CODEWITHSUNDEEP
c#.netwcfweb-serviceswindows-services
John Russell
John Russell

Reputation: 2247

Clients polling service for activity

I have a Windows service that continuously performs calculation tasks. There are multiple calculation steps performed as part of a single calculation task.

This service is required to log/track the calculation tasks and steps so that multiple clients can poll and observe the activity.

To meet this requirement, I've added a simple Dictionary<Guid, List<string>> (_activity) to my service that collects the activity messages. The key is a Guid representing the CalculationTask.Id, and the value is a List<string> holding the messages from the individual calculation steps.

I naively added a method to the service that is invoked by the clients (via wcf) to poll for activity messages. The client would pass in a lastPolledTimeStamp in an attempt to tell the service, "give me all of your messages since I last polled"...

private readonly Dictionary<Guid, List<string>> _activity = new Dictionary<Guid, List<string>>();
public List<string> GetActivity(DateTime lastPolledTimeStamp)
{
   var snapshot = _activity.ToList();

   return snapshot.SelectMany(x => x.Value)
      .Where(x => x.TimeStamp > lastPolledTimeStamp)
      .OrderBy(x => x.TimeStamp)
      .ToList();
}

Here are my problems and questions:

(1) When the client and service are on different machines, attempts to filter the results by date/time comparisons will not work. And I can't simply return everything in _activity each time GetActivity is called - there's too much data. There has to be some sort of filtering. What other options do I have?

(2) Another "problem" is knowing when an a key/value pair in the _activity dictionary can be removed. The clients are polling every 5 seconds. I want to keep the key/value pair around long enough so that clients have a chance to pick it up... Perhaps I should include another timer that executes to periodically clean up the old entries in _activity. Thoughts?

Upvotes: 0

Views: 224

Answers (2)

Rawling
Rawling

Reputation: 50184

For your first problem, return the server's time with each response, and get the client to send this timestamp, rather than their own time, when making a request.

For the second problem: if the calculations/steps may be producing messages at different rates, rather than have a fixed-size buffer for each set of updates, I'd stick with running a cleanup job every so often, deleting messages older than [a small multiple of the client's ping interval].

Upvotes: 1

Matt Burland
Matt Burland

Reputation: 45155

For your second problem, I'd implement a circular array structure which will automatically overwrite old entries. If the rate that messages are written is constant, this should be easy. If it's variable, you'll have to figure out what size to make the array based on the expected maximum number of messages per 5 second period.

Upvotes: 0

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