel")
+public R2dbcMessageSource r2dbcMessageSourceSelectMany() {
+    R2dbcMessageSource r2dbcMessageSource = new R2dbcMessageSource(this.r2dbcEntityTemplate,
+            "SELECT * FROM person WHERE name='Name'");
+    r2dbcMessageSource.setPayloadType(Person.class);
+    r2dbcMessageSource.setUpdateSql("UPDATE Person SET name='SomeOtherName' WHERE id = :id");
+    r2dbcMessageSource.setBindFunction(
+				(DatabaseClient.GenericExecuteSpec bindSpec, Person o) -> bindSpec.bind("id", o.getId()));}
+    return r2dbcMessageSource;
+}
+----
+====
+
+With Java DSL a configuration for this channel adapter is like this:
+
+====
+[source, java]
+----
+@Bean
+IntegrationFlow r2dbcDslFlow(R2dbcEntityTemplate r2dbcEntityTemplate) {
+    return IntegrationFlows
+        .from(R2dbc.inboundChannelAdapter(r2dbcEntityTemplate,
+            (selectCreator) ->
+                    selectCreator.createSelect("person")
+                        .withProjection("*")
+                        .withCriteria(Criteria.where("id").is(1)))
+                    .expectSingleResult(true)
+                    .payloadType(Person.class)
+                    .updateSql("UPDATE Person SET id='2' where id = :id")
+                    .bindFunction((DatabaseClient.GenericExecuteSpec bindSpec, Person o) ->
+                            bindSpec.bind("id", o.getId())),
+            e -> e.poller(p -> p.fixedDelay(100)))
+        .<Mono<?>>handle((p, h) -> p, e -> e.async(true))
+        .channel(MessageChannels.flux())
+        .get();
+}
+----
+====
 
 [[r2dbc-outbound-channel-adapter]]
 === R2DBC Outbound Channel Adapter
 
-TBD
+The `R2dbcMessageHandler` is a `ReactiveMessageHandler` implementation to perform an `INSERT` (default), `UPDATE` or `DELETE` query in database using a provided `R2dbcEntityOperations`.
+The `R2dbcMessageHandler.Type` can be configured statically or via a SpEL expression against request message.
+The query to execute can be based on the `tableName`, `values` and `criteria` expression options or (if `tableName` is not provided) the whole message payload is treated as an `org.springframework.data.relational.core.mapping.Table` entity to perform SQL against.
+The package `org.springframework.data.relational.core.query` is registered as an import into a SpEL evaluation context for direct access to the `Criteria` fluent API which is used for `UPDATE` and `DELETE` queries.
+The `valuesExpression` is used in the `INSERT` and `UPDATE` and must be evaluated to the `Map` for column-value pairs to perform a change in the target table against request message.
+
+A typical configuration for this channel adapter might look like this:
+
+====
+[source, java]
+----
+@Bean
+@ServiceActivator(inputChannel = "toR2dbcChannel")
+public R2dbcMessageHandler r2dbcMessageHandler(R2dbcEntityTemplate r2dbcEntityTemplate) {
+    R2dbcMessageHandler messageHandler = new R2dbcMessageHandler(r2dbcEntityTemplate)
+    messageHandler.setValuesExpression(new FunctionExpression<Message<?>>(Message::getPayload));
+    messageHandler.setQueryType(R2dbcMessageHandler.Type.UPDATE);
+    messageHandler.setCriteriaExpression(
+        EXPRESSION_PARSER.parseExpression("T(Criteria).where('id).is(headers.personId)));
+    return messageHandler;
+}
+----
+====
+
+With Java DSL a configuration for this channel adapter is like this:
+
+====
+[source, java]
+----
+.handle(R2dbc.outboundChannelAdapter(r2dbcEntityTemplate)
+        .queryType(R2dbcMessageHandler.Type.UPDATE)
+        .tableNameExpression("payload.class.simpleName")
+        .criteria((message) -> Criteria.where("id").is(message.getHeaders().get("personId")))
+        .values("{age:36}"))
+----
+====