nesper group by prev

Program.cs

using com.espertech.esper.client;
using System;
using System.Threading;

namespace UseNesper
{
    class PerformanceCounter
    {
        public string ServerName { get; set; }
        public double Value { get; set; }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var configuration = new Configuration();

            configuration.AddEventType<PerformanceCounter>();

            var engine = EPServiceProviderManager.GetProvider("Engine", configuration);

            var statement = engine.EPAdministrator.CreateEPL(@"
select ServerName, Value - prev(1, Value) as Value
from PerformanceCounter.std:groupwin(ServerName).win:time_batch(5 seconds)
");

            statement.Events += (sender, e) =>
            {
                var now = DateTime.Now;

                Console.WriteLine("Batch:");

                if (e.OldEvents != null)
                {
                    foreach (var bean in e.OldEvents)
                    {
                        Console.WriteLine("{0} OldEvents: ServerName={1} Value={2}", now, bean.Get("ServerName"), bean.Get("Value") ?? "(null)");
                    }
                }

                if (e.NewEvents != null)
                {
                    foreach (var bean in e.NewEvents)
                    {
                        Console.WriteLine("{0} NewEvents: ServerName={1} Value={2}", now, bean.Get("ServerName"), bean.Get("Value") ?? "(null)");
                    }
                }
            };

            var connectionFailures = 0;

            while (!Console.KeyAvailable)
            {
                var p = new PerformanceCounter
                {
                    ServerName = "a",
                    Value = connectionFailures
                };

                Console.WriteLine("{0} Post Performance Counter Value={1}", DateTime.Now, connectionFailures);

                engine.EPRuntime.SendEvent(p);

                connectionFailures += 10;

                Thread.Sleep(500);
            }
        }
    }
}