document parallel processing

docs/utilities/batch.md

@@ -30,6 +30,7 @@ stateDiagram-v2
 
 * Reports batch item failures to reduce number of retries for a record upon errors
 * Simple interface to process each batch record
+* Parallel processing of batches
 * Integrates with Java Events library and the deserialization module 
 * Build your own batch processor by extending primitives
 
@@ -110,16 +111,9 @@ You can use your preferred deployment framework to set the correct configuration
 while the `powertools-batch` module handles generating the response, which simply needs to be returned as the result of
 your Lambda handler.
 
-A complete [Serverless Application Model](https://aws.amazon.com/serverless/sam/) example can be found
-[here](https://github.com/aws-powertools/powertools-lambda-java/tree/main/examples/powertools-examples-batch) covering
-all of the batch sources.
-
-For more information on configuring `ReportBatchItemFailures`, 
-see the details for [SQS](https://docs.aws.amazon.com/lambda/latest/dg/with-sqs.html#services-sqs-batchfailurereporting), 
-[Kinesis](https://docs.aws.amazon.com/lambda/latest/dg/with-kinesis.html#services-kinesis-batchfailurereporting),and
-[DynamoDB Streams](https://docs.aws.amazon.com/lambda/latest/dg/with-ddb.html#services-ddb-batchfailurereporting). 
-
+A complete [Serverless Application Model](https://aws.amazon.com/serverless/sam/) example can be found [here](https://github.com/aws-powertools/powertools-lambda-java/tree/main/examples/powertools-examples-batch) covering all the batch sources.
 
+For more information on configuring `ReportBatchItemFailures`, see the details for [SQS](https://docs.aws.amazon.com/lambda/latest/dg/with-sqs.html#services-sqs-batchfailurereporting), [Kinesis](https://docs.aws.amazon.com/lambda/latest/dg/with-kinesis.html#services-kinesis-batchfailurereporting), and [DynamoDB Streams](https://docs.aws.amazon.com/lambda/latest/dg/with-ddb.html#services-ddb-batchfailurereporting). 
 
 
 !!! note "You do not need any additional IAM permissions to use this utility, except for what each event source requires."
@@ -150,12 +144,10 @@ see the details for [SQS](https://docs.aws.amazon.com/lambda/latest/dg/with-sqs.
         public SQSBatchResponse handleRequest(SQSEvent sqsEvent, Context context) {
             return handler.processBatch(sqsEvent, context);
         }
-
-
+
         private void processMessage(Product p, Context c) {
             // Process the product
         }
-
     }
     ```
 
@@ -276,7 +268,6 @@ see the details for [SQS](https://docs.aws.amazon.com/lambda/latest/dg/with-sqs.
         private void processMessage(Product p, Context c) {
             // process the product
         }
-
     }
     ```
 
@@ -475,6 +466,46 @@ see the details for [SQS](https://docs.aws.amazon.com/lambda/latest/dg/with-sqs.
         }
     ```
 
+## Parallel processing
+You can choose to process batch items in parallel using the `BatchMessageHandler#processBatchInParallel()` 
+instead of `BatchMessageHandler#processBatch()`. Partial batch failure works the same way but items are processed
+in parallel rather than sequentially. 
+
+This feature is available for SQS, Kinesis and DynamoDB Streams but cannot be 
+used with SQS FIFO. In that case, items will be processed sequentially, even with the `processBatchInParallel` method.
+
+!!! warning
+    Note that parallel processing is not always better than sequential processing, 
+    and you should benchmark your code to determine the best approach for your use case. 
+
+!!! info
+    To get more threads available (more vCPUs), you need to increase the amount of memory allocated to your Lambda function.
+    While the exact vCPU allocation isn't published, from observing common patterns customers see an allocation of one vCPU per 1024 MB of memory.
+
+=== "Example with SQS"
+
+    ```java hl_lines="13"
+    public class SqsBatchHandler implements RequestHandler<SQSEvent, SQSBatchResponse> {
+
+        private final BatchMessageHandler<SQSEvent, SQSBatchResponse> handler;
+
+        public SqsBatchHandler() {
+            handler = new BatchMessageHandlerBuilder()
+                    .withSqsBatchHandler()
+                    .buildWithMessageHandler(this::processMessage, Product.class);
+        }
+
+        @Override
+        public SQSBatchResponse handleRequest(SQSEvent sqsEvent, Context context) {
+            return handler.processBatchInParallel(sqsEvent, context);
+        }
+
+        private void processMessage(Product p, Context c) {
+            // Process the product
+        }
+    }
+    ```
+
 
 ## Handling Messages
 
@@ -490,7 +521,7 @@ In general, the deserialized message handler should be used unless you need acce
 
 === "Raw Message Handler"
 
-    ```java
+    ```java hl_lines="4 7"
     public void setup() {
         BatchMessageHandler<SQSEvent, SQSBatchResponse> handler = new BatchMessageHandlerBuilder()
                 .withSqsBatchHandler()
@@ -505,7 +536,7 @@ In general, the deserialized message handler should be used unless you need acce
 
 === "Deserialized Message Handler" 
 
-    ```java
+    ```java hl_lines="4 7"
     public void setup() {
         BatchMessageHandler<SQSEvent, SQSBatchResponse> handler = new BatchMessageHandlerBuilder()
                 .withSqsBatchHandler()
@@ -529,20 +560,20 @@ provide a custom failure handler.
 Handlers can be provided when building the batch processor and are available for all event sources.
 For instance for DynamoDB:
 
-```java
-    BatchMessageHandler<DynamodbEvent, StreamsEventResponse> handler = new BatchMessageHandlerBuilder()
-                .withDynamoDbBatchHandler()
-                .withSuccessHandler((m) -> {
-                    // Success handler receives the raw message
-                    LOGGER.info("Message with sequenceNumber {} was successfully processed",
-                        m.getDynamodb().getSequenceNumber());
-                })
-                .withFailureHandler((m, e) -> {
-                    // Failure handler receives the raw message and the exception thrown.
-                    LOGGER.info("Message with sequenceNumber {} failed to be processed: {}"
-                    , e.getDynamodb().getSequenceNumber(), e);
-                })
-                .buildWithMessageHander(this::processMessage);
+```java hl_lines="3 8"
+BatchMessageHandler<DynamodbEvent, StreamsEventResponse> handler = new BatchMessageHandlerBuilder()
+    .withDynamoDbBatchHandler()
+    .withSuccessHandler((m) -> {
+        // Success handler receives the raw message
+        LOGGER.info("Message with sequenceNumber {} was successfully processed",
+            m.getDynamodb().getSequenceNumber());
+    })
+    .withFailureHandler((m, e) -> {
+        // Failure handler receives the raw message and the exception thrown.
+        LOGGER.info("Message with sequenceNumber {} failed to be processed: {}"
+        , e.getDynamodb().getSequenceNumber(), e);
+    })
+    .buildWithMessageHander(this::processMessage);
 ```
 
 !!! info