community[patch]: Implement TTL for DynamoDBChatMessageHistory (#15478)

- **Description:** Implement TTL for DynamoDBChatMessageHistory, 
  - **Issue:** see #15477,
  - **Dependencies:** N/A,

---------

Co-authored-by: Piyush Jain <[email protected]>

libs/community/langchain_community/chat_message_histories/dynamodb.py

@@ -38,6 +38,11 @@ class DynamoDBChatMessageHistory(BaseChatMessageHistory):
             This may also contain global and local secondary index keys.
         kms_key_id: an optional AWS KMS Key ID, AWS KMS Key ARN, or AWS KMS Alias for
             client-side encryption
+        ttl: Optional Time-to-live (TTL) in seconds. Allows you to define a per-item
+            expiration timestamp that indicates when an item can be deleted from the
+            table. DynamoDB handles deletion of expired items without consuming
+            write throughput. To enable this feature on the table, follow the
+            [AWS DynamoDB documentation](https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/time-to-live-ttl-how-to.html)
     """
 
     def __init__(
@@ -49,6 +54,8 @@ def __init__(
         key: Optional[Dict[str, str]] = None,
         boto3_session: Optional[Session] = None,
         kms_key_id: Optional[str] = None,
+        ttl: Optional[int] = None,
+        ttl_key_name: str = "expireAt",
     ):
         if boto3_session:
             client = boto3_session.resource("dynamodb", endpoint_url=endpoint_url)
@@ -66,6 +73,8 @@ def __init__(
         self.table = client.Table(table_name)
         self.session_id = session_id
         self.key: Dict = key or {primary_key_name: session_id}
+        self.ttl = ttl
+        self.ttl_key_name = ttl_key_name
 
         if kms_key_id:
             try:
@@ -134,7 +143,15 @@ def add_message(self, message: BaseMessage) -> None:
         messages.append(_message)
 
         try:
-            self.table.put_item(Item={**self.key, "History": messages})
+            if self.ttl:
+                import time
+
+                expireAt = int(time.time()) + self.ttl
+                self.table.put_item(
+                    Item={**self.key, "History": messages, self.ttl_key_name: expireAt}
+                )
+            else:
+                self.table.put_item(Item={**self.key, "History": messages})
         except ClientError as err:
             logger.error(err)