Introduction Link to heading
This post covers a simple and efficient solution for implementing liveness probes in pods running console applications (think background services or utility DaemonSets). A common question I see in Kubernetes forums is:
“How do I use health probes with console apps?”
The typical responses often suggest:
- Sidecar Container with HTTP Server: Requires coding the server, building a sidecar container, and managing additional infrastructure. While effective, this can be overkill for lightweight applications.
- Exec Probes with Shell Scripts: Feels hacky and can waste resources, depending on what the script does.
Pods are meant to be lightweight, and both solutions can deviate from that principle.
Instead, we’ll use a simple TCP listener to create an efficient liveness probe. Here’s the code:
using System;
using System.Net;
using System.Net.Sockets;
using System.Text;
using System.Threading;
class Program
{
static void Main(string[] args)
{
int port = 666; // You can run this on a different port if necessary
Thread tcpServerThread = new Thread(() => StartTcpServer(port));
tcpServerThread.IsBackground = true;
tcpServerThread.Start();
Console.WriteLine("Application is running...");
while (true)
{
// do some work
Thread.Sleep(1000);
}
}
static void StartTcpServer(int port)
{
TcpListener server = null;
try
{
server = new TcpListener(IPAddress.Any, port);
server.Start();
Console.WriteLine($"Liveness server started on port {port}.");
while (true)
{
TcpClient client = server.AcceptTcpClient();
NetworkStream stream = client.GetStream();
byte[] buffer = new byte[256];
int bytesRead = stream.Read(buffer, 0, buffer.Length);
string request = Encoding.ASCII.GetString(buffer, 0, bytesRead).Trim();
if (request.Equals("ping", StringComparison.OrdinalIgnoreCase))
{
byte[] response = Encoding.ASCII.GetBytes("pong\n");
stream.Write(response, 0, response.Length);
}
client.Close();
}
}
catch (Exception ex)
{
Console.WriteLine($"Error in liveness server: {ex.Message}");
}
finally
{
server?.Stop();
}
}
}
The StartTcpServer() method starts a simple TCP server in a thread running concurrent to the main thread. Use a tcpSocket liveness probe in your Kubernetes manifest. Kubernetes will then attempt to establish a TCP connection at configured intervals. You can test it by doing the same, or using a tool like netcat:
echo "ping" | nc localhost 666
‘pong’ should be returned.
Simply add this TCP server to your application and then configure the liveness probe. Examples can be found in the Kubernetes docs linked below.