ConcurrencyMode Enum
Definition
Important
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Specifies whether a service class supports single-threaded or multi-threaded modes of operation.
public enum class ConcurrencyMode
public enum ConcurrencyMode
type ConcurrencyMode =
Public Enum ConcurrencyMode
- Inheritance
Fields
Multiple | 2 | The service instance is multi-threaded. No synchronization guarantees are made. Because other threads can change your service object at any time, you must handle synchronization and state consistency at all times. |
Reentrant | 1 | The service instance is single-threaded and accepts reentrant calls. The reentrant service accepts calls when you call another service; it is therefore your responsibility to leave your object state consistent before callouts and you must confirm that operation-local data is valid after callouts. Note that the service instance is unlocked only by calling another service over a WCF channel. In this case, the called service can reenter the first service via a callback. If the first service is not reentrant, the sequence of calls results in a deadlock. For details, see ConcurrencyMode. |
Single | 0 | The service instance is single-threaded and does not accept reentrant calls. If the InstanceContextMode property is Single, and additional messages arrive while the instance services a call, these messages must wait until the service is available or until the messages time out. |
Examples
The following code example demonstrates the different between using Single, Reentrant, and Multiple. This sample does not compile without a real implementation behind it, but does demonstrate the kind of threading guarantees that WCF makes and what that means for your operation code.
using System;
using System.ServiceModel;
[ServiceContract]
public interface IHttpFetcher
{
[OperationContract]
string GetWebPage(string address);
}
// These classes have the invariant that:
// this.slow.GetWebPage(this.cachedAddress) == this.cachedWebPage.
// When you read cached values you can assume they are valid. When
// you write the cached values, you must guarantee that they are valid.
// With ConcurrencyMode.Single, WCF does not call again into the object
// so long as the method is running. After the operation returns the object
// can be called again, so you must make sure state is consistent before
// returning.
[ServiceBehavior(ConcurrencyMode = ConcurrencyMode.Single)]
class SingleCachingHttpFetcher : IHttpFetcher
{
string cachedWebPage;
string cachedAddress;
readonly IHttpFetcher slow;
public string GetWebPage(string address)
{
// <-- Can assume cache is valid.
if (this.cachedAddress == address)
{
return this.cachedWebPage;
}
// <-- Cache is no longer valid because we are changing
// one of the values.
this.cachedAddress = address;
string webPage = slow.GetWebPage(address);
this.cachedWebPage = webPage;
// <-- Cache is valid again here.
return this.cachedWebPage;
// <-- Must guarantee that the cache is valid because we are returning.
}
}
// With ConcurrencyMode.Reentrant, WCF makes sure that only one
// thread runs in your code at a time. However, when you call out on a
// channel, the operation can get called again on another thread. Therefore
// you must confirm that state is consistent both before channel calls and
// before you return.
[ServiceBehavior(ConcurrencyMode = ConcurrencyMode.Reentrant)]
class ReentrantCachingHttpFetcher : IHttpFetcher
{
string cachedWebPage;
string cachedAddress;
readonly SlowHttpFetcher slow;
public ReentrantCachingHttpFetcher()
{
this.slow = new SlowHttpFetcher();
}
public string GetWebPage(string address)
{
// <-- Can assume that cache is valid.
if (this.cachedAddress == address)
{
return this.cachedWebPage;
}
// <-- Must guarantee that the cache is valid, because
// the operation can be called again before we return.
string webPage = slow.GetWebPage(address);
// <-- Can assume cache is valid.
// <-- Cache is no longer valid because we are changing
// one of the values.
this.cachedAddress = address;
this.cachedWebPage = webPage;
// <-- Cache is valid again here.
return this.cachedWebPage;
// <-- Must guarantee that cache is valid because we are returning.
}
}
// With ConcurrencyMode.Multiple, threads can call an operation at any time.
// It is your responsibility to guard your state with locks. If
// you always guarantee you leave state consistent when you leave
// the lock, you can assume it is valid when you enter the lock.
[ServiceBehavior(ConcurrencyMode = ConcurrencyMode.Multiple)]
class MultipleCachingHttpFetcher : IHttpFetcher
{
string cachedWebPage;
string cachedAddress;
readonly SlowHttpFetcher slow;
readonly object ThisLock = new object();
public MultipleCachingHttpFetcher()
{
this.slow = new SlowHttpFetcher();
}
public string GetWebPage(string address)
{
lock (this.ThisLock)
{
// <-- Can assume cache is valid.
if (this.cachedAddress == address)
{
return this.cachedWebPage;
// <-- Must guarantee that cache is valid because
// the operation returns and releases the lock.
}
// <-- Must guarantee that cache is valid here because
// the operation releases the lock.
}
string webPage = slow.GetWebPage(address);
lock (this.ThisLock)
{
// <-- Can assume cache is valid.
// <-- Cache is no longer valid because the operation
// changes one of the values.
this.cachedAddress = address;
this.cachedWebPage = webPage;
// <-- Cache is valid again here.
// <-- Must guarantee that cache is valid because
// the operation releases the lock.
}
return webPage;
}
}
Remarks
ConcurrencyMode is used in conjunction with the ConcurrencyMode property to specify whether a service class supports single-threaded or multi-threaded modes of operation. A single-threaded operation can be either reentrant or non-reentrant.
The following table shows when Windows Communication Foundation (WCF) permits an operation to be invoked while another one is in progress, depending upon the ConcurrencyMode.
ConcurrencyMode Value | Can a new operation be invoked? |
---|---|
Single | Never. |
Reentrant | Only while invoking another service or a callback. |
Multiple | Always. |