diff --git a/spring-integration-core/src/main/java/org/springframework/integration/store/MessageGroup.java b/spring-integration-core/src/main/java/org/springframework/integration/store/MessageGroup.java
index b13ba286be..698bd902ba 100644
--- a/spring-integration-core/src/main/java/org/springframework/integration/store/MessageGroup.java
+++ b/spring-integration-core/src/main/java/org/springframework/integration/store/MessageGroup.java
@@ -1,45 +1,70 @@
 package org.springframework.integration.store;
 
-import java.util.Collection;
-
 import org.springframework.integration.core.Message;
 
+import java.util.Collection;
+
+/**
+ * A group of messages that are correlated with each other and should be processed in the same context. The group is
+ * divided into marked and unmarked messages. The marked messages are typically already processed, the unmarked messages
+ * are to be processed in the future.
+ * <p/>
+ * The message group allows implementations to be mutable, but this behavior is optional. Implementations should take
+ * care to document their thread safety and mutability.
+ */
 public interface MessageGroup {
 
 	/**
 	 * Add a message to the internal list. This is needed to avoid hitting the
-	 * underlying store or copying the internal list. Use with care.
+	 * underlying store or copying the internal list.
 	 */
 	boolean add(Message<?> message);
 
 	/**
-	 * @return internal message list, modification is allowed, but not
-	 *         recommended
+	 * @return unmarked messages in the group at time of the invocation
 	 */
 	Collection<Message<?>> getUnmarked();
 
 	/**
-	 * @return internal message list, modification is allowed, but not
-	 *         recommended
+	 * @return marked messages in the group at the time of the invocation
 	 */
 	Collection<Message<?>> getMarked();
 
 	/**
-	 * @return the correlation key that links these messages together according
+	 * @return the correlation key that links these messages together, typically according
 	 *         to a particular CorrelationStrategy
 	 */
 	Object getCorrelationKey();
 
+	/**
+	 * @return true if the group is complete (i.e. no more messages are expected to be added)
+	 */
 	boolean isComplete();
 
+	/**
+	 * @return the size of the sequence expected 0 if unknown
+	 */
 	int getSequenceSize();
 
+	/**
+	 * @return the total number of messages (marked and unmarked) in this group
+	 */
 	int size();
 
+	/**
+	 * Mark all unmarked messages in the group. A MessageGroupProcessor typically invokes this method after
+	 * processing all unmarked messages.
+	 */
 	void mark();
 
+	/**
+	 * @return a single message from the group
+	 */
 	Message<?> getOne();
-	
+
+	/**
+	 * @return the timestamp (milliseconds since epoch) associated with the creation of this group
+	 */
 	long getTimestamp();
 
 }
\ No newline at end of file
diff --git a/spring-integration-core/src/main/java/org/springframework/integration/store/SimpleMessageGroup.java b/spring-integration-core/src/main/java/org/springframework/integration/store/SimpleMessageGroup.java
index f59f48e82c..1a2f19f096 100644
--- a/spring-integration-core/src/main/java/org/springframework/integration/store/SimpleMessageGroup.java
+++ b/spring-integration-core/src/main/java/org/springframework/integration/store/SimpleMessageGroup.java
@@ -22,7 +22,8 @@ import java.util.concurrent.LinkedBlockingQueue;
 
 /**
  * Represents a mutable group of correlated messages that is bound to a certain {@link MessageStore} and correlation
- * key. The group will grow during its lifetime, when messages are <code>add</code>ed to it.
+ * key. The group will grow during its lifetime, when messages are <code>add</code>ed to it. This MessageGroup is thread
+ * safe.
  *
  * @author Iwein Fuld
  * @author Oleg Zhurakousky
diff --git a/spring-integration-core/src/main/java/org/springframework/integration/store/SimpleMessageStore.java b/spring-integration-core/src/main/java/org/springframework/integration/store/SimpleMessageStore.java
index 73837dd87a..b35c207ed6 100644
--- a/spring-integration-core/src/main/java/org/springframework/integration/store/SimpleMessageStore.java
+++ b/spring-integration-core/src/main/java/org/springframework/integration/store/SimpleMessageStore.java
@@ -13,17 +13,17 @@
 
 package org.springframework.integration.store;
 
+import org.springframework.integration.core.Message;
+import org.springframework.integration.core.MessagingException;
+import org.springframework.integration.util.UpperBound;
+import org.springframework.util.Assert;
+
 import java.util.HashSet;
 import java.util.Iterator;
 import java.util.UUID;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
 
-import org.springframework.integration.core.Message;
-import org.springframework.integration.core.MessagingException;
-import org.springframework.integration.util.UpperBound;
-import org.springframework.util.Assert;
-
 /**
  * Map-based implementation of {@link MessageStore} and {@link MessageGroupStore}. Enforces a maximum capacity for the
  * store.
@@ -38,7 +38,7 @@ public class SimpleMessageStore extends AbstractMessageGroupStore implements Mes
 
 	private final ConcurrentMap<UUID, Message<?>> idToMessage;
 
-	final ConcurrentMap<Object, SimpleMessageGroup> correlationToMessageGroup;
+	private final ConcurrentMap<Object, SimpleMessageGroup> correlationToMessageGroup;
 
 	private final UpperBound individualUpperBound;