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Web application + SQL server triggers / thresholds

We have a web application that is hosted in IIS. In our database that serves the application we have all kinds of different data values. We are trying to figure out a way to have an email sent to a client if a certain data value exists or exceeds a threshold value.
Generic Example:
Say we have a table that lists widgets and their 'in inventory' quantity. Every time someone sells a widget, this quantity value would be depleted. We want to send an email to the manager when the widget quantity gets below 5 and tell him to reorder more widgets.
We don't want to have sql triggers that check the quantity any time a 'depletion' transaction takes place. Instead, we want some type of background monitoring process that checks the level of the widgets on a timed basis. How can we accomplish this? Windows Service / WinForm application? Something built into IIS that will run ASP.net C# code?

Polling based monitoring should be your last resort. It uses too many resources for a simple task and most of the time it will only see that it's not the case to do anything. And it doesn't even scale when your data grows.
Instead, you should focus on the code that changes those values and act then, on in spot. And the check will also be lighter: only one item being checked not all, and only once, not every x seconds/minutes/hours/...
Apart from the architectural considerations, to answer your question, just as Jonathan said: anything that can read a database and send emails will do, but I'd consider a Windows Service for this job because that's what they were made for: background jobs running all the time, unrelated to the host users. You also get some extra benefits like automatic startup and recovery options.

Anything that can read the database and send an email could accomplish this - console app, winforms app, web app -- it doesn't really matter.
It may be more efficient to monitor when the values are changed (what changes them? A web application?) and have that application also send notifications

Multi-User Database Desktop Application Design

First, I apologize for the seemingly dumb question I'm not very strong with databases.
I'm re-designing a desktop application in C# for use over a local network that basically is a highly specialized ticket tracking system. Essentially when a user launches the application they'll be asked for their credentials to gain access to the system and then the application will query the central database for data (currently a MySQL server running on a local machine), displaying it on the screen.
My question is if four users are connected and two users enter new data, what is the most efficient method of letting each user know of the new data? Would it be simply to query the database and update the application with the new data on a timer? Or would creating a server application to sit in between the user and the database server to perform queries itself and notify each connected user of updated data?

See it all depends how important is it to notify the clients in real time about the changes in your database. If your clients have no issue with a delay of minute or two you can probably go for the timer approach. But if they really wish the data to be real time (delay of less than 1-2 sec), go for the other approach. Create a separate service which polls the database and notify the client application for any update. For this you can make use of socket listners.
Hope that helps !!

4 users? On a local LAN? Using simple, indexed queries? Just poll the DB from the clients. Kick off a thread at application start up and have it run a query every 2-5 seconds, then notify the user using whatever is appropriate for background threads updating GUIs in .NET.
This is straightforward, don't over think it. The "hardest" part is the asynchronous notification of the user (which depending on your GUI layout and required actions is probably not a big deal either, thus the quotes).

Database Scheduled manipulation

I've wrote a Restaurant Management application.
I have a Database based on SQL Server 2005 which has one table named OrdersItems. Every 5 minutes I want to read all rows of this table and based on a specific criteria Update some fields.
I don't want to do that in my main application and I prefer to have an Alternative engine to perform this.
Which method is the best method to perform such task ? Also note that this Table (OrdersItems) is under process every time because main application must be always running and get new Restaurant Orders.

You can create a SQL Server Agent job that does the update every five minutes.
If you are using SQL Server Express edition, you can't use SQL Server Agent because it's only included in the "bigger" versions of SQL Server.
In this case, you can create your jobs manually using batch files and Windows Task Scheduler.

I definitely agree with Christian and dougajmcdonald's points about using SQL Task/ Maintenance. However since you included c# in your tags an alternative is to create a Windows Service.
The benefits of this approach
Can run on a machine that doesn't have the SQL Server Agent installed (including express editions)
Can be run outside the context of a user login.
Has standard stop start pause continue mechanism that's well understood.
If the service itself fails will likely result in an event log
This answer contains a template for a windows service that periodically gets data and executes. You may simply want to change the DoStuff method to execute a stored procedure

Create a dialog timer and let it activate a stored procedure. This has the advantage of being fully contained inside the database (no external process), it does not require SQL Agent (runs on Express) and is completely crash resilient at the point it will survive database detach/attach and backup/restore operations (the scheduled job will run after recovery on the new restored database).

I would expect a SQL Task / Maintenance plan would be the best for this.
You can set them up for whatever interval you want, specifying a SQL statement, maintenance task etc you want to run.
You can also setup alerts etc if you want to know when it fails for example.

Deploy a cron job on a server with access to the database which is started every 5 minutes and processes your data, using transactions. I see one problem there: If the amount of data to be processed is large, it could quite work more than five minutes.

Windows Service Vs Simple Program

Let me give a back ground for everybody before I go to my problem. My company hosts website for many clients, my company also contracts some of the work to another company.
So when we first set up a website with all the informations to our clients, we pass that information to the other company we contracted and three of us have the same data. Problem is once the site is up and running, our clients will change some data and when ever they do that we should be able to update our contracted company.
The way we transfer data to the contracted company is by using a web service (httppost, xml data). Now my question is what it the best way to write a program which sends updated data to the contracted company everytime our clients change some data.
1) Write a windows service having a timer inside my code where every 30min or so connects to the database and find all changes and send it to the contracted company
2) Write the same code as #1 (with out the timer in it) but this time make it a simple program and let windows scheduler wake it every 30min
3) Any other suggestion you may have
Techenologies available for me are VS 2008, SQLServer 2005

Scheduled task is the way to go. Jon wrote up a good summary of why services are not well suited for this sort of thing: http://weblogs.asp.net/jgalloway/archive/2005/10/24/428303.aspx

A service is easy to create and install and is more "professional" feeling so why not go that way? Using a non-service EXE would also work of course and would be slightly easier to get running (permissions, etc.) but I think the difference in setup between the two is nearly negligible.

One possible solution would be to add a timestamp column to your data tables.
Once this is done, you can have one entry in each table that has the last collected time by your contracted company. They can pull all records since that last time and update their records accordingly.

A Windows Service is more self contained, and you can easily configure it to start up automatically when the OS is starting up. You might also need to create additional configuration options, as well as some way to trigger the synchronization immediately.
It will also give you more room to grow your functionality for the service in the future.
A standalone app should be easier to develop though, however you are reliant on the windows scheduler to execute the task always. My experience has been that it is easier to mess up things with the windows scheduler and have it not run, for example in cases where you reboot the OS but no user has logged in.
If you want a more professional approach go with the service, even though it might mean a little bit more work.

A windows service makes more sense in this case. Think about what happens after your server is restarted:
With a Windows Application you need to have someone restart the application, or manually copy a shortcut to the startup folder to make sure the application gets launched
OR,
With a Windows Service you set it to start automatically and forget about it. When the machine reboots your service starts up and continues processing.
One more consideration, what happens when there is an error? A Windows application would likely show an error dialog and wait for input before continuing; whereas a service would log the error in the event log and carry on.

How do I get a Windows Form client to update every time a SQL Server table changes?

I have a form with a list that shows information from a database. I want the list to update in real time (or almost real time) every time something changes in the database. These are the three ways I can think of to accomplish this:
Set up a timer on the client to check every few seconds: I know how to do this now, but it would involve making and closing a new connection to the database hundreds of times an hour, regardless of whether there was any change
Build something sort of like a TCP/IP chat server, and every time a program updates the database it would also send a message to the TCP/IP server, which in turn would send a message to the client's form: I have no idea how to do this right now
Create a web service that returns the date and time of when the last time the table was changed, and the client would compare that time to the last time the client updated: I could figure out how to build a web service, but I don't how to do this without making a connection to the database anyway
The second option doesn't seem like it would be very reliable, and the first seems like it would consume more resources than necessary. Is there some way to tell the client every time there is a change in the database without making a connection every few seconds, or is it not that big of a deal to make that many connections to a database?

Try the SqlDependency class. It will fire an OnChange event whenever the results of its SqlCommand change.
EDIT:
Note that if there are large numbers of copies of your program running, it can generate excessive server load. If your app will be publicly available, it might not be a good idea.
Also note that it can fire the event on different threads, so you'll need to use Control.BeginInvoke to update your UI.

You can use Event Notifications in SQL Server to raise events in .Net letting you know that the data has changed. See article linked below.
Eric
SQL Server Event Notification

With ASP.Net you can cache query results in memory and setup a dependency that registers with the SQL Server. When something within the data changes the cache is refreshed automatically. Perhaps looking into this might point you in a good direction.
http://msdn.microsoft.com/en-us/library/ms178604.aspx