Refreshing Windows program when other users make changes?

Question

Scenario:

• 4 users launch separate instances of the same client program (Winforms) that is connected to a database based To-Do list.
• The first user selects the 3rd To-Do list item.

How do I update/refresh the other 3 users screens to reflect that item #3 is no longer available?

My thought was a table that contains the last update date-time stamp. Then a timer would check every few seconds to see if there have been any changes.

UPDATE1:

Thanks to all - there are definitely a number of valid answers.

I went with simpler version of the scenario that Icemanind recommended.

Answer 1

You could implement a 'Push' system where when 1 user updates something, the server sends an update message to all connected clients.

Answer 2

I'd opt for an isDirty timestamp/flag on the items. No need to get all the items again and no need to create a difficult push system. Reread the items since last call every now and then.

Answer 3

As Lucas suggested you can implement a 'Push' style system that whenever an entity is modified it is 'Pushed' to the other connected users. This can be a bit complex. Working with a legacy system the way we handle this is through a 'Change Number' column but really it can be anything that is updated each time the record is modified.

When a user attempts to modify an entity we query the database to row-lock that entity where the 'Change Number' reflects the 'Change Number' the user currently has.

If the lock is successful the user is able to update/delete the entity. When they are done they 'Save/Commit' and 'Change Number' on the entity is increased.

If they fail to get the row-lock and the 'Change Number' was the same, we display a message that the entity they requested is in use by another user. If the 'Change Number' was different then the message states they must refresh their view.

Answer 4

Yes. The best way to do this is to implement a "push" type system. Here is how it would work. Whenever someone clicks something on the client end, the client would send a message to the server. The server would need to receive this signal and then the server would send out a refresh message to all clients connected to the server.

I don't know your client or server is coded, but you'll want to create a thread on the server that "listens" for incoming messages from clients and once it receives a message, puts it into a queue, the returns to listening for more messages. A second thread on the server needs to process the messages in the queue.

On the client side, you'll also want a second thread that listens for incoming messages from the server. Once a message is received, it can process the message and take whatever action is necessary.

A pretty decent tutorial on client/server and socket programming can be found here: http://www.codeproject.com/KB/IP/serversocket.aspx

Of course, it is a guide. You will need to modify it as you see fit.

Hope this makes sense and good luck!

Answer 5

If you're using SQL Server as your store, and ADO.NET or something on top of that for your data access, you should also check into SQL Dependency.

Basically, you can create a query from a table, and you can instruct SQL Server to notify you if any of the data in that query result changes.

See some intro articles on the topic:

• Using SqlDepedency for data change events
• SqlDepedency: creating a smarter cache
• Using SqlDependency to monitor SQL Database changes
• Query notifications in ADO.NET 2.0

It requires a bit of initial setup effort, but the huge advantage here is: you'll be notified automagically when something you're interested in (which is part of your query result set) changes, and you can react to that. No messy polling or anything....