How to Prioritize Outgoing Messages with WCF

Question

I would like to be able to prioritize the outgoing data/messages from a WCF service.

Here's the basic scenario:

1. Client requests from server a data stream. The data stream is live, large, and potentially unending (equipment monitoring data). We'll call this HighPriorityDataStream.
2. Client requests additional data. We'll call this LowPriorityData.

The bandwidth is limited (think dial-up modem or satellite). It is very important that the current HigPriorityDataStream not be interrupted or delayed when a request for LowPriorityData is made.

I have a sockets-based legacy system already where this is accomplished by manually controlling the order that data is placed into the socket buffer. High-priority data is placed in the buffer, and if there's room left over, lower priority data is added to fill the rest of the buffer.

I'm trying to reengineer this process with WCF... I don't know of any out-of-the box solutions and am thinking I may need to write a custom channel behavior, but I'd like to pick the brains of the community before I go that route :)

Answer 1

Hello,

I think there is no general out-of-the box solution. The solution is dependend on your other requirements. Do you want to control bandwith per client or per whole server (all clients)? Do you want to call low priority operation from the same proxy or do you start new proxy for new operation? Do you want to run more high priority operations at the same time? Do you want to prioritize incomming requests?

The easiest solution expects that you control bandwith per client, you are reusing same proxy for all calls, only one high priority operation can be completed at the same time and requests are processed in FIFO order. Than you just mark your service implementation with [ServiceBehavior(InstanceContextMode = InstanceContextMode.PerSession, ConcurrencyMode = ConcurrencyMode.Single)] (this should be default setting for services exposed over NET.TCP). This settings will reuse the same service instance for all calls from the same client proxy but only one call will be processed at time (other will wait in queue until they are processed or time outed).

Best regards, Ladislav

Answer 2

After a lot of poking around (thanks Ladislav for your thoughtful ideas), I've come to the conclusion that I'm asking the communication layer to solve a buisness-layer problem. To better state the problem, there are multiple connections and one data source. The datasource must prioritize which data it gathers from it's own data sources (live data streams and also persisted databases) and send the data back to the various clients based on their priority. To be clear, the clients have a relative priority based on their role-based identity, the data sources have a priority (prefer live data over persisted data) and individual fields within a data source have a priority order (all else being equal, field X must always be sent before field Y).

This is all firmly business-logic and the solution we adopted as a set of priority queues that autoamatically sorted the input data items based on these priority requirements and then served each request in that order.