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WCF Concurrency (Single, Multiple, and Reentrant) and Throttling

, 4 Mar 2013 CPOL
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An article on WCF concurrency and throttling.

Table of contents

Introduction and goal

In this article, we will concentrate on WCF concurrency and throttling. We will first try to understand what is WCF concurrency and the three important types of WCF concurrency. We will then see a small sample of WCF concurrency with Single and Multiple. We will then go through 9 combinations of WCF concurrency and instancing. Finally we will try to understand how to configure throttling using the WCF ‘web.config’ file.




In order to understand WCF concurrency well, it’s important to understand the concepts around WCF instancing. So before moving ahead with this article, please do read about WCF instancing from here.

Why do we need concurrency in WCF?

If you search on the web for the dictionary meaning of concurrency, you will find the following definition:

“Happening at the same time”

WCF concurrency helps us configure how WCF service instances can serve multiple requests at the same time. You will need WCF concurrency for the below prime reasons; there can be other reasons as well but these stand out as important reasons:

  • Increase throughput: Many times you want to increase the amount of work your WCF service instance does at any moment of time. In other words, you would like to increase the throughput. Throughput means how much work a given thing can do.
  • By default, a WCF service handles only one request at a given moment of time.

  • Integration with a legacy system: Many times your WCF services interact with legacy systems like VB6, COM, etc. It’s very much possible that these systems are not multithreaded, in other words they handle only one request at any given time. So even though your WCF service has concurrent capabilities, you would still like to handle one request at a time. This is achieved by using throttling in combination with WCF concurrency capabilities.

WCF concurrency and instancing – Two different things

In our previous article, we had discussed about WCF instances. WCF instancing and WCF concurrency are two different things. WCF instance dictates how objects are created while WCF concurrency dictates how many requests can be handled by WCF objects.

I do understand that many of our developer friends know this difference, but as you go deeper into WCF concurrency, there is lot of connection with WCF instancing also, so I wanted to just emphasize the differences.

WCF instancing gives three levels of WCF object instance controls: per call, per session, and single. In case you are not aware of this, read my article: 3 way to do WCF instance management. In similar lines, WCF also has three ways of implementing WCF concurrency.

Three types of WCF concurrency

There are three ways by which you can handle concurrency in WCF: single, multiple, and reentrant. To specify WCF concurrency, we need to use the ServiceBehavior tag as shown below, with the appropriate ‘ConCurrencyMode’ property value.

Single: A single request has access to the WCF service object at a given moment of time. So only one request will be processed at any given moment of time. The other requests have to wait until the request processed by the WCF service is completed.

Multiple: In this scenario, multiple requests can be handled by the WCF service object at any given moment of time. In other words, requests are processed at the same time by spawning multiple threads on the WCF server object. So you have great throughput here but you need to ensure concurrency issues related to WCF server objects.

Reentrant: A single request thread has access to the WCF service object, but the thread can exit the WCF service to call another WCF service or can also call a WCF client through callback and reenter without deadlock.

Sample code demonstration

In order to demonstrate this, let’s create a simple sample code as shown below. We will create a simple WCF service with a method name Call. When a client calls this WCF service, it will display the following details:

  • Client name that made the call. This value will be provided as an input when the client wants to make a call to the WCF service.
  • Instance number, this will represent the number of WCF instances which are serving the request.
  • Thread number which is executing the method.
  • Time when the Call method was actually called.

Below is the service contract of the Call method. Please note that OperationContract is defined with IsOneWay as true.

public interface IHelloWorldService
    void Call(string ClientName);

Below is a simple implementation of the service contract IHelloWorldService interface. It has an instance variable i which helps us maintain the instance counter and a simple Console.Writeline which displays the client name, instance number, thread number, and time when the method was called.

This method waits for 5 seconds using the ‘Thread.Sleep’ function.

public class HelloWorldService : IHelloWorldService
    // maintain instance count 
    public int i;
    public void Call(string ClientName)
        // increment instance counts

        // display client name, instance number , thread number and time when 
        // the method was called

        Console.WriteLine("Client name :" + ClientName + " Instance:" + 
          i.ToString() + " Thread:" + Thread.CurrentThread.ManagedThreadId.ToString() + 
          " Time:" + DateTime.Now.ToString() + "\n\n");

        // Wait for 5 seconds

This WCF service will be self hosted using WsHttpBinding as shown below. We have chosen self hosting so that we can monitor the time, threads, and number of instances of the WCF service.

static void Main(string[] args)
    //Create a URI to serve as the base address
    Uri httpUrl = new Uri("http://localhost:8010/MyService/HelloWorld");

    //Create ServiceHost
    ServiceHost host = new ServiceHost(typeof(ClassLibrary1.HelloWorldService), httpUrl); 

    //Add a service endpoint
    host.AddServiceEndpoint(typeof(ClassLibrary1.IHelloWorldService), new WSHttpBinding(), "");

    //Enable metadata exchange
    ServiceMetadataBehavior smb = new ServiceMetadataBehavior();
    smb.HttpGetEnabled = true;

    //Start the Service

    Console.WriteLine("Service is host at " + DateTime.Now.ToString());
    Console.WriteLine("Host is running... Press <Enter> key to stop");

From the client side, we will make five continuous calls simultaneously on the WCF service. Below is the code snippet for this:

Console.WriteLine("Enter Client name");
string str = Console.ReadLine();
ServiceReference1.HelloWorldServiceClient obj = 
             new ServiceReference1.HelloWorldServiceClient();

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

If you run the above project and monitor your WCF server service, you will see the screenshot. There are two important points to note:

  • When you run using per call new instances are created every time the service runs. Instance 1 indicates new instances created for every method call.
  • By default the concurrency is single so the same thread is used to service all five requests which are sent from the WCF client side. You can see the value of the thread is always 4.
  • The most important factor is time. As concurrency is configured as single, it will be called one after another sequentially. You can notice this from the time difference of method calls of 5 seconds.

Let’s go and change the concurrency mode to multiple. In order to change the concurrency mode to multiple, we need to specify Multiple as the concurrency mode as shown in the below code snippet.

If you run the client now, you can see different threads (i.e., thread 4, 5, 6, 7, and 8) are spawned to serve the request and the time of the method calls are almost same. In other words, the methods are executed concurrently on different threads.

In short, you are now having a higher throughput in less amount of time.

9 combinations of Instancing and Concurrency

There are 9 combinations of concurrency and instancing as shown in the below table. In the sections below, we will discuss this in more detail.

Single (Default) Multiple Reentrant
(Single instance for all clients)
Single thread for all clients. Multiple threads for all clients. Single threads for all clients, locks are released when calls diverted to other WCF services.
PerSession (Default)
(Multiple instance per client)
Single thread for every client. Multiple threads for every request. Single threads for all clients, locks are released when calls diverted to other WCF services.
PerCall (Multiple instances for every method call) Single thread for every client. Single thread for every client Single threads for all clients, locks are released when calls diverted to other WCF services.

Instance mode = Per Call and Concurrency = Single

With Per Call, new WCF instances are created for every method call made to the WCF server service. The default concurrency is Single so only one thread will be used to serve all instances.

Below is a simple pictorial representation of what will happen in Per Call and Single concurrency scenarios:

  • For every client instance, a single thread will be allocated.
  • For every method call, a new service instance will be created.
  • A single thread will be used to serve all WCF instances generated from a single client instance.

If you refer the previous sample, you can see how threads are the same and the halt of 5 seconds on the WCF service.

Instance mode = Per Call and Concurrency = Multiple

                 ConcurrencyMode = ConcurrencyMode.Multiple)]
public class HelloWorldService : IHelloWorldService

In this combination, multiple instances are created for every call but multiple threads serve every method call to a WCF service instance. You can see in the below figure we have two WCF service instances and every instance has multiple WCF service objects created for every method call. Every method call is handled by multiple threads.

In the above sample, if you remember, we have multiple threads serving every method call and the time difference between every method call is reduced considerably. The method calls are fired approximately at the same time.

Instance mode = Per Session and Concurrency = Single

                 ConcurrencyMode = ConcurrencyMode.Single)]
public class HelloWorldService : IHelloWorldService

In this combination, one WCF service instance is created for every WCF client session because the WCF instance mode is set to per session. All the method calls are executed in a sequential manner one by one. In other words, only one thread is available for all method calls for a particular service instance.

If you run the sample code with Per Session and Single combination mode, you will find the same thread executes all requests with the same WCF instance per session.

Instance mode = Per Session and Concurrency = Multiple

                 ConcurrencyMode = ConcurrencyMode.Multiple)]
public class HelloWorldService : IHelloWorldService

Instance mode ‘PerSession’ concurrency is the default setting in WCF

In this combination, one WCF instance is created for every WCF client session and every method call is run over multiple threads. Below is a pictorial representation:

If you run the sample code attached with this article, you will find the same instance with every method call running on different methods.

To get a better idea, you can run different client exe instances with different names as shown below. You will notice how every client gets its own WCF service instance with every method allocated to run on different threads.

Instance mode = Single and Concurrency = Single

                 ConcurrencyMode = ConcurrencyMode.Single)]
public class HelloWorldService : IHelloWorldService

In this combination, only one instance of a WCF service instance is created which serves all requests which are sent from all WCF clients. These requests are served using only one thread.

You can see in the below figure approximately one thread is assigned for every WCF client call and only one instance of the WCF service is created.

Instance mode = Single and Concurrency = Multiple

                 ConcurrencyMode = ConcurrencyMode.Multiple)]
public class HelloWorldService : IHelloWorldService

In this combination, one WCF service instance is created for serving all WCF clients. All requests are served using multiple / different threads.

You can see from the output that we have 6 threads serving 10 requests, as compared to previously we had approximately only two threads.


In this mode, only one thread runs to serve all requests. If your WCF service makes an outbound call to some other WCF service, or makes a client call, it releases the thread lock. In other words, until the outbound call is completed, other WCF clients can not make calls.

Throttling behavior

WCF throttling helps you to put an upper limit on the number of concurrent calls, WCF instances, and concurrent sessions. WCF provides three ways by which you can define upper limits: MaxConcurrentCalls, MaxConcurrentInstances, and MaxConcurrentSessions.

  • MaxConcurrentCalls: Limits the number of concurrent requests that can be processed by WCF service instances.
  • MaxConcurrentInstances: Limits the number of service instances that can be allocated at a given time. When it’s PerCall services, this value matches the number of concurrent calls. For PerSession services, this value equals the number of active session instances. This setting doesn’t matter for Single instancing mode, because only one instance is ever created.
  • MaxConcurrentSessions: Limits the number of active sessions allowed for the service.

All the above three settings can be defined in the servicebehaviors tag as shown in the below XML snippet:


<behavior name="serviceBehavior"> 

<serviceThrottling maxConcurrentCalls="16"
   maxConcurrentInstances="2147483647" maxConcurrentSessions="10" /> 



Default values for WCF throttling

Below is a simple table which shows the default settings for throttling for different WCF versions:

MaxConcurrentSessions MaxConcurrentCalls  MaxConcurrentInstances 
WCF 3.0 / 3.5 6 26 10
WCF 4.0 16 * processorcount MaxConcurrentCalls MaxConcurrentCalls
MaxConcurrentSessions 100 * processorcount
100 * processorcount

Finally do not forget to visit my site which covers 500 .NET interview questions and answers on various topics. 


Other articles written by me on WCF

You can refer to some of my other articles written on WCF:


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Shivprasad koirala
India India

I am a Microsoft MVP for ASP/ASP.NET and currently a CEO of a small
E-learning company in India. We are very much active in making training videos ,
writing books and corporate trainings. Do visit my site for 
.NET, C# , design pattern , WCF , Silverlight
, LINQ , ASP.NET , ADO.NET , Sharepoint , UML , SQL Server  training 
and Interview questions and answers

Comments and Discussions

QuestionYet another screenshot question PinmemberMember 377413518-Mar-15 3:52 
QuestionIs simultaneous call correct? PinmemberMember 838475017-Jan-15 21:30 
AnswerRe: Is simultaneous call correct? PinmemberMember 377413518-Mar-15 3:43 
Questionwrong screenshot for Instance mode = Per Session and Concurrency = Multiple PinmemberGyozo Kudor19-Dec-14 0:48 
AnswerRe: wrong screenshot for Instance mode = Per Session and Concurrency = Multiple Pinmembersaxenaabhi619-Jan-15 15:15 
QuestionQuestion about "Instance mode = Per Call and Concurrency = Single" PinmemberJuanReyes8423-Oct-14 20:00 
AnswerRe: Question about "Instance mode = Per Call and Concurrency = Single" Pinmembersaxenaabhi619-Jan-15 15:20 
QuestionMy vote is 5 for this beautiful Article Pinmembernomi ali 905418-Oct-14 23:49 
GeneralMy vote of 5 [modified] PinmemberDiego Parolin16-Oct-14 23:06 
GeneralMy vote of 5 Pinmemberluis r carrizo8-Aug-14 13:07 
GeneralExcellent article!! Pinmemberluis r carrizo8-Aug-14 13:06 
GeneralMy vote of 5 PinprofessionalNormDroid28-Mar-14 2:47 
QuestionGood Article PinprofessionalNormDroid28-Mar-14 2:47 
QuestionThis article is right on! PinmemberDave Difilippo27-Mar-14 10:13 
Questioncongratulation, one little mistake though [modified] PinmemberNockawa26-Jan-14 14:41 
QuestionConcurrency Problem PinmemberMuhammad Adeel Sheikh26-Nov-13 1:54 
QuestionWCF 4.0 Pinmembersundarsrini13-Sep-13 13:59 
Questionwcf 4.0 Pinmembersundarsrini13-Sep-13 13:55 
AnswerRe: wcf 4.0 Pinmembersundarsrini13-Sep-13 13:56 
GeneralMy vote of 3 Pinmemberjoy_live11-Sep-13 1:23 
GeneralReentrant description seems wrong [modified] PinmemberDeepakAggarwal.Net16-Jun-13 20:58 
GeneralMy vote of 5 PinmemberMember 32777605-Jun-13 2:59 
GeneralMy vote of 4 Pinmembercore.s.logic25-May-13 21:40 
GeneralMy vote of 5 Pinmembervendettamit16-Apr-13 23:00 
GeneralMy vote of 5 Pinmemberholylust13-Apr-13 5:23 
GeneralMy vote of 5 PinmemberdgDavidGreene6-Mar-13 19:49 
SuggestionWrong table header for Default values for WCF throttling Pinmembert@rn27-Feb-13 18:06 
GeneralMessage Removed Pinmemberkiquenet.com4-Mar-13 21:15 
GeneralRe: Wrong table header for Default values for WCF throttling PinmvpShivprasad koirala4-Mar-13 21:35 
QuestionWrong screenshot PinmemberAndrew Sirotenko15-Jan-13 13:34 
The screenshot in case PerSession/Multiple case seems wrong because only one instanse of service was created for serving 2 different clients.
But the article describes all the concurrency aspects as best as it could be. My thanks to the author.
GeneralMy vote of 5 PinmemberTejas Vaishnav19-Dec-12 0:23 
GeneralMy vote of 5 Pinmemberthalaiselvam6-Dec-12 23:44 
QuestionSingle Instance, Multiple concurrency... A question on channel states PinmemberLuca Dominici20-Sep-12 6:53 
QuestionTimeout Exception? PinmemberAmin J. Ismaili15-Aug-12 5:55 
QuestionConcurrencyMode in web.config? Pinmembertsp4axl11-Jul-12 11:26 
GeneralMy vote of 5 Pinmembermytroshyn21-Jun-12 10:54 
QuestionWhy havn't the difrrent result with TCP Channel? Pinmembercl.kcitwm6-Jun-12 22:41 
Question5 PinmemberStephen Hewison 224-May-12 4:06 
GeneralMy vote of 5 Pinmembermanoj kumar choubey16-Apr-12 4:12 
QuestionQ: Combination of Per Session and Multiple. Pinmemberkunal_g11-Apr-12 3:45 
GeneralI got the same question and your opinion perfectly makes sense for me. [EOM] PinmemberScott_Rhee24-Jun-13 20:38 
QuestionVery nice & Useful topic... Pinmemberykiranbabu8-Apr-12 7:35 
GeneralNice topic. PinmemberAbhijit Roy9915-Mar-12 2:27 
GeneralMy vote of 5 PinmemberSanek111115-Mar-12 9:00 
GeneralMy vote of 5 PinmemberSledgeHammer012-Mar-12 11:49 
QuestionSame instance? PinmvpPaulo Zemek2-Mar-12 6:29 
GeneralMy vote of 5 Pinmemberashutosh k. shukla1-Mar-12 21:35 
QuestionDefault InstanceContextMode is PerSession NOT PerCall PinmemberMember 82372731-Mar-12 13:01 
GeneralMy vote of 5 PinmemberRajeshjoseph8-Feb-12 8:17 
QuestionMax concurrent wcf calls from silverlight Pinmemberaashish3213-Feb-12 5:49 

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