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I need to implement a notification mechanism for a system that has one manager and multiple consumers/clients. A manager should poll a database and fire an event whenever there are changes in the data. Now, it'd be easy if all clients would be interested in the same data, and it would be sufficient to implement a single event and subscribe all clients to that event. However, clients should only receive the events for the data they are responsible for.

For example, there are multiple clients that add new customers. This happens through the manager in a thread-safe way. Now, these clients that created the customers need to know of any changes that happen only to those customers. The manager polls the Customers table every N seconds and gets a list of all customers that changed. Then, the manager will need to "route" (for a lack of a better word) the notifications to the interested clients.

Will this have to be implemented with some sort of a callback that each client will have to supply to the manager? This sounds like something I need, but I dont know how I can pass parameters to this callback (here, these are the customers Im interested in, dont bother me when you have updates for any other customer)

Im using C#, .NET 2.0. Thanks!

kateroh
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This is a good description of the Observer pattern. Typically a client registers interest with the manager for a set of data that is relevant for it, providing a means of notification (this would be your callback). A client may also unregister if it's no longer interested in previously-useful data. Then the manager's job is to propagate changes to all interested Observers (i.e. clients).

In C#, the required infrastructure is available as first-class language features - events and delegates. There is good (if simple) example code here.

In .Net 4 this convenience is taken a step further with ObservableCollection<T> available to automate the notification process.

By the way - I would avoid polling the database if possible. Is there no way you can get notified on the necessary changes in your DB? In C#/SQL Server you can use SqlDependency.

Steve Townsend
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  • Events are not an option here, because the subscriber needs to specify for which customers it wants to receive notifications. The OP probably needs something like `void Subscribe(int customerId, CustomerChangedEventHandler handler)`. You can't do that using events – Thomas Levesque Nov 11 '10 at 02:12
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    Events are an option. You can pass around event handlers (ie `CustomerChangedEventHandler`). The manager class would add the handlers to a `Dictionary>`. Then the manager can handler the event and, based on the `customerId`, call the list of event handlers associated with the customer. The client code wouldn't have any idea it wasn't directly handling the events. It would appear as if they were. – Enigmativity Nov 11 '10 at 02:33
  • Thanks for the pointer to the Observers pattern. Reading it now and figuring out how I can implement it myself. I'm on .NET 2.0 so ObservableCollection is not an option. As for SqlDependency, change notification was introduced in SQL2005, but deprecated in 2008 (2005 Sp3). ServiceBroker is not an option due to complexity and inflexibility (ie changing from SQL to SQL Azure for example). – kateroh Nov 11 '10 at 19:18