Receive multipart response and treat each part as soon as received

Question

Current situation: an existing SQL Server stored procedure I have no control upon returns 10 large strings in separate resultsets in about 30 seconds (~3 seconds per dataset). The existing ASP.NET Web API controller method that collects these strings only returns a response once all strings are obtained from the stored procedure. When the client receives the response, it takes another 30 seconds to process the strings and display the results, for a total of 1 minute from request initiation to operation completion.

Contemplated improvement: somehow transmit the strings to the client as soon as each is obtained from the SqlDataReader, so the client can work on interpreting each string while receiving the subsequent ones. The total time from request initiation to completion would thus roughly be halved.

I have considered the WebClient events at my disposal, such as DownloadStringCompleted and DownloadProgressChanged, but feel none is viable and generally think I am on the wrong track, hence this question. I have all kinds of ideas, such as saving strings to temporary files on the server and sending each file name to the client through a parallel SignalR channel for the client to request in parallel, etc., but feel I would both lose my time and your opportunity to enlighten me.

Answer 1

I would not resort to inverting the standard client / server relationship using a "server push" approach. All you need is some kind of intermediary dataset. It could be a singleton object (or multiple objects, one per client) on your server, or another table in an actual database (perhaps NoSql).

The point is that the client will not directly access the slow data flow you're dealing with. Instead the client will only access the intermediary dataset. On the first request, you will start off the process of migrating data from the slow dataset to the intermediary database and the client will have to wait until the first batch is ready.

The client will then make additional requests as he processes each result on his end. If more intermediary results are already available he will get them immediately, otherwise he will have to wait like he did on the first request.

But the server is continuously waiting on the slow data set and adding more data to the intermediate data set. You will have to have a way of marking the intermediate data as having already been sent to the client or not. You will probably want to spawn a separate thread for the code that moves data from the slow data source to the intermediate one.