Data block types in Task Parallel Library

The following code sample shows data block types in Task Parallel Libary (TPL). The code is written in C# and can be run in a simple Console Application. Dataflow in TPL makes it possible to build actor-based programming and orchestrate coarse-grained dataflow and pipelining tasks, maintaining robustness and supporting concurrency-enabled applications. This makes it easier to construct high-performance, low latency systems.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Threading.Tasks.Dataflow;

namespace TplDataFlowSimpleTests
{
    class Program
    {

        static void Main(string[] args)
        {
            ActionBlockPostingAndFaulting();

            //BufferBlockPostingAndReceiving();

            //BroadCastPostAndMultipleReceive();

            //WriteOnceBlockParallellPostsAndSingleReceive();

            //ActionBlockPostingCompleting();

            //TransformBlockPostingAndProducingOutput(); 

            //TransformManyBlockPostingsAndReceive(); 

            //BatchBlockSeveralBatches();

            //JoinBlockAritmeticCombinations(); 

            //BatchedJoinBlockPropagatingValuesAndException();

            Console.WriteLine("Press the any key to continue ..");
            Console.ReadKey();

        }

        private static void BatchedJoinBlockPropagatingValuesAndException()
        {
            Func<int, int> doWork = n =>
            {
                if (n < 0)
                    throw new ArgumentOutOfRangeException();
                return n;
            };

            var batchedJoinBlock = new BatchedJoinBlock<int, Exception>(7);

            foreach (int i in new int[] {5, 6, -7, -22, 13, 55, 0})
            {
                try
                {
                    batchedJoinBlock.Target1.Post(doWork(i));
                }
                catch (ArgumentOutOfRangeException e)
                {
                    batchedJoinBlock.Target2.Post(e);
                }
            }

            var results = batchedJoinBlock.Receive();

            foreach (int n in results.Item1)
            {
                Console.WriteLine(n);
            }

            foreach (Exception e in results.Item2)
            {
                Console.WriteLine(e.Message);
            }
        }

        private static void JoinBlockAritmeticCombinations()
        {
            var joinBlock = new JoinBlock<int, int, char>(new GroupingDataflowBlockOptions{ Greedy = true});

            joinBlock.Target1.Post(3);
            joinBlock.Target1.Post(6);

            joinBlock.Target2.Post(5);
            joinBlock.Target2.Post(4);

            joinBlock.Target3.Post('+');
            joinBlock.Target3.Post('-');

            for (int i = 0; i < 2; i++)
            {
                var data = joinBlock.Receive();

                switch (data.Item3)
                {
                    case '+':
                        Console.WriteLine("{0} + {1} = {2}", data.Item1, data.Item2, data.Item1 + data.Item2);
                        break;
                    case '-':
                        Console.WriteLine("{0} - {1} = {2}", data.Item1, data.Item2, data.Item1 - data.Item2);
                        break;
                    default:
                        Console.WriteLine("Unknown operator '{0}'.", data.Item3);
                        break;
                } //switch 
            } //for 

        }

        private static void BatchBlockSeveralBatches()
        {
            var batchBlock = new BatchBlock<int>(10);

            for (int i = 0; i < 13; i++)
            {
                batchBlock.Post(i);
            }

            batchBlock.Complete();


            Console.WriteLine("The elements of the first batch are: [{0}] ", string.Join(",", batchBlock.Receive()));
            Console.WriteLine("The elements of the second batch are: [{0}]", string.Join(",", batchBlock.Receive()));
        }

        private static void TransformManyBlockPostingsAndReceive()
        {
            var transformManyBlock = new TransformManyBlock<string, char>(s => s.ToCharArray());

            transformManyBlock.Post("Hello");
            transformManyBlock.Post("World");

            for (int i = 0; i < ("Hello" + "World").Length; i++)
            {
                Console.WriteLine(transformManyBlock.Receive());
            }
        }

        private static void TransformBlockPostingAndProducingOutput()
        {
            var transformBlock = new TransformBlock<int, double>(n => Math.Sqrt(n));

            transformBlock.Post(10);
            transformBlock.Post(20);
            transformBlock.Post(30);

            for (int i = 0; i < 3; i++)
            {
                Console.WriteLine(transformBlock.Receive());
            }
        }

        private static void ActionBlockPostingCompleting()
        {
            var cts = new CancellationTokenSource();
            var actionBlock = new ActionBlock<int>(n =>
            {
                Console.WriteLine(n);
                Thread.Sleep(1000);
                if (n > 50)
                    cts.Cancel();
            }, new ExecutionDataflowBlockOptions{ MaxDegreeOfParallelism = 2, MaxMessagesPerTask = 1, CancellationToken = cts.Token });

            for (int i = 0; i < 10; i++)
            {
                actionBlock.Post(i*10); 
         
            }

            actionBlock.Complete();
            try
            {
                actionBlock.Completion.Wait(cts.Token);
            }
            catch (OperationCanceledException ex)
            {
                Console.WriteLine(ex.Message);
            }
        }

        private static void WriteOnceBlockParallellPostsAndSingleReceive()
        {
            var writeOnceBlock = new WriteOnceBlock<string>(null);

            Parallel.Invoke(
                () => writeOnceBlock.Post("Message 1"),
                () => writeOnceBlock.Post("Message 2"),
                () => writeOnceBlock.Post("Message 3"));

            Console.WriteLine(writeOnceBlock.Receive());
        }

        private static void BroadCastPostAndMultipleReceive()
        {
            var broadcastBlock = new BroadcastBlock<double>(null);

            broadcastBlock.Post(Math.PI);

            for (int i = 0; i < 3; i++)
            {
                Console.WriteLine(broadcastBlock.Receive());
            }
        }

        private static void BufferBlockPostingAndReceiving()
        {
            var bufferBlock = new BufferBlock<int>();

            for (int i = 0; i < 3; i++)
            {
                bufferBlock.Post(i);
            }

            for (int i = 0; i < 4; i++)
            {
                try
                {
                    Console.WriteLine(bufferBlock.Receive(new TimeSpan(0, 0, 2)));
                }
                catch (TimeoutException tie)
                {
                    Console.WriteLine("Exception of type: {0} with message: {1}", tie.GetType().Name, tie.Message);
                }
            }
        }

        private static void ActionBlockPostingAndFaulting()
        {
            var throwIfNegative = new ActionBlock<int>(n =>
            {
                Console.WriteLine("n = {0}", n);
                if (n < 0)
                    throw new ArgumentOutOfRangeException();
            });

            throwIfNegative.Completion.ContinueWith(
                task => { Console.WriteLine("The status of the completion task is '{0}'", task.Status); });

            throwIfNegative.Post(0);
            throwIfNegative.Post(-1);
            throwIfNegative.Post(1);
            throwIfNegative.Post(2);

            throwIfNegative.Complete();

            try
            {
                throwIfNegative.Completion.Wait();
            }
            catch (AggregateException ae)
            {
                ae.Handle(e =>
                {
                    Console.WriteLine("Encountered {0}: {1}", e.GetType().Name, e.Message);
                    return true;
                });
            }

          
        }
    }
}

Producer-consumer scenario with BlockingCollection of Task Parallell Library

The BlockingCollection of Task Parallel Library or TPL supports Producer-Consumer scenarios well. This is a scenario or pattern, where you want to start processing items as soon as they are available. BlockingCollection makes this easy, as long as you follow the convention. In this article, a simple scenario with five producers and one consumer is presented. We create an array of tasks (Task[]) that we wait on, using the Task.WaitAll(Task[]) method. This creates a barrier in our main method of which we call the CompleteAdding() method of BlockingCollection to signal that we have no more items to add. The consumer, which here is a single Task, will use standard foreach iteration and call the method GetConsumingEnumerable() on the BlockingCollection to get the collection to iterate over. Note that we will be inside the foreach loop until the CompleteAdding() method is called on the BlockingCollection, i.e. the iteration is halted until CompleteAdding() is called and no more items are available.

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace TestBlockingCollectionSecond
{

    public class Node
    {

        public int ManagedThreadId { get; set; }

        public int Value { get; set; }

        public override string ToString()
        {
            return string.Format("Inside ManagedThreadId {0}, Value: {1}", ManagedThreadId, Value);
        }

    }

    class Program
    {
        static void Main(string[] args)
        {

            BlockingCollection<Node> bc = new BlockingCollection<Node>();

            Console.WriteLine("Producer-consumer scenario BlockingCollection.\nFive producers, one consumer scenario");

            var rnd = new Random();

            var producers = new List<Task>();

            for (int i = 0; i < 5; i++)
            {

                var producer = Task.Factory.StartNew(() =>
                {                  
                    for (int j = 0; j < 5; j++)
                    {
                        Thread.Sleep(rnd.Next(100,300));
                        bc.Add(new Node { ManagedThreadId = Thread.CurrentThread.ManagedThreadId, Value = rnd.Next(1, 100) });
                    }
                });

                producers.Add(producer);

            }

            var consumer = Task.Factory.StartNew(() =>
            {
                foreach (Node item in bc.GetConsumingEnumerable())
                {
                    Console.WriteLine(item);
                }

                Console.WriteLine("Consumer all done!");             
            });

            Task.WaitAll(producers.ToArray());

            bc.CompleteAdding();
       

            Console.WriteLine("Hit any key to exit program");
            Console.ReadKey();         


        }
    }
}

If you are not sure when to signal CompleteAdding(), you can keep taking items from the BlockingCollection, using one single consumer or multiple, but remember to catch InvalidOperationException, in case there are no more items available, that is - after CompleteAdding() method has been called on the BlockingCollection. Note that the while loop below is wrapped in a consumer Task that is once again started before the Task.WaitAll call on the producers array, to start consuming right away. Our exit condition here is the flag bc.IsAddingCompleted is set to true, i.e. a call to CompleteAdding is performed. The benefit of using the GetConsumingEnumerable() here is having not to deal with exceptions and boolean flag of completed adding items in the consumer Block, since calling the Take() method on the collection after the CompleteAdding() method is called will throw an InvalidOperationException.

            var consumer = Task.Factory.StartNew(() => {

                try{
                while (!bc.IsAddingCompleted){
                    Console.WriteLine("BlockingCollection element: " + bc.Take());
                }
                }
                Console.WriteLine("Consumer all done!");
                catch (InvalidOperationException ioe){
                    //Console.WriteLine(ioe.Message);
                }
            });

Logging the SQL of Entity Framework exceptions in EF 6

If you use EF 6, it is possible to add logging functionality that will reveal why an exception in the data layer of your app or system occured and in addition creating a runnable SQL that you might try out in your testing/production environment inside a transaction that is rollbacked for quicker diagnosis-cause-fix cycle! First off, create a class that implements the interface IDbCommandInterceptor in the System.Data.Entity.Infrastructure.Interception namespace. This class is then added using in your ObjectContext / DbContext class (this is a usually a partial class that you can extend) using the DbInterception.Add method. I add this class in the static constructor of my factory class inside a try-catch block. The important part is that you call the DbInterception.Add method and instantiate the class you create. Let's consider a code example of this. I am only focusing on logging exceptions, other kind of interceptions can of course be performed. Here is the sample class for logging exceptions, I have replaced the namespaces of the system of mine with the more generic "Acme":

using System;
using System.Data;
using System.Data.Common;
using System.Data.Entity.Infrastructure.Interception;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using MySoftware.Common.Log;



namespace Acme.Data.EntityFramework
{
    
    /// <summary>
    /// Intercepts exceptions that is raised by the database running an operation and propagated to the eventlog for logging 
    /// </summary>
    public class AcmeDbCommandInterceptor : IDbCommandInterceptor
    {

        public void NonQueryExecuted(DbCommand command, DbCommandInterceptionContext<int> interceptionContext)
        {
            LogIfError(command, interceptionContext);
        }

        public void NonQueryExecuting(DbCommand command, DbCommandInterceptionContext<int> interceptionContext)
        {
            LogIfError(command, interceptionContext);          
        }

        public void ReaderExecuted(DbCommand command, DbCommandInterceptionContext<DbDataReader> interceptionContext)
        {
            LogIfError(command, interceptionContext);          
        }

        public void ReaderExecuting(DbCommand command, DbCommandInterceptionContext<DbDataReader> interceptionContext)
        {
            LogIfError(command, interceptionContext);
        }

        public void ScalarExecuted(DbCommand command, DbCommandInterceptionContext<object> interceptionContext)
        {
            LogIfError(command, interceptionContext);            
        }

        public void ScalarExecuting(DbCommand command, DbCommandInterceptionContext<object> interceptionContext)
        {
            LogIfError(command, interceptionContext);           
        }

        private void LogIfError<TResult>(DbCommand command, DbCommandInterceptionContext<TResult> interceptionContext)
        {
            try
            {
                if (interceptionContext != null && interceptionContext.Exception != null)
                {
                    bool isLogged = false;
                    try
                    {
                        LogInterpolatedEfQueryString(command, interceptionContext);
                        isLogged = true; 
                    }
                    catch (Exception err)
                    {
                        LogRawEfQueryString(command, interceptionContext, err);
                    }
                    if (!isLogged)
                        LogRawEfQueryString(command, interceptionContext, null);
                   
                }
            }
            catch (Exception err)
            {
                Debug.WriteLine(err.Message);
            }
        }

        /// <summary>
        /// Logs the raw EF query string 
        /// </summary>
        /// <typeparam name="TResult"></typeparam>
        /// <param name="command"></param>
        /// <param name="interceptionContext"></param>
        /// <param name="err"></param>
        private static void LogRawEfQueryString<TResult>(DbCommand command, DbCommandInterceptionContext<TResult> interceptionContext,
            Exception err)
        {
            if (err != null)
                Debug.WriteLine(err.Message);

            string queryParameters = LogEfQueryParameters(command);

            EventLogProvider.Log(
                string.Format(
                    "Acme serverside DB operation failed: Exception: {0}. Parameters involved in EF query: {1}. SQL involved in EF Query: {2}",
                    Environment.NewLine + interceptionContext.Exception.Message,
                    Environment.NewLine + queryParameters,
                    Environment.NewLine + command.CommandText
                    ), EventLogProviderEnum.Warning);
        }

        /// <summary>
        /// Return a string with the list of EF query parameters 
        /// </summary>
        /// <param name="command"></param>
        /// <returns></returns>
        private static string LogEfQueryParameters(DbCommand command)
        {
            var sb = new StringBuilder(); 
            for (int i = 0; i < command.Parameters.Count; i++)
            {
                if (command.Parameters[i].Value != null)
                    sb.AppendLine(string.Format(@"Query param {0}: {1}", i, command.Parameters[i].Value));
            }
            return sb.ToString();
        }

        private static DbType[] QuoteRequiringTypes
        {
            get
            {
                return new[]
                {
                        DbType.AnsiString, DbType.AnsiStringFixedLength, DbType.String,
                            DbType.StringFixedLength, DbType.Date, DbType.Date, DbType.DateTime,
                            DbType.DateTime2, DbType.Guid
                };
            }
        }


        private static void LogInterpolatedEfQueryString<TResult>(DbCommand command,
            DbCommandInterceptionContext<TResult> interceptionContext)
        {
            var paramRegex = new Regex("@\\d+");
            string interpolatedSqlString = paramRegex.Replace(command.CommandText,
                m => GetInterpolatedString(command, m));

            EventLogProvider.Log(string.Format(
                "Acme serverside DB operation failed: Exception: {0}. SQL involved in EF Query: {1}",
                Environment.NewLine + interceptionContext.Exception.Message,
                Environment.NewLine + interpolatedSqlString),
                EventLogProviderEnum.Warning);

        }

        private static string GetInterpolatedString(DbCommand command, Match m)
        {
            try
            {
                int matchIndex;
                if (string.IsNullOrEmpty(m.Value))
                    return m.Value;
                int.TryParse(m.Value.Replace("@", ""), out matchIndex);
                    //Entity framework will usually build parametrized queries with @1, @2 and so on .. 
                if (matchIndex < 0 || matchIndex >= command.Parameters.Count)
                    return m.Value;

                //Ok matchIndex from here 
                DbParameter dbParameter = command.Parameters[matchIndex];
                var dbParameterValue = dbParameter.Value;
                if (dbParameterValue == null)
                    return m.Value;

                try
                {
                    return GetAdjustedDbParameterValue(dbParameter, dbParameterValue);
                }
                catch (Exception err)
                {
                    Debug.WriteLine(err.Message);
                }
            }
            catch (Exception err)
            {
                Debug.WriteLine(err.Message);
            }

            return m.Value;
        }

        /// <summary>
        /// There are some cases where one have to adjust the Db Parametre value in case it is a boolean 
        /// </summary>
        /// <param name="dbParameter"></param>
        /// <param name="dbParameterValue"></param>
        /// <returns></returns>
        private static string GetAdjustedDbParameterValue(DbParameter dbParameter, object dbParameterValue)
        {
            if (QuoteRequiringTypes.Contains(dbParameter.DbType))
                return string.Format("'{0}'", dbParameterValue); //Remember to put quotes on parameter value 

            if (dbParameter.DbType == DbType.Boolean)
            {
                bool dbParameterBitValue;
                bool.TryParse(dbParameterValue.ToString(), out dbParameterBitValue);
                return dbParameterBitValue ? "1" : "0"; //BIT
            }

            return dbParameterValue.ToString(); //Default case (not a quoted value and not a bit value)
        }
    }
}

The code above uses a class EventLogProvider that will record to the Event Log of the system running the Entity Framework code in the data layer, usually the application server of your system. Here is the relevant code for logging to the Eventlog:

using System.Diagnostics;
using System;
using Acme.Common.Security;

namespace Acme.Common.Log
{
    public enum EventLogProviderEnum
    {
        Warning, Error
    }

    public static class EventLogProvider
    {
        private static string _applicationSource = "Acme";
        private static string _applicationEventLogName = "Application";

        public static void Log(Exception exception, EventLogProviderEnum logEvent)
        {
            Log(ErrorUtil.ConstructErrorMessage(exception), logEvent);
        }

        public static void Log(string logMessage, EventLogProviderEnum logEvent)
        {
            try
            {
                if (!EventLog.SourceExists(_applicationSource))
                    EventLog.CreateEventSource(_applicationSource, _applicationEventLogName);

                EventLog.WriteEntry(_applicationSource, logMessage, GetEventLogEntryType(logEvent));
            }
            catch { } // If the event log is unavailable, don't crash.
        }

        private static EventLogEntryType GetEventLogEntryType(EventLogProviderEnum logEvent)
        {
            switch(logEvent)
            {
                case EventLogProviderEnum.Error:
                    return EventLogEntryType.Error;
                case EventLogProviderEnum.Warning:
                default:
                    return EventLogEntryType.Warning;
            }
        }
    }
}

It is also necessary to add an instance of the db interception class in your ObjectContext or DbContext class as noted. Example:

try {
DbInterception.Add(new AcmeDbCommandInterceptor());
}
catch (Exception err){
 //Log error here (consider using the EventLogProvider above for example)
}

The interpolated string will often be the one that is interesting when EF queries fail. Entity Framework (EF) uses stored procedures and parameters to prevent SQL injection attacks. To get a SQL you can actually run, you will usually interpolate the EF query CommandText and look at the parameters, that is named as @0, @1, @2 and so on.. I use a Regex here to search after this. Note that my code uses a lot of try-catch in case something goes wrong. You also do NOT want to run any heavy code here, as the DbInterception will run on ANY query. I only do further processing IF an exception has occured, to avoid bogging down the system with performance drain. I also first try to get the interpolated EF query string that I can run in my Production or Test environment, usually inside a BEGIN TRAN.. and ROLLBACK statement just to see why the database call failed. In addition, the code will try to log the Raw EF Query in form of logging the EF query CommandText and the command parameters, but without the interpolation technique. I have also done some adjustment, by adding single quotes around strings and considering booleans as the value 0 or 1 (BIT). This code is new and there might be some additional adjustments here. The bottom line to note here is that it is important to LOG the EF query SQL to INFER the real REASON why the SQL query FAILED, i.e. a quicker DIAGNOSE-INFER-FIX cycle leading to more success on your projects, if you use .NET and Entity Framework (version 6 or newer)!