Pipelining web service response

Question

I am designing a web service that wraps a very large data source and I would be very grateful for any suggestions whether my design is appropriate or I am missing something substantially better.

So here is the problem: We have several data sources that all provide the same interface with the "most important" method being RowIterator select(Table table, String where) . Now, functionally everything is going fine for all our implementations but the problem is that the web service that we need to wrap around one of the sources would (in a naive implementation) upon receiving a query

1. wait for the wrapped data source to return the whole result set
2. marshal the whole result set before sending it to the client
3. at the client side unmarshal the whole result set before returning it to the caller

Only after this sequence would the caller be able to see the first row. This is a quite disappointing behavior as the caller has to wait unnecessessarily for the whole result set twice. I want to have some pipelining, instead. The caller must be able to see the first results while the service is still sending rows. Now I am planning to overcome this by implementing some kind of paging that is encapsulated in my client-side row iterator. The service would maintain a session id (with a timeout) that is created upon receiving a query and can be used to fetch chunks of data. The session id could already be returned before sending the actual query to the wrapped data source. The client would then fetch chunks (pages) until a chunk is empty or smaller than the expected (= requested) chunk size.

So, in this design the caller would be able to see the first results while the service is still sending rows. However, I am wondering whether there is a way to efficiently pipeline results on a per-row basis using a SOAP web service?

Also, would it be possible to return the results to the caller without repeatedly asking for more results?

Answer 1

In the end I used MTOM to transmit the data in binary and used blocking queues at the client and the server to achieve the desired parallelism. I sketched this here: Streaming large SOAP attachments