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The basis of a channel stack is that there is a series of objects that share a common interface for communication. This leaves message encoders with something of a problem as the primitive operations for encoding and decoding messages are different than the primitive operations for sending and receiving messages. Message encoders avoid this problem by being contained within a channel rather than acting as a peer in the channel stack. However, this is different than the model used for bindings, which make the message encoding binding element a peer of the other channel binding elements. To make things harder, bindings use a "no lookahead" construction process where constructing an object cannot speculate about future construction by looking into the unprocessed information in the binding. That results in a subtle dance between a message encoder and a channel that wants to use a message encoder. During the construction process, a binding gives you only a context for storing computed information and the ability to initiate the next phase of the construction process. When we go to build a message encoder, it's not possible to actually build anything because the channel that will hold the message encoder doesn't exist yet. The message encoder doesn't know how or why it is going to be used. Instead, what a message encoding binding element does to build is add itself to the BindingParameters in the context. The message encoding binding element then initiates the next phase of the construction process. At some point in the future, a channel may look at the context to see if it contains a previously saved binding element, which can now be built. The delay in instantiation is what allows channels to have a different order for physical containment than the logical order of the binding elements. Next time: Quotas for Copying Messages Read More...
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I've created a custom implementation of GetProperty for my binding but now I'm getting errors when I go to use the channels. Why is the validation for these channels failing? This is an implementation problem that I've talked about in the past. There is a requirement that the values queried from design time objects, such as bindings and binding elements, match the values queried from runtime objects, such as a channels. If the two don't agree on a property value, then you can experience problems ranging from an error creating the channel to mysterious failures sending and receiving messages. This problem most commonly happens when you override an existing property value on the binding but forget to override the property in the same way on the channels. Use this picture illustrating GetProperty to follow the chain of property values. However, this problem can also happen even when you're not overriding an existing value. If you write a GetProperty implementation but forget to delegate to the inner channel or binding element, then the chain of GetProperty calls terminates right there. Any further channels or binding elements do not get to contribute to GetProperty evaluations. This can easily cause a discrepancy between the design time and runtime values of a property despite the fact that you didn't explicitly modify that property. In most cases, this is a severe enough problem to fail very quickly. If you've written a GetProperty method and are suddenly seeing property value mismatches for properties that you didn't touch though, then make sure that your GetProperty method is delegating any unhandled properties to the right place. Next time: Preventing Anonymous Access Read More...
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The story from yesterday: there is an important setting for composite duplex that is only settable through the binding context. Unfortunately, proxy clients automatically create and use their own binding context so there is no convenient time to poke in a replacement value for the setting. What kind of a workaround can we come up with? Since there is no convenient way to intercept the binding context at the producer, we'll instead need to intercept the binding context at the consumer. The consumer of the binding context is the channel construction process of the binding. In particular, we need to modify the ListenUri setting before we reach the BuildChannelListener method of the composite duplex binding element. That means our opportunities for interception are going to be in methods such as CanBuildChannelFactory, CanBuildChannelListener, BuildChannelFactory, and BuildChannelListener. The binding context instance gets passed along from binding element to binding element so it is safe to modify the setting at any opportunity along the way. We don't have to worry about finding the correct instance of the binding context. Every instance that we see is one that we want to modify. The solution that I came up with for this problem was to write a binding element that modified the ListenUri as the binding context went by. Other than this change, the binding element simply delegates every method to the next binding element down. You can position the binding element in the channel stack so that your customization is always reached before the composite duplex binding element. The binding context that composite duplex receives contains your modifications, giving you full control over the ListenUri setting that was otherwise hidden. This solution is generally reusable for other situations where you need to modify the binding context or channel construction process. I won't say that there are many such situations, but it is a tool for you to use. An interesting coincidence about this problem was that two different people on consecutive days with different scenarios were looking for a solution. Mike Taulty was the second person and he's done a write up of why he was interested in composite duplex and the code I gave him for the ListenUri modifying binding element . Mike added configuration support, which I hadn't bothered with. From configuration, you'll need to build out a complete custom binding. From code, it's simpler because you can start with an existing binding and Read More...
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This is just a style convention that helps you avoid doing some thinking while writing custom channel classes. During the channel construction process, there's a flow of information from the binding (design time), through the channel factory and listener, and down to the channel (run time). The channel needs to know about the configuration changes that the user has made in setting up the binding. As we've seen before, there's some difficulty with storing this information outside the channel , so we can't rely on referencing the information indirectly through another object after creation. Here's the convention that we've sort of settled on in most of our channels. When creating the channel factory or listener (jointly referred to as the channel manager), the constructor takes two arguments. The constructor gets the this instance of the binding element that is building the channel manager. Inside the constructor, the configuration settings of the binding element are copied to private fields of the channel manager. The channel manager doesn't hold on to the binding element after initialization is complete. The constructor gets the binding context instance that the binding element was passed. The binding context instance allows the channel manager to fish out the inner channel manager and access binding parameters. When creating the channel, the constructor takes as many arguments as it needs to pass all of the private data that was being stored in the channel manager. It can be helpful to group some of these arguments using a specializeed interface if you need to pass around the same information to a lot of different places. The channel manager does not give the this instance to the channel normally. An exception is if you're using the ChannelBase class for your channel, which requires an instance of ChannelManagerBase. In that case, the channel manager gives its this instance to the channel solely for that purpose. The channel constructor takes that instance as a ChannelManagerBase rather than using a specialized type. Next time: Mixed Mode Addressing Read More...
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I drew this picture for myself while working on the guidelines for implementing GetProperty . I figured that other people might find it useful as well. One of the guidelines is that queryable properties on design time objects should flow to the corresponding run time objects. The black boxes show the classes that support GetProperty and the black arcs show the anticipated flow of properties. The red boxes show the classes that don't support GetProperty today but might make sense to have support in the future. Read More...
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This post is just some quick thinking about guidelines for implementing the GetProperty method. These guidelines are still in development so think of this as a draft rather than real guidance at this time. Background: We provide an extensibility point called GetProperty on many of the components you can build using the channel model. The exact list of components that support GetProperty are bindings, binding elements, channel factories, channel listeners, channels, and message encoders. GetProperty allows someone to execute a strongly-typed query against your component to inspect its configuration settings. GetProperty takes a type parameter T and either returns some instance of T or returns default(T) to indicate that the type is unknown. DO expose configurable settings for your component using GetProperty. GetProperty should be used in preference to ad-hoc methods for searching through a stack of components. DO NOT require the use of GetProperty to access mandatory or intrinsic configuration settings. The types supported by GetProperty are not discoverable and any setting that your component considers mandatory should be accessible through the object model. GetProperty can be used as an alternative way of accessing the same value. DO support in your runtime objects every property that was available on your design time objects. If a channel factory exposes a setting, then every channel produced by that factory should expose a corresponsing setting that tells you what the value was when the channel was created. DO create specialized interfaces that capture each class of information that you want people to query on. Specialized interfaces promote the reuse of a particular type for semantically identical settings. DO reuse existing interfaces if your information has exactly the same semantics as the previous use. Conversely, if your settings don't have the same semantics, don't reuse a type just because it's more convenient to do so. DO NOT key properties off of commonly used types with a generic meaning. Types that appear frequently, such as IDisposable, will not have consistent semantics for all of their potential uses. DO NOT key properties off of value or enumerated types. We enforce this by placing a restriction on the generic type parameter. DO delegate down to inner channels, channel factories, and channel listeners when they exist and your component does not know about the requested type. We have a pipeline model that supports the composition of many Read More...
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