Return HTTP OK Response after user logic execution

samples/listener/onMessage_HeavyProcessing.bal

@@ -0,0 +1,34 @@
+// Copyright (c) 2021, WSO2 Inc. (http://www.wso2.org) All Rights Reserved.
+//
+// WSO2 Inc. licenses this file to you under the Apache License,
+// Version 2.0 (the "License"); you may not use this file except
+// in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+import ballerinax/slack.'listener as slack;
+
+slack:ListenerConfiguration configuration = {
+    port: 9090,
+    verificationToken: "VERIFICATION_TOKEN"
+};
+
+listener slack:Listener slackListener = new (configuration);
+
+service /slack on slackListener {
+    remote function onMessage(slack:MessageEvent eventInfo) returns error? {
+        _ = @strand { thread: "any" } start userLogic(eventInfo);
+    }
+}
+
+function userLogic(slack:MessageEvent eventInfo) returns error? {
+    // Write your logic here
+}

slack/modules/listener/Module.md

@@ -95,5 +95,28 @@ service /slack on slackListener {
     }
 }
 ```
+> **NOTE:**
+If the user's logic inside any remote method of the connector listener throws an error, connector internal logic will 
+covert that error into a HTTP 500 error response and respond to the webhook (so that event may get redelivered), 
+otherwise it will respond with HTTP 200 OK. Due to this architecture, if the user logic in listener remote operations
+includes heavy processing, the user may face HTTP timeout issues for webhook responses. In such cases, it is advised to
+process events asynchronously as shown below.
+
+```ballerina
+import ballerinax/slack.'listener as slack;
+slack:ListenerConfiguration configuration = {
+    port: 9090,
+    verificationToken: "VERIFICATION_TOKEN"
+};
+listener slack:Listener slackListener = new (configuration);
+service /slack on slackListener {
+    remote function onMessage(slack:MessageEvent eventInfo) returns error? {
+        _ = @strand { thread: "any" } start userLogic(eventInfo);
+    }
+}
+function userLogic(slack:MessageEvent eventInfo) returns error? {
+    // Write your logic here
+}
+```
 
 ## Please check the [Samples directory](https://github.com/ballerina-platform/module-ballerinax-slack/tree/master/samples) for more examples.

slack/modules/listener/http_service.bal

@@ -87,7 +87,6 @@ service class HttpService {
         } else if (eventOrVerification == EVENT_CALLBACK) {
             http:Response response = new;
             response.statusCode = http:STATUS_OK;
-            check caller->respond(response);
 
             json eventTypeJson = check payload.event.'type;
             string eventType = eventTypeJson.toString();
@@ -118,6 +117,7 @@ service class HttpService {
                 TeamJoinEvent slackEvent = check payload.cloneWithType(TeamJoinEvent);
                 check self.handleTeamEvents(eventType, slackEvent);
             }
+            check caller->respond(response);
         } else {
             return error("Unidentified Request Type");
         }