Tuesday, 18 November 2008

A Simple Multi-threaded Logger

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Diagnostics;
using System.Threading;
using System.IO;

namespace SudhirMurthy.Logger {

public interface ILog {

void LogToDisk(string msg);

}

interface ILoggerService {
ILog ILog {
get;
}
}

public class LogFile : ILog {

static List<string> messages;

//WriteToDisk delegate
private delegate void WriteToDisk(object sender, string fileName);

//WriteToDisk event
private static event WriteToDisk writeToDisk;

public static void WriteLog(string fileName) {
//Raise the event
writeToDisk(new object(), (fileName));
}

public void SendToDisk(object sender, string fileName) {

StreamWriter sw = File.CreateText(fileName);
foreach (string s in messages) {
sw.WriteLine(s);
}
sw.Close();
}

public LogFile() {
messages = new List<string>();
WriteToDisk mydelegate = new WriteToDisk(SendToDisk);
writeToDisk = new WriteToDisk(SendToDisk);
}

public List<string> Messages {
get {
return messages;
}
}

#region ILog Members

public void LogToDisk(string msg) {
Monitor.Enter(this);
messages.Add(msg);
Monitor.Exit(this);
}

#endregion
}

public class LoggerService : ILoggerService {

//TODO: Need to make it threadsafe..
private static ILog logInstance = null;

public LoggerService() {
logInstance = new LogFile();
}

#region ILoggerService Members

public ILog ILog {
get {
return logInstance;
}
}

#endregion
}

public class LoggerClient1 {

LoggerService logService;

public LoggerClient1() {
logService = new LoggerService();
}

public void Log() {
logService.ILog.LogToDisk(string.Format(
"I am Logging: {0}", this.ToString()));
}
}

public class LoggerClient2 {
LoggerService logService;
public LoggerClient2() {
logService = new LoggerService();

}
public void Log() {
logService.ILog.LogToDisk(string.Format(
"I am Logging: {0}", this.ToString()));
}
}

public class TestApp {

LoggerClient1 a;
LoggerClient2 b;

public TestApp() {

a = new LoggerClient1();
b = new LoggerClient2();
}

public void logClient1() {
for (int i = 0; i < 1000000; i++) {
a.Log();
}
}

public void logClient2() {
for (int i = 0; i < 1000000; i++) {
b.Log();
}
}
}

class Program {

static void Main(string[] args) {

TestApp app = new TestApp();
Thread t1 = new Thread(new ThreadStart(app.logClient1));
Thread t2 = new Thread(new ThreadStart(app.logClient2));


try {
t1.Start();
t2.Start();
} catch (Exception) {
throw;
}
t1.Join();
t2.Join();

//The main thread should technically wait until
//t1 and t2 should have finished and then execute
//the following
LogFile.WriteLog("log.txt");
}

}

}



I have demonstrated a simple multi-threaded logging mechanism here. The threads t1 and t2 share the static instance variable 'List<string> messages' while logging. The LoggerClient classes LoggerClient1 and LoggerClient2 each use the LoggerService to log messages. The log is finally written to disk by the the main thread. The LogFile class uses the Monitor.Enter(this) and Monitor.Exit(this) methods to allow locking while shared access to writing the log. This synchronizes the threads t1 and t2 to perform shared access writes without corrupting or overlapping the write operations. Another important thing to notice here is the methods t1.Join() and t2.Join() which is used to make the main thread wait until the threads t1 and t2 have finished executing.


If you try removing the Monitor.Enter(this) and Monitor.Exit(this) lines from the above code, you end up getting weird line numbers in your log.txt file. (enable line no's in visual studio and see log.txt). i.e the count of the log will never be as expected (2 million in the above case). By putting the above lines back, you get the expected value which is 2million lines of log. :). It takes about 5-6 seconds on my dual core for threads t1 and t2 to finish logging and the main thread to write to disk. :).  A bloated 93.4 MB file!!



My thread on MSDN Forums.



C0de

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