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Lesson 2: Working with Instances
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Lesson 2: Working with Instances
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The instance mode WCF uses is just the start of working with instances. You can manipulate a number of details to improve the performance and scalability of a WCF service. WCF provides throttling and quota capabilities that can help prevent denial of service (DoS) attacks as well as ensure that the servers aren t overloaded by handling requests. Along the same lines, you can control the activation and deactivation of the instances used to process requests to a degree that is finer than the default functionality. Not only does WCF allow for performance to be protected, some attributes can be set to demarcate operations. The demarcation ensures that, where necessary, some operations cannot be completed before or after other operations. This is not a complete workflow management function, but it does allow a service to ensure that a particular operation is called first and that no operations can be called after a finalize operation has been performed.
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After this lesson, you will be able to: Protect a WCF service by setting the throttling and quota parameters. Demarcate service operations. Manage instance activation and deactivation at a very granular level. Estimated lesson time: 50 minutes
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Protecting the Service
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When WCF is deployed in the real world (where real is defined as a distributed environment in which requests arrive at a pace that is outside of your control), a number of potential problems can arise. Some of the performance differences associated with the different instancing modes have already been covered. However, beyond pure performance problems, WCF services have to contend with some of the same problems that a Web site has to contend with. This includes the potential for being flooded with client requests, similar to a denial of service attack. Denial of service attacks are attempts to deplete the resources required by the service to process incoming requests to the point that no additional resources are available. The type of depleted resources can include any scarce resource the service uses. WCF provides a number of ways to mitigate the problem through either throttling requests or applying quotas to the resource.
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Throttling
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The goal of throttling is twofold. First, it prevents the service host from being overrun by a flood of requests. Second, it enables the load on the WCF service (and the server on which the
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Sessions and Instancing
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service is running) to be smoothed out. In both cases, the intent is to place a limit on the number of incoming requests so that the service will be able to handle them in a timely manner. The default WCF setting for throttling is to have none at all. When throttling is engaged, WCF will check the current counters for each request that arrives. If the configured settings are exceeded, WCF automatically places the request in a queue. As the counters come down below the threshold, the requests are then retrieved from the queue in the same order and presented to the service for processing. The result of this is that, in many cases, the observed behavior for a service that has reached its maximum is to have the client request time out. Three settings in the service behavior control the number of requests the service host will be allowed to process simultaneously. Each of these is defined in the ServiceThrottlingBehavior section of the configuration file. The following paragraphs describe the three settings and are followed by an example of how you can configure them. MaxConcurrentCalls The MaxConcurrentCalls value specifies the number of simultaneous calls the service will accept. The default value is 16 calls. Of the three settings, this is the only one that covers all the types of requests that arrive. MaxConcurrentSessions The MaxConcurrentSessions value determines the maximum number of channels requiring sessions that the service will support. The default value for this setting is 10 session-aware channels. Any attempt to create a channel beyond this maximum will throw a TimeoutException. Because this setting is concerned with session-aware channels only, if the binding is not session-aware (such as the basicHttpBinding), this setting has no impact on the number of requests that can be processed. MaxConcurrentInstances The MaxConcurrentInstances setting sets the maximum number of instances of the service implementation object that will be created. The default value for this setting is Int32.MaxValue, and the impact this value has on the service depends on the mode. If the mode is per call, this is the same as MaxConcurrentCalls because each call gets its own instance. If the mode is per session, the setting works the same as MaxConcurrentSessions. For singleton mode, the value of the number of instances is always 1, so the setting is really only useful when the IInstanceContextProvider is being used. The following segment from a configuration file demonstrates how you can configure these settings:
<behaviors> <serviceBehaviors> <behavior name="throttlingBehaviort"> <serviceThrottling maxConcurrentCalls="10" maxConcurrentInstances="10" maxConcurrentSessions="5"/> </behavior> </serviceBehaviors> </behaviors>
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