Best approach to Triggering Event on SQL database

Question

Quite simply I have a SQL database with a .net front end and I need to set off event's when an specific time is reached . I call them alarms , so a user can set an alarm for say 12:30am on x day and when that time is reached on x day a popup would appear reminding them.

What is the best way to achieve this , should I poll the database or try to use SQL triggers?. I have not attempted this before so , if anyone has experience could you shed some light on the best approach? and any potholes that I should try to avoid falling into !.

Any insight = great

Yes sorry triggers where in my head for another project (lack of sleep).

To clarify , Multiple clients may be connected (usually 2 will always be on ,but it might be the case that everything goes down and none are connected) So I will need to think about how the alarms/popups happen. When they receive a popup alarm , they can choose to dismiss or "action" it , to say they have done the task.

Depending on the information on the alarm , different users should be notified i.e some people won't get the alarm if chosen , (hopefully achieve this through active directory logins and keep a table of current users & assign each to an area so people will only see alarms in there area, while others may just receive all )

Thanks for all the input : It sounds like I will want a central program or service?.

I could have a service that keeps polling the database checking alarm times and current active users, if found it updates a notification table which the user pages polls for every couple of seconds(10)(Clarify: The alarm times , should be put in well in advance and the alarm notification does not need to be seconds accurate). How does that sound?.

In terms of load , I can't see more than around 15-20 using it at the one time but usually only around 5-6.

Any glaringly obvious omissions or concerns?.

Answer 1

I don't think that you understand triggers correctly. They fire off based on actions taken (for example, a row is inserted into a table). They are not time-based. I don't think that this is what you're looking for based on your explanation.

Who will be seeing these alarms? Will they literally be pop-up windows on a client machine? If a pop-up appears in the woods and no one is there to read it, was there still an alarm?

My first though, given the limited information, would be to have the client load any alarm data when it is first opened. If any alarms were missed (i.e. they should have gone off when the person wasn't logged into the system) then they would pop up immediately (preferably with some visual cue that they were in the past). Any other alarms that are scheduled in the near future can then be set to fire off at the appropriate time.

The application could repoll the server every so often (once a minute? once every 10 minutes?) to see if there are any new alarms set in the system. The downside here is that if someone goes in and creates an alarm for the immediate time then it might not pop-up until the next poll of the database. This will help you determine the business rules on how often the polling needs to be done.

Some additional notes on this approach... you'll need to mark alarms as having been "handled" so that they don't pop up again. Also, you need to make sure that the clients' clocks are synchrponized with the server. Plus, do you need to consider differing time zones?

Another possible approach would be to have a central server that is polling the database and pushes out alarms to the clients. This can save each of the clients from having to constantly poll the database (you only have one poller), but it won't catch "missed" alarms. If you always have a bunch of clients connected and need to send out "worldwide" alerts then this might be the route. If alarms are user-specific then I'd go with the first approach.

Answer 2

Triggers are activated when something happens in the database.

I've used all of the following in the past...

1. Using an external application

If there is an app that needs to know about the event, time the event in the App.

If there are multiple clients, you'll need to co-ordinate them. Either with a central master application, or some synchronisation process.

2. Agents

MS SQL Server has the ability to set up time and repeating events that fire off some SQL.

These are useful if there isn't an external client application that needs this event to be fired.

If you have multiple client applications, it's like option 1, but SQL Server becomes the central app and the clients poll the server to know what is going on.

3. Infinite Loop

Instead (or as well as) Agents, you could write a never ending loop and use WAIT type commands to pause until the next event is due.

NOTE

What is best depends on what you are actually going to do with these events. Polling is not a problem, but it is generally best to have a single point controlling the firing of events.

Do note, you don't (usually) want to have connections from a client to a SQL Server standing open forever. And you can't open a connection form SQL Server to a bunch of client apps. So the apps will need to poll the server regardless. But you can be intelligent about the polling.

Answer 3

One of approach will be to client listening DATBASE all time and to rise event locally when something changed in database. Here is my SO about that

EDIT:

Also there is great SQL command waitfor if you want to delay executing something for future