[ServiceBehavior(InstanceContextMode = InstanceContextMode.PerSession)]
public class service1:IService1
{
   public static List<service1> servicesInstances = new List<service1>();
   ParallelOptions paralleOptions = new ParallelOptions();
   public static init()
   {
      TimerCallback processDataTimerDelegate = new TimerCallback(ProcessData);
      Timer processDataTimerDelegateItem = new Timer(processDataTimerDelegate , new object(), 0, 1000);

      Thread DispatchMWThread = new Thread(new ThreadStart(dispatchData));
      DispatchMWThread.IsBackground = true;
      DispatchMWThread.Priority = ThreadPriority.Highest;
      DispatchMWThread.Start();

      paralleOptions.MaxDegreeOfParallelism = -1;
   }
   public void login()
   {
      Monitor.TryEnter(servicesInstances);
      servicesInstances.Add((service1)OperationContext.Current.InstanceContext.GetServiceInstance());
      Monitor.Exit(servicesInstances);
   }
   public static void ProcessData(object obj)
   {
      //some code to process data for all clients
   }
   public static void dispatchData()
   {
      //When data processed is ready. Dispatch data concurrently
      Parallel.foreach(serviceInstances, delegate(service1)
      {
         service1.callbackData();
      }
   }

<serviceBehaviors>
    <behavior name="service1.mybehaviour">
      <serviceMetadata httpGetEnabled="false"/>
      <serviceDebug includeExceptionDetailInFaults="true"/>
      <serviceThrottling maxConcurrentCalls="5000" maxConcurrentInstances="5000" maxConcurrentSessions="5000"/>
    </behavior>
  </serviceBehaviors>

<netTcpBinding>
    <binding name="MyServiceBinding" listenBacklog="2500" maxConnections="5000" portSharingEnabled="true" maxReceivedMessageSize="4194304" maxBufferPoolSize="0">
      <readerQuotas maxDepth="2147483647" maxStringContentLength="2147483647" maxArrayLength="2147483647" maxBytesPerRead="2147483647" maxNameTableCharCount="2147483647"/>
      <reliableSession ordered="true" inactivityTimeout="00:20:00" enabled="false"/>
      <security mode="None">
        <message clientCredentialType="None"/>
      </security>
    </binding>
 </netTcpBinding>

[OperationContract(IsOneWay = true)]
    Task OnPushData(data);

public async Task PublishData()
    {
        communicationCallback = (ICommunicationObject)service.Callback;
        if (communicationCallback.State == CommunicationState.Opened)
        {
                await service.Callback.OnPushData(data);
        }
    }