Configure connection draining

Remove backend server

The following figure shows the state transitions of ECS01 after it is removed from a server group with connection draining enabled.

Health check failure

WebSocket sessions

Step 1: Enable connection draining

A server group was created as a prerequisite. This step shows how to enable connection draining for an existing server group. You can also enable this feature when you create one.

1. Log on to the ALB console.

2. In the top navigation bar, select the region where the server group is deployed.

3. In the left-side navigation pane, choose ALB > Server Groups.

4. On the Server Groups page, find the target server group and click its ID.

5. On the Details tab, in the Basic Information section, click Modify Basic Information.

6. In the Modify Basic Information dialog box, click Advanced Settings and enable Connection Draining.

7. Set Timeout Period to 300 seconds and click Save.

Step 2: Verify the results

Configure the server

1. Log on to the ECS01 instance. For more information, see Connection methods.

2. Run the following commands to create and enter a WebSocket directory:

   mkdir WebSocket
   cd WebSocket

3. Run the following command to install dependencies:

   pip install tornado
   pip install websocket-client

4. Run the following command to edit the server.py configuration file:

   vim server.py

   1. Press i to enter edit mode and add the following configurations to start a WebSocket service:

      #!/usr/bin/env python3
      # encoding=utf-8
      
      import tornado.websocket
      import tornado.ioloop
      import tornado.web
      from datetime import datetime
      
      
      # WebSocket handler
      class WebSocketHandler(tornado.websocket.WebSocketHandler):
          def open(self):
              current_time = datetime.now()
              formatted_time = current_time.strftime("%Y-%m-%d %H:%M:%S")
              print("Time:", formatted_time, "WebSocket connection opened")
      
          def on_message(self, message):
              current_time = datetime.now()
              formatted_time = current_time.strftime("%Y-%m-%d %H:%M:%S")
              print("Time:", formatted_time, "Received message:", message)
              self.write_message("Server received your message: " + message)
      
          def on_close(self):
              current_time = datetime.now()
              formatted_time = current_time.strftime("%Y-%m-%d %H:%M:%S")
              print("Time:", formatted_time, "WebSocket connection closed")
      
      # Routing
      application = tornado.web.Application([
          (r"/websocket", WebSocketHandler),
      ])
      
      if __name__ == "__main__":
          print("WebSocket Server Start on 8080 ...")
          application.listen(8080)
          tornado.ioloop.IOLoop.current().start()
      

   2. After you modify the configuration file, press the Esc key, enter :wq, and then press Enter to save and close the file.

5. Go to the directory where server.py is stored and run the following command to start the WebSocket service:

   python3 server.py

   The following response indicates that the WebSocket backend service is started.

   Websocket Server Start on 8080 ...

Add ECS01 to the server group

1. Log on to the ALB console.

2. In the top navigation bar, select the region where the server group is deployed.

3. In the left-side navigation pane, choose ALB > Server Groups.

4. On the Server Groups page, find the target server group, and click Modify Backend Server in the Actions column.

5. On the Backend Servers tab, click Add Backend Server. In the Add Backend Server panel, select ECS01 and click Next.

6. In the Ports/Weights step, select the ECS01 instance, set the port to 8080, and then click OK.

Configure the client

1. Log on to the client and open the command-line interface (CLI). Run the following commands to create and enter a WebSocket directory:

   mkdir WebSocket
   cd WebSocket

2. Run the following command to install dependencies:

   pip install websocket-client

3. Run the following command to edit the client.py file:

   vim client.py

   1. Press i to enter edit mode and add the following configurations to start a WebSocket client to access the service:

      #!/usr/bin/env python3
      # encoding=utf-8
      
      import websocket
      import time
      from datetime import datetime
      
      def on_message(ws, message):
          print("Received message from server:", message)
      
      if __name__ == "__main__":
          ws = websocket.WebSocket()
          ws.connect("ws://<domain_name>:80/websocket") # Replace <domain_name> with your actual domain name.
          print("WebSocket connection opened")
          try:
              while True:
                  current_time = datetime.now()
                  formatted_time = current_time.strftime("%Y-%m-%d %H:%M:%S")
                  print("Sending time:", formatted_time)
                  ws.send("Hello, Server!")
                  result = ws.recv()
                  on_message(ws, result)
                  time.sleep(1)
          except Exception:
              print("WebSocket connection closed")
      

   2. After you modify the configuration file, press the Esc key, enter :wq, and then press Enter to save and close the file.

4. Go to the directory where client.py is stored and run the following command to access the ECS01 backend server:

   python3 client.py

   The following response indicates that the access is successful.

   WebSocket connection opened
   Sending time: 2024-04-28 17:00:53
   Received message from server: Server received your message: Hello, Server!
   Sending time: 2024-04-28 17:00:54
   Received message from server: Server received your message: Hello, Server!

   The ECS01 backend server returns the following response.

   WebSocket Server Start on 8080 ...
   Time: 2024-04-28 17:00:53 WebSocket connection opened
   Time: 2024-04-28 17:00:53 Received message: Hello, Server!
   Time: 2024-04-28 17:00:54 Received message: Hello, Server!

Remove the backend server

1. Log on to the ALB console.

2. In the top navigation bar, select the region where the server group is deployed.

3. In the left-side navigation pane, choose ALB > Server Groups.

4. Find the target server group and click its ID.

5. Click the Backend Servers tab, find the target backend server ECS01, and then click Remove in the Actions column.

6. In the Remove dialog box, click OK.

Wait for connection draining

The Timeout Period is set to 300 seconds. The test results show that ALB terminates the session about 300 seconds after ECS01 is removed.

• The client returns the following response.

  Sending time: 2024-04-28 17:06:23
  Received message from server: Server received your message: Hello, Server!
  Sending time: 2024-04-28 17:06:24
  WebSocket connection closed

• The ECS01 server instance returns the following response.

  Time: 2024-04-28 17:06:22 Received message: Hello, Server!
  Time: 2024-04-28 17:06:23 Received message: Hello, Server!
  Time: 2024-04-28 17:06:23 WebSocket connection closed

HTTP sessions

In HTTP scenarios, the response that the client receives varies based on the settings of connection draining timeout, connection request timeout, and backend server processing time.

• If the connection draining timeout is shorter than the backend server processing time, the connection is interrupted before ECS01 sends a response. In this case, the client receives a 500 error response.

• If the backend server processing time is longer than the connection request timeout, the request to ECS01 times out. In this case, the client receives a 504 error response.

This topic uses a connection draining timeout of 15 seconds and a backend server processing time of 30 seconds as an example. In this scenario, the connection is interrupted before ECS01 finishes sending the response, and the client receives a 500 error response.

Step 1: Enable connection draining

A server group was created as a prerequisite. This step shows how to enable connection draining for an existing server group. You can also enable this feature when you create one.

1. Log on to the ALB console.

2. In the top navigation bar, select the region where the server group is deployed.

3. In the left-side navigation pane, choose ALB > Server Groups.

4. On the Server Groups page, find the target server group and click its ID.

5. On the Details tab, in the Basic Information section, click Modify Basic Information.

6. In the Modify Basic Information dialog box, click Advanced Settings and enable Connection Draining.

7. Set Timeout Period to 15 seconds and click Save.

Step 2: Set ALB request timeout

1. Log on to the ALB console.

2. In the top navigation bar, select the region where the ALB instance is deployed.

3. On the Instances page, find the target ALB instance and click its ID.

4. Click the Listener tab, find the target HTTP listener, and then click its ID.

5. In the Basic Information area, click Modify Listener.

6. In the Modify Listener dialog box, click Modify next to Advanced Settings.

7. Set Connection Request Timeout to 60 seconds (the default value) and click Save.

Step 3: Configure DNS resolution

In a production environment, use your own domain name and point it to the DNS name of the ALB instance by creating a CNAME record.

1. In the left navigation pane, choose ALB > Instances.

2. On the Instances page, copy the DNS name of your ALB instance.

3. Add a CNAME record to map your domain name to the DNS name of the ALB instance.

   Note

   If your domain name is not registered with Alibaba Cloud, you must first add it to the Alibaba Cloud DNS console before you can configure DNS records. For more information, see Domain Name Management. If your domain name is registered with Alibaba Cloud, you can proceed to the next steps.

   1. Log on to the Alibaba Cloud DNS console.

   2. On the Authoritative DNS Resolution page, find your domain name and click Settings in the Operations column.

   3. On the Settings page, click Add Record.

   4. In the Add Record panel, configure the following parameters to create the CNAME record, and then click OK.

      Configuration	Description
      Record Type	Select CNAME from the drop-down list.
      Hostname	The prefix for your domain name. This tutorial uses @. Note To use the root domain, set the host to @.
      Query Source	Select Default.
      Record Value	Paste the copied DNS name of the ALB instance.
      TTL	The Time to Live (TTL) is the amount of time the record is cached on a DNS server. This tutorial uses the default value.

Step 4: Verify the results

Configure the server

1. Log on to the ECS01 instance. For more information, see Connection methods.

2. Run the following commands to create and enter an HTTP directory:

   mkdir http
   cd http

3. Run the following command to edit the http_server.py configuration file:

   vim http_server.py

   1. Press i to enter edit mode and add the following configurations to start an HTTP server service:

      #!/usr/bin/env python3
      # encoding=utf-8
      
      from http.server import SimpleHTTPRequestHandler, HTTPServer
      from datetime import datetime
      import time
      
      class DelayedHTTPRequestHandler(SimpleHTTPRequestHandler):
          def do_GET(self):
              current_time = datetime.now()
              formatted_time = current_time.strftime("%Y-%m-%d %H:%M:%S")
              print("Time:", formatted_time, "GET request received. Responding after a 30-second delay....")
              time.sleep(30) # Use the time.sleep function to simulate the processing time of the backend server.
              SimpleHTTPRequestHandler.do_GET(self)
      
      PORT = 8080
      server = HTTPServer(("", PORT), DelayedHTTPRequestHandler)
      print(f"Serving HTTP on 0.0.0.0 port {PORT} (http://0.0.0.0:{PORT}/) ...")
      server.serve_forever()
      

   2. After you modify the configuration file, press the Esc key, enter :wq, and then press Enter to save and close the file.

4. Go to the directory where http_server.py is stored and run the following command to start the HTTP server service:

   python3 http_server.py

   The following response indicates that the HTTP server backend service is started.

   Serving HTTP on 0.0.0.0 port 8080 (http://0.0.0.0:8080/) ...

Add ECS01 to the server group

1. Log on to the ALB console.

2. In the top navigation bar, select the region where the server group is deployed.

3. On the Server Groups page, find the target server group, and click Modify Backend Server in the Actions column.

4. On the Backend Servers tab, click Add Backend Server. In the Add Backend Server panel, select ECS01 and click Next.

5. In the Ports/Weights step, select the ECS01 instance, set the port to 8080, and then click OK.

Configure the client

1. Log on to the client and open the CLI. Run the following command to access the backend server ECS01:

   curl http://<domain_name>:80/ -v

   The following response indicates that ALB can access the backend service.

   * About to connect() to www.example.com port 80 (#0)
   *   Trying 10.X.X.225...
   * Connected to www.example.com (10.X.X.225) port 80 (#0)
   > GET / HTTP/1.1
   > User-Agent: curl/7.29.0
   > Host: www.example.com
   > Accept: */*

   The server returns the following response.

   Serving HTTP on 0.0.0.0 port 8080 (http://0.0.0.0:8080/) ...
   Time: 2024-02-07 13:57:33 Received a GET request. Responding after a 30-second delay....

Remove the backend server

1. Log on to the ALB console.

2. In the top navigation bar, select the region where the server group is deployed.

3. In the left-side navigation pane, choose ALB > Server Groups.

4. Find the target server group and click its ID.

5. Click the Backend Servers tab, find the target backend server ECS01, and then click Remove in the Actions column.

6. In the Remove dialog box, click OK.

Wait for connection draining

The test results show that the client receives a 500 error response when the connection draining timeout set for ALB is shorter than the backend server processing time.

* About to connect() to www.example.com port 80 (#0)
*   Trying 10.X.X.224...
* Connected to www.example.com (10.XX.XX.224) port 80 (#0)
> GET / HTTP/1.1
> User-Agent: curl/7.29.0
> Host: www.example.com
> Accept: */*
> 
< HTTP/1.1 500 Internal Server Error
< Date: Wed, 07 Feb 2024 06:02:24 GMT
< Content-Type: text/html
< Content-Length: 186
< Connection: close
< Via: HTTP/1.1 SLB.87
< 
<html>
<head><title>500 Internal Server Error</title></head>
<body bgcolor="white">
<center><h1>500 Internal Server Error</h1></center>
<hr><center>nginx</center>
</body>
</html>
* Closing connection 0