How should I continuously observe a Web service-provided database query? - web-services

Question

I'm developing a Web application that is supposed to query a Web service (ASP.NET Web API) for certain data, and visualize the results. The queried data may change while the client application is running, as items may be added to or removed from the corresponding database collection. Either the client itself or other clients may modify the collection (via the Web service). The database server, RavenDB, has the ability to notify its client (the Web service) of changes.
What I'm wondering is how should clients be kept up-to-date as data changes in the Web service? Specifically, if a change takes place in the Web service's database so that a client's view of the data becomes outdated, the client should receive fresh query results. Would it be a good idea to maintain a persistent connection to clients, e.g. via SignalR, and simply notify them each time changes are made to the database so that each client can re-query for data? Should these change notifications be throttled in case they become too frequent?
Example Scenario
The database contains the following items (JSON notation):
[{"Id": "2", "User": "usera"}, {"Id": "1", "User": "usera"},
{"Id": "3", "User": "userb"}, {"Id": "4", "User": "usera"}]
Client A requests items where User == "usera", paginated to max 2 items and sorted on Id; the service returns the following set:
[{"Id": "1", "User": "usera"}, {"Id": "2", "User": "usera"}]
Then client B tells the service to delete the following item: {"Id": "2", "User": "usera"}, so that the database becomes:
[{"Id": "1", "User": "usera"}, {"Id": "3", "User": "userb"},
{"Id": "4", "User": "usera"}]
The question now is, how does the Web service notify client A that it should re-query for new data? That is, client A should refresh its view to contain the following:
[{"Id": "1", "User": "usera"}, {"Id": "4", "User": "usera"}]

Answer 1

What you said sounds right. You can have Web API and SignalR hosted side-by-side. You can use Web API to retrieve the data and SignalR to notify clients whenever the data changes. You can either notify clients that the data has changed so that they can re-query or you could actually send the changes to clients so that they can avoid re-querying the API.
You could also go with a different model where the client polls the server every say 15 or 30 seconds and updates the visualized results. This has the advantage of not requiring a persistent-connection and being easier to implement. But changes will take longer to propagate to clients and you may end up consuming more bandwidth if the result set is large or if changes are infrequent (since the polling will happen regardless of whether there are actually any changes).