Awaiting an event to complete before it gets fired again

Question

I have the following scenario, I have an event that keeps on receiving xml's that need to be pushed into DB, the pushing code and event looks something like below :

async void signalrClient_OnXMLReceived(object value, MessageResponseEventArgs e)
{
    await SaveXML();
}

async task SaveXML(string xmlDoc)    
{    
    await SaveCustomer(GetCustomerDataFrmXML(xmlDoc));
    await SavePlantInfo(GetPlantDataFrmXML(xmlDoc))

   if(NotJobexists(GetJob(xmlDoc))
   {
    await SaveJob(GetJobInfo(xmlDoc))
   }
}

The problem is as soon as I encounter the first await in the save method, the control is taken back to the event where it receives new xml and calls save again, so My question is , how do I make sure that I execute all the code in the save method before the event gets fired again.

I do not have control over the event when it gets called, but as and when it calls I want to make sure that I handle the event fully before it gets called next time, how can I do it?

UPDATE : Thanks all for your reply, The reason why I wanted to handle the event fully before it gets fired again is, if I get one more xml which is similar to the first one in the event, I was actually inserting both which will cause the duplicate entries in DB due to the async nature, finally I reordered my saves and checked if duplicate exists before couple of await save methods. This fixed the issue. I didn't tried the queue or Recative framework suggestions as this needed to be fixed quickly.

Answer 1

"..I want to make sure that I handle th event fully before it gets called next time, how can I do it?"

Don't make async the method at all should meet your requirement.

Answer 2

You need a Queue in the event handler, where you push XML that arrives, and you need a timer, where on every tick you get an element from that queue and save it to DB .

Considering that Queue is FIFO structure, you will also maintain an order of the arrived XML s, in case it's important to you.

Answer 3

Microsoft's Reactive Framework would handle this nicely for you.

Instead of handling events in the normal way you create an observable from the event:

var xmlreceiveds =
    Observable.FromEventPattern<EventHandler<MessageResponseEventArgs>>(
        h => signalrClient.OnXMLReceived += h, 
        h => signalrClient.OnXMLReceived -= h);

You can now get a steady stream of events that are guaranteed to complete before the next event comes.

You handle then like this:

var xmlreceiveds =
    Observable
        .FromEventPattern<EventHandler<MessageResponseEventArgs>>(
            h => signalrClient.OnXMLReceived += h, 
            h => signalrClient.OnXMLReceived -= h);

xmlreceiveds.Subscribe(ep =>
{
    SaveCustomer(GetCustomerDataFrmXML(ep.EventArgs.XmlDoc));
    SavePlantInfo(GetPlantDataFrmXML(ep.EventArgs.XmlDoc));

    if (NotJobexists(GetJob(ep.EventArgs.XmlDoc)))
    {
        SaveJob(GetJobInfo(ep.EventArgs.XmlDoc));
    }
});

You can easily make this be handled on a different thread by including an ObserveOn method call, like so:

var xmlreceiveds =
    Observable
        .FromEventPattern<EventHandler<MessageResponseEventArgs>>(
            h => signalrClient.OnXMLReceived += h, 
            h => signalrClient.OnXMLReceived -= h)
        .ObserveOn(Scheduler.Default);