December 31, 2009

Happy New Year!

Instead of a technical post, today I just want to wish a happy new year to everyone! The promised follow-up post of my AOP series will go online on January 4 in the next decade.

All the Best and I hope to see you again next year!

December 28, 2009

Spring.NET AOP - behind the scenes (1)

On the forums I see one topic coming up quite often: "Can I advise XY?". In this multipart post series I will describe the motivation for AOP and the way Spring.NET AOP (and most others like Castle.DynamicProxy and LinFu) technically work in .NET to give you a better understanding and provide you the knowledge to help answering such questions yourself.

The example: Retrying operations

Instead of the usual logging example I'd like to show you another useful feature: Retrying operations. For the sake of simplicity let's assume we're calling a webservice method for calculating the sum of two integers.

CalculatorWebService calc = new CalculatorWebService("http:/...");
int sum = calc.Add(2, 5);

Since webservices usually involve calling over the network, they are inherently unsafe and might fail for various reasons. In our application, we would like to retry webservice operations 3 times before giving up, with a 1 second delay between retries.

There are a couple of ways to implement this requirement, the most direct approach probably by deriving our own class from the webservice client class:

public class RetryingCalculatorWebService : CalculatorWebService
{
  public RetryingCalculatorWebService(string url):base(url) {}   

  public override int Add(int x, int y)
  {
    int retries = 0;
    while (true)
    {
      try
      {
        return base.Add(x, y);
      }
      catch (Exception ex)
      {
        retries++;
        if (retries >= 3)
        {
          throw; // retry threshold exceeded, giving up// wait a second
      }
    }
  }
}

There are of course a couple of issues with that approach, the 2 most important are:

  • The code for retrying the operation effectively buries our single line of actual business code. This makes it hard identifying what the code actually does:
public class RetryingCalculatorWebService : CalculatorWebService
{
  public RetryingCalculatorWebService(string url):base(url) {}   

  public override int Add(int x, int y)
  {
    int retries = 0;
    while(true)
    {
      try
      {
        return base.Add(x, y);
      }
      catch(Exception ex)
      {
        retries++;
        if (retries >= 3)
        {
          throw; // retry threshold exceeded, giving up
        }
        Thread.Sleep(1000); // wait a second
      }
    }
  }
}
  • When implementing this requirement across all our webservice clients we not only end up scattering the same lines of code all over our codebase. A change in the requirement might cause us having to change all our webservice client classes causing a huge amount of work.

image

A manually implemented a solution

A structured way for non-intrusively adding behavior to exisiting code is described in the GoF book, the pattern is called "Decorator", the basic idea being wrapping the actual target method with additional code. Thus you do not need to modify any existing code, instead you "chain" the various required behaviours, each behaviour implemented in its own class. In contrast to the GoF-pattern, nowadays composition is favoured over inheritance, thus instead of using the inheritance approach, let's introduce an interface to easily chain our behaviors:

public interface ICalculator
{
  int Add(int x, int y);
}
ICalculator calc = ...;
int sum = calc.Add(2, 5);

Now we can easily implement our business logic and the infrastructure constraint separately and chain them later as needed:

public class CalculatorWebService : ICalculator
{
  public CalculatorWebService(string url) { ... }   

  public int Add(int x, int y)
  {
    // perform actual webservice call
    return ...
  }
}
public class CalculatorRetryDecorator : ICalculator
{
  private ICalculator next;

  public CalculatorRetryDecorator(ICalculator next) { this.next = next; }

  public int Add(int x, int y)
  {
    int retries = 0;
    while (true)
    {
      try
      {
        return next.Add(x, y);
      }
      catch (Exception ex)
      {
        retries++;
        if (retries >= 3)
        {
          throw; // retry threshold exceeded, giving up
        }
        Thread.Sleep(1000); // wait a second
      }
    }
  }
}

Notice how the CalculatorRetryDecorator delegates the actual work to the next calculator in the chain. Now, whenever our requirements force us to retry calculator operations, we just "chain" the implementations together:

ICalculator calc = new CalculatorRetryDecorator( new CalculatorWebService( "http://..." ) );
int sum = calc.Add(2, 5);

Now, when we discover that our performance is to slow, well - implement a caching decorator that caches method results for a certain amount of time using the same approach and add it to the decorator chain:

ICalculator calc = new CalculatorCacheDecorator( new CalculatorRetryDecorator( new CalculatorWebService( "http://..." ) ) );
int sum = calc.Add(2, 5);

When we call the Add() method,  our call graph looks like this:

calculatordecorator_callgraph

This is only the beginning

We already achieved a lot by separating concerns into different classes. Still our solution has some significant weaknesses:

  • When we add new methods to our ICalculator, we need to extend all of our decorators
  • We want to reuse the retry logic for other services

In my next post I will address those issues and show you how we can implement a more generic solution.

Here you can download the example code for this post.

December 24, 2009

A little Christmas present - for .NET

Just read Eberhard Wolff's newest blog post and simply couldn't resist to borrow the idea and show you, how the same things can be done using Spring.NET. First, here's the .NET version of Eberhard's example:

public class MyRepository
{ }

public class MyService
{
    public MyRepository MyRepository { get; set; }
}

Here it is, the smallest possible Spring.NET object. Notice, due to Java's lack of properties the .NET version is even smaller. Still, no attributes, no Spring dependencies - pure code.

How do you wire them? Spring.NET's XML configuration unfortunately lacks Spring.Java's component-scanning as demonstrated in Eberhard's post. But using the new code-based configuration I presented recently, you easily can wire your objects by writing

appContext.Configure(cfg => cfg
     .Scan(scan => scan
        .TheCallingAssembly()
        .Include(t => t.FullName.EndsWith("Service") || t.FullName.EndsWith("Repository"))
        .With<AutowiringConvention>()
));

The example code can be downloaded at SmallestSpringObject.zip

Merry Christmas!

December 21, 2009

NetMX & Spring.NET - configure your Apps at runtime

Ever wished you could change some settings of your application during runtime? Easily retrieve runtime information from your components? You tried WMI and Performance Counters?

Well I did and to put it polite: I don't like WMI. Performance Counters are a nice way to retrieve peformance information about an application, but when it comes to changing an application's settings or behavior at runtime, you need another way. WMI is still COM-based, although a WMI.NET binding is available via the System.Management components. Alas this binding is not complete and functions are spread across different namespaces for historical reasons. To put a long story short: It is nothing for the fainthearted.

Brief Introduction to Java Management Extensions (JMX)

In the Java universe for this purpose there is JMX. Basically it allows an application to expose management objects via a JMX server. A monitoring client can then connect to this server and dynamically obtain runtime information about the exposed management objects. In contrast to WMI there is no schema involved, everything is discovered dynamically. For instance the server application could expose the following management bean:

public interface SampleMBean
{
    public String getHello();
    public void setHello(String helloMessage);
    public String sayHello();
}

public class Sample implements SampleMBean {

    private String hello = "Hello World(s)!";

    public Sample() {
    }

    public String getHello() {
        return hello;
    }

    public void setHello(String helloMessage) {
      hello = helloMessage;
    }

    public String sayHello() {
      return hello;
    }
}

Using the JConsole tool, one can easily connect to this application's virtual machine and remotely retrieve/change the properties as well as invoke the exposed method(s):

image

 image

The server application may expose its management objects over a variety of different connectors, as it is shown in this image (copied from the Wikipedia article about JMX)

And Now: .NET Management Extensions

Thanks to the amazing work of Szymon Pobiega, the power of JMX is available for the .NET platform as well. The NetMX implementation is available from CodePlex. The really nice part is, that it also comes with a working implementation of the JSR262 JMX connector, so that it is possible to connect to your .NET application using JConsole (yes, that's true and I'm going to proof that ;-)). Of course NetMX also comes with a .NET version of the console, please check out the project for more. I grabbed the sources and compiled them. After playing around a while, I decided to implement a simple usage scenario.

The Sample Scenario

Every now and then I wanted to increase decrease the logging level of my applications (among other things ...). Using NetMX it should be easy. And - well it was! Using Spring.NET, it was easy to weave a logging advice around my components, using NetMX it was easy to expose a management object for this logging advice, so that the log settings can be changed at runtime. The picture below illustrates the scenario setup I aimed for:

image The Logging Advice intercepts every call to the service object and logs the method calls to the log system. By default, logging is turned off, using JConsole I want to change those settings during runtime.

My sample application again is the MovieFinder application. To make the effect visible, the client code retrieves the movielist every second and prints the timestamp + the moviename on the console. Also the logging system is configured to write to the console, but disabled by default.

The Solution

The sources for my experiment can be fetched from my subversion repository. What you also need is the JSR262 enhanced version of JConsole, which is included in the samples of the JSR262 Connector download. Unpack the connector download and checkout $/jsr262-ri-ea4/samples/index.html for more.

So launching the application results in a screen similar to this:

image

Now launch JConsole and connect to our .NET application using the url "service:jmx:ws://127.0.0.1:9998/jmxws":

imageExpanding the tree shown in the left pane, you should see this:

image  Now let's change some settings. Double-Click into the "Value" column and enter the following:

imageé voila! Our console has significantly changed now:

image

We just reconfigured the logging advice on the fly to also log all method calls to our service and also dumps the arguments passed in!

Here's the configuration snipped required to make this happen (using Spring.NET CodeConfig from my last post):

var appContext = new GenericApplicationContext(false);
appContext.Configure()
   .FromConfiguration<MovieFinderConfiguration>()
    .EnableLogging(log =>
    {
       log.TypeFilter = type => type.IsDefined(typeof(ServiceAttribute), true);
       log.Logger.LogLevel = LogLevel.Off;
    })
   .EnableNetMX(netmx =>
   {
     netmx.AddConnector<Jsr262ConnectorServerProvider>(new Uri("http://0.0.0.0:9998/jmxws"));
     netmx.ExportMBean<SimpleLoggingManager>();
   });

Enjoy and again thanks to Szymon for his great work!

December 17, 2009

Merry XMLless! CodeConfig for Spring.NET

Straight to the point: XML is not meant for humans. Fullstop. The only way for humans to deal with XML is when it is hidden behind proper tooling support. Without a tool hiding XAML you wouldn't write XAML code by hand, would you? Currently being on a greenfield project with my team collegues not familar with Spring.NET it quickly turned out that XML configuration can be quite a hurdle, burying the gain of power in the pain of configuration. Using the simple MovieFinder example from the Spring.NET quickstart examples, I would like to introduce you to a new way to do familar things.

Introducing MovieFinder

In the following I will use the following - very simple - example to show you different ways of wiring the components. A MovieLister can be used to obtain a list of movies directed by a particular director. To access persistent storage, it uses the MovieFinder repository component: sample

Here's how the client code might look like:

IMovieLister lister = ...
var movies = lister.MoviesDirectedBy("Roberto Benigni")

The traditional way of wiring

The traditional way of configuring the Spring.NET container looks like this:

<objects xmlns='http://www.springframework.net'>

  <object id='movieLister' type='MovieFinder.Core.MovieLister, MovieFinder.Core'>
    <constructor-arg index='0' ref='movieFinder' />
  </object>
  
  <object id='movieFinder' type ='MovieFinder.Data.SimpleMovieFinder, MovieFinder.Data' />    
  
</objects>
Code Example: Traditional Configuration

There are a couple of problems with that approach, here are some:

  • not pleasant to read
  • much 'noise', xml syntax hiding the actual wiring structure
  • lack of refactoring support
  • unmanageable for large object numbers

You can imagine that in large applications without any tool support this can be become very painful. Already less known is the fact that Spring is able to autowire components. Here's the example to autowire components based on type-matching contructor arguments (see the reference documentation for other values of 'default-autowire'):

<objects xmlns='http://www.springframework.net' default-autowire='constructor'>

  <object id='movieLister' type='MovieFinder.Core.MovieLister, MovieFinder.Core' />  
  <object id='movieFinder' type ='MovieFinder.Data.SimpleMovieFinder, MovieFinder.Data' />    
  
</objects>
Code Example: Using constructor autwiring

This saves you from having to manually specifiying all dependencies. Of course, renaming your classes still may break this configuration.

Mark Pollack blogged an extensive post about configuring the Spring.NET container, including the even less known container API for configuring the container.

Going fluent

A lot of frameworks out there have already introduced a configuration style known as fluent API. A nice example for the Windsor container can be found here. There's a lot more, Fluent NHibernate being another very populare example.

Recently, Tom Farnbauer released such a fluent configuration API for Spring.NET, Recoil for Spring.NET. Using Recoil, our MovieFinder example could look like this:

public class MyWiringContainer : WiringContainer
{
    public override void SetupContainer()
    {
        Define( () => new MovieFinder() )
             .As<IMovieFinder>;

        Define( () => new MovieLister( Wire<IMovieFinder>() ) )
             .As<IMovieLister>;
    }
} var myContext = new GenericApplicationContext(); myContext.Configure() .With<MyWiringContainer>(); myContext.Refresh();
Code Example: Wiring using Recoil for Spring.NET

Unfortunately I quickly found while introducing Recoil into my current project, that the API has some flaws and - after all - when looking at fluent apis, they usually add as much codenoise to your configuration as xml already does. Also fluent APIs tend to be less extensible than other approaches. Imho Fluent NHibernate suffers this fate a lot (probably others too, but this is the latest example crossing my way).

Of course we already achieved at least 1 important goal: We are safe against refactoring. Any class moves or renames will automatically be reflected in our configuration - because it is code.

Back to the whiteboard

I didn't find any of those approaches really satisfying. After all, all I want to do is

var movieFinder = new MovieFinder();
var movieLister = new MovieLister( movieFinder );

So lets start with the least minimal configuration container I can think of. A simple class, who's member methods return the objects you are asking for:

public class MovieFinderConfiguration 
{
   public IMovieFinder MovieFinder() 
  {
     return new MovieFinder();
  }

  public IMovieLister MovieLister()
  {
    return new MovieLister( MovieFinder() )
  }
}
Code Example: Wiring using ... plain C#

This already does a lot of what we want:

  • it is readable
  • it doesn't cause more codenoise than your standard code would have
  • we have full refactoring support
  • we automatically get the whole object graph once we call MovieLister():
var container = new MovieFinderConfiguration();
var movieLister = container.MovieLister();
var movies = movieLister.MoviesDirectedBy("Roberto Bergnini");
Code Example: Obtaining the object graph from our C# "container"

Unfortunately our minimal container is lacking a couple of things:

  • no support for managing the object lifecycle - everytime we call MovieLister() it will return a new graph
  • no support for applying aspects to care for crosscutting concerns
  • no support for all the other features you get from the containers out there

New on stage: CodeConfig for Spring.NET

Rooting in a great idea of Rod Johnson, Chris Beams started working on bringing this idea to the Java world, the result is JavaConfig (and since yesterday's release of our mother-project integral part of Spring 3.0, congrats at this point to the Java team!). Mark Pollack implemented the first POC for Spring.NET, due to the needs in my current project, I decided to take that POC and continue develop it.

Yet again, Spring.NET surprised me. Due to being incredibly flexible, it was almost a breeze to merge the above mentioned concept with the container infrastructure. How does our MovieFinder look like using CodeConfig? Simple:

[Configuration]
public class MovieFinderConfiguration 
{
   public virtual IMovieFinder MovieFinder() 
  {
     return new MovieFinder();
  }

  public virtual IMovieLister MovieLister()
  {
    return new MovieLister( MovieFinder() )
  }
}
Code Example: The CodeConfig configuration - not much difference!

Note the additional [Configuration] attribute and the "virtual" keyword added to the methods. Now feed this configuration into the Spring.NET application context and retrieve an instance of the IMovieLister:

var appContext = new GenericApplicationContext();
appContext.Configure()
    .FromConfiguration<MovieFinderConfiguration>();
ApplicationContext.Refresh();

var movieLister = appContext.GetObject<IMovieLister>();
var movies = movieLister.MoviesDirectedBy("Roberto Bergnini");
Code Example: retrieving an IMovieFinder from an CodeConfig-configured application context

Neat, isn't it? This means you can write your configuration in the simplest possible way and still can leverage the full power of the IoC container. Enabling logging on your services? No problem:

appContext.Configure() 
.FromConfiguration<MovieFinderConfiguration>()
.EnableLogging(cfg => { cfg.TypeFilter = type => type.IsDefined(typeof(ServiceAttribute), true); cfg.Logger.LogExecutionTime = true; cfg.Logger.LogMethodArguments = true; cfg.Logger.LogReturnValue = true; cfg.Logger.LogLevel = LogLevel.Trace; });

Using Conventions for Configuration

You don't want to configure each object manually in a large application? Use component-scanning, a feature that I shamelessly stole from StructureMap:

appContext.Configure() 
    .Scan(s => s                
        .AssemblyOfType<MovieLister>()
        .AssemblyOfType<SimpleMovieFinder>()
        .Include(t => t.IsDefined(typeof (ComponentAttribute), true))
        .With<TypeNamingConvention>()
        .With<AutowiringConvention>()
    );

Sources, Documentation and Next Steps

At this moment, you can find the sources in my public SVN repository at XP-Dev. Note, that the code will move to SpringSource's CodeConfig for Spring.NET repository here at a later stage.

For a quick overview of what is already possible, check out the various configuration examples for the MovieFinder example there. Documentation is missing, but reading the JavaConfig reference will give you a good overview of the features. For the component-scanning feature read Jeremy Miller's introduction on assembly scanning.

Beware that this is still a moving target, consider it not being more than a preview yet. But I'd love you to grab the sources, play around and let me know what you think - keen on hearing your feedback!

The first milestone of CodeConfig is scheduled for end of January - and don't forget to check, later this day, Spring.NET 1.3.0 GA will be released.

Merry XMLless!

December 14, 2009

Post-Rational

Due to my boss requesting an architecture review, I just find myself trying to put together all those pen&paper discussions I had with people on the team and rethinking the decisions I have made on how the project is set up and the application is structured.

While doing this, I find myself in a rather interesting situation: I realize that quite a lot of my decisions were based on intuition (which is of course likely driven by experience). Those decisions were not the result of a traditional, concious Analysis-Thesis-Synthesis approach. Much more, the process obviously happened somewhere in the back of my mind and at some point it comes back to the surface and presents me a solution that just "feels" right. Luckily most of those solutions proof useful over time and withstand all kinds of project challenges.

Nevertheless it leaves me thoughtfully. Should I break my habits and switch to e.g. famous +/- lists? How do I explain certain decisions? "I have a good feeling" is hardly an explanation my boss will eat...

I think Kevlin Hennley mentioned it in his talk "Five Considerations for Software Architects" that there is nothing wrong about making intuitive decisions. After all intuition comes a lot from experience. But "I had the idea under the shower" is likely an explanation that not all people will accept. People are different, Myers-Briggs spent a lot of time on this - and a lot of people don't take intuition as an argument. It always makes sense to rationalize your decisions afterwards, so that everyone can understand them. If your decisions proof right, it should be easy to rationalize them anyway.

December 10, 2009

Sniff HTTP traffic with ASP.NET Development Webserver

During developing webapplications or -services, every now and then you will face a strong desire to be able to see the HTTP traffic that is sent back and forth between a client and the webserver.

Tracing local HTTP traffic is not easy

You will quickly find out, that this task sounds easier than it actually is when your client and your server reside on the same machine. There are tools like Fiddler (and tons of others, but this is my favorite), but they all suffer the same problem: Requests to localhost/127.0.0.1 cannot be captured because they are optimized by the windows network stack and bypass the usual hooks used by capturing tools. Thus you either need to use your network card's IP address for submitting requests or - if you don't have a local ipaddress when e.g. you use DHCP and are disconnected from the network - you need to install the MS Loopback Adapter and use this adapter's IP address (see installing MS Loopback Adapter)

ASP.NET Development Webserver is locked down on "localhost"

If you are like me and like the ASP.NET Development Webserver (aka "Cassini" or "WebDev.Webserver") that comes with Visual Studio (and recently also with the Windows 7 SDK), you can't use it to capture traffic. Probably due to legal issues, the ASP.NET Development WebServer (aka Cassini) contains code that binds the TCP socket to the 127.0.0.1 address only and also contains a check, that the requesting client resides on localhost. In this case you have 2 choices:

a) setup a Webapplication in IIS this is possible but nasty when you want to do it in a build script

b) follow the instructions below to patch the WebDev.WebHost.dll on your system

Steps to patch WebDev.WebServer

Note: All steps below assume you have .NET 3.5 Service Pack 1 and the Windows 7 SDK installed. But with basic familarity of IL code you shouldn't have much problems applying those steps to e.g. the version of WebDev.WebServer that comes with VS 2005 (note, that this version resides under the installroot of VS!)

Step 1

Create a new directory "C:\patchcassini" that we will use to work and change to this directory on the commandline. You can create any directory you want, but I will refer to it as C:\patchcassini below.

Step 2

Copy the WebDev.WebHost.dll from the GAC to your working directory and disassemble into an IL script. The following batch script shows how this is done:

set SDKHOME=C:\Program Files\Microsoft SDKs\Windows\v7.0




cd C:\patchcassini

rem refresh copy from GAC
copy /Y C:\Windows\assembly\GAC_32\WebDev.WebHost\9.0.0.0__b03f5f7f11d50a3a\WebDev.WebHost.dll .

REM create backup
copy WebDev.WebHost.dll WebDev.WebHost.original.dll

REM disassemble
"%SDKHOME%\bin\ildasm.exe" WebDev.WebHost.original.dll /out=WebDev.WebHost.il

This will create 2 new files in your folder: WebDev.WebHost.il and WebDev.WebHost.res. Note that the script also creates a backup of the original assembly

Step 3

Patch the generated IL script. There are 2 things you have to do:

1) Open the script in any editor and find & replace all occurrences of [System]System.Net.IPAddress::Loopback with [System]System.Net.IPAddress::Any

2) Find the method body of Connection::get_IsLocal() (just search for this string), you will see something like

.method assembly hidebysig specialname
        instance bool get_IsLocal() cil managed
{
 ....
} // end of method Connection::get_IsLocal

Replace the whole method body with the code as shown below:

.method assembly hidebysig specialname
        instance bool  get_IsLocal() cil managed
{
.maxstack  2
IL_0014:  ldc.i4.1
IL_0015:  ret
} // end of method Connection::get_IsLocal
Step 4

You need to recompile the IL script and reinstall the patched dll into the GAC. Since we also modified a signed dll, we need to turn off signature validation for this dll. The batch script below shows how this is done:

set FRAMEWORKHOME=C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727
set SDKHOME=C:\Program Files\Microsoft SDKs\Windows\v7.0

REM recompile the patched IL script into WebDev.WebHost.dll
%FRAMEWORKHOME%\ilasm.exe /output=WebDev.WebHost.dll /quiet /resource=WebDev.WebHost.res /debug /dll WebDev.WebHost.il

rem Remove validation
"%SDKHOME%\bin\sn.exe" -Vr WebDev.WebHost.dll

rem Install into GAC, forcing overriding any existing assembly
"%SDKHOME%\bin\gacutil.exe" /i WebDev.WebHost.dll /f

Enjoy!

After patching and reinstalling the WebDev.WebHost.dll, hitting F5 in VS to launch your webapplication allows you to access the Webserver using any local IP address and thus allows tools like Fiddler to capture the traffic.

Hint: You can also make your life easier by registering the Webserver in the context menu of any folder. Just use the registry script below:

Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\Directory\shell\webdev.webserver.9]
@="ASP.NET Webserver 2008"

[HKEY_CLASSES_ROOT\Directory\shell\webdev.webserver.9\command]
@="\"C:\\Program Files\\Common Files\\Microsoft Shared\\DevServer\\9.0\\WebDev.WebServer.EXE\"  /port:81 /vpath:\"/\" /path:\"%1\""

This allows you to launch the Webserver using a rightclick of your mouse on any arbitrary folder:

image

Hope this helps!

December 08, 2009

Have a cup of coffee with a friend

A friend recently told me a great story about priorities in your life. Not directly related to software development, I still think it might apply to one or another and I simply like this story very much. When things in your life seem almost too much to handle, when 24 hours in a day just are not enough, remember the mayonnaise jar and 2 cups of coffee.

A professor stood before his philosophy class and had some items in front of him. When the class began, wordlessly, he picked up the very large, empty mayonnaise jar and proceeded to fill it with golf balls.
When no more golf balls would fit in the jar he asked the students if the jar was full. They agreed it was full.

Then the professor picked up a box of pebbles and poured them into the jar. He shook the jar lightly. The pebbles rolled into open areas between the golf balls. He asked his students again if the jar was full. They agreed it was.

The professor next picked up a box of sand and poured it into the jar. The sand filled up everything else. He once more asked the students if the jar was full. The students responded with a unanimous "yes".

The professor then produced two cups of coffee from under the table. He poured the coffee into the jar, effectively filling the empty spaces between the sand. The students laughed.

"Now" said the professor, as the laughter subsided, "I want you to recognize that this jar represents your life. The Golf balls are the important things - God, Family, Children, Health, Friends, Passions - things that if everything else was lost and only they remained, your life would still be full. The pebbles are other things that matter - like your Job, House, Car, etc. The sand is everything else - the small stuff. If you put the sand in the jar first," he continued, "There is no room for the pebbles or the golf balls. The same is true for life. If you spend all your time and energy on the small stuff, you will never have room for the things that are important to you.

So pay attention to the things that are critical to your happiness. Play with your children. Take time to get that check-up. Take your partner out to dinner. Play another 18 holes. There will always be time to clean the house and fix the disposal. Take care of the golf balls first - the things that really matter. Set your priorities. The rest is just sand."

One of the students raised her hand and asked what the coffee represented. The professor smiled. "I am glad you asked. It just goes to show that no matter how full your life may seem, there's always room for a couple cups of coffee with friends."