[[rabbit-receiving-batch]]
= Receiving Batched Messages

With the RabbitMQ binder, there are two types of batches handled by consumer bindings:

[[batches-created-by-producers]]
== Batches Created by Producers

Normally, if a producer binding has `batch-enabled=true` (see xref:rabbit/rabbit_overview/prod-props.adoc[Rabbit Producer Properties]), or a message is created by a `BatchingRabbitTemplate`, elements of the batch are returned as individual calls to the listener method.
Starting with version 3.0, any such batch can be presented as a `List<?>` to the listener method if `spring.cloud.stream.bindings.<name>.consumer.batch-mode` is set to `true`.

[[consumer-side-batching]]
== Consumer-side Batching

Starting with version 3.1, the consumer can be configured to assemble multiple inbound messages into a batch which is presented to the application as a `List<?>` of converted payloads.
The following simple application demonstrates how to use this technique:

[source, properties]
----
spring.cloud.stream.bindings.input-in-0.group=someGroup

spring.cloud.stream.bindings.input-in-0.consumer.batch-mode=true

spring.cloud.stream.rabbit.bindings.input-in-0.consumer.enable-batching=true
spring.cloud.stream.rabbit.bindings.input-in-0.consumer.batch-size=10
spring.cloud.stream.rabbit.bindings.input-in-0.consumer.receive-timeout=200
----

[source, java]
----
@SpringBootApplication
public class Application {

	public static void main(String[] args) {
		SpringApplication.run(Application.class, args);
	}

	@Bean
	Consumer<List<Thing>> input() {
		return list -> {
			System.out.println("Received " + list.size());
			list.forEach(thing -> {
				System.out.println(thing);

				// ...

			});
		};
	}

	@Bean
	public ApplicationRunner runner(RabbitTemplate template) {
		return args -> {
			template.convertAndSend("input-in-0.someGroup", "{\"field\":\"value1\"}");
			template.convertAndSend("input-in-0.someGroup", "{\"field\":\"value2\"}");
		};
	}

	public static class Thing {

		private String field;

		public Thing() {
		}

		public Thing(String field) {
			this.field = field;
		}

		public String getField() {
			return this.field;
		}

		public void setField(String field) {
			this.field = field;
		}

		@Override
		public String toString() {
			return "Thing [field=" + this.field + "]";
		}

	}

}
----

[source]
----
Received 2
Thing [field=value1]
Thing [field=value2]
----

The number of messages in a batch is specified by the `batch-size` and `receive-timeout` properties; if the `receive-timeout` elapses with no new messages, a "short" batch is delivered.

IMPORTANT: Consumer-side batching is only supported with `container-type=simple` (the default).

If you wish to examine headers of consumer-side batched messages, you should consume `Message<List<?>>`; the headers are a `List<Map<String, Object>>` in a header `AmqpInboundChannelAdapter.CONSOLIDATED_HEADERS`, with the headers for each payload element in the corresponding index.
Again, here is a simple example:

[source, java]
----
@SpringBootApplication
public class Application {

	public static void main(String[] args) {
		SpringApplication.run(Application.class, args);
	}

	@Bean
	Consumer<Message<List<Thing>>> input() {
		return msg -> {
			List<Thing> things = msg.getPayload();
			System.out.println("Received " + things.size());
			@SuppressWarnings("unchecked")
			List<Map<String, Object>> headers =
					(List<Map<String, Object>>) msg.getHeaders().get(AmqpInboundChannelAdapter.CONSOLIDATED_HEADERS);
			for (int i = 0; i < things.size(); i++) {
				System.out.println(things.get(i) + " myHeader=" + headers.get(i).get("myHeader"));

				// ...

			}
		};
	}

	@Bean
	public ApplicationRunner runner(RabbitTemplate template) {
		return args -> {
			template.convertAndSend("input-in-0.someGroup", "{\"field\":\"value1\"}", msg -> {
				msg.getMessageProperties().setHeader("myHeader", "headerValue1");
				return msg;
			});
			template.convertAndSend("input-in-0.someGroup", "{\"field\":\"value2\"}", msg -> {
				msg.getMessageProperties().setHeader("myHeader", "headerValue2");
				return msg;
			});
		};
	}

	public static class Thing {

		private String field;

		public Thing() {
		}

		public Thing(String field) {
			this.field = field;
		}

		public String getfield() {
			return this.field;
		}

		public void setfield(String field) {
			this.field = field;
		}

		@Override
		public String toString() {
			return "Thing [field=" + this.field + "]";
		}

	}

}
----

[source]
----
Received 2
Thing [field=value1] myHeader=headerValue1
Thing [field=value2] myHeader=headerValue2
----