April 4, 2011, 1:47 a.m.
posted by orel
There are multiple aspects of communication with any given service, and there are many possible communication patterns: messages can be synchronous request/reply or asynchronous fire-and-forget; messages can be bidirectional; messages can be delivered immediately or queued; and the queues can be durable or volatile. There are many possible transport protocols for the messages, such as HTTP (or HTTPS), TCP, P2P (peer network), IPC (named pipes), or MSMQ. There are a few possible message encoding options: you can chose plain text to enable interoperability, binary encoding to optimize performance, or MTOM (Message Transport Optimization Mechanism) for large payloads. There are a few options for securing messages: you can choose not to secure them at all, to provide transport-level security only, to provide message-level privacy and security, and of course there are numerous ways for authenticating and authorizing the clients. Message delivery might be unreliable or reliable end-to-end across intermediaries and dropped connections, and the messages might be processed in the order they were sent or in the order they were received. Your service might need to interoperate with other services or clients that are only aware of the basic web service protocol, or they may be capable of using the score of WS-* modern protocols such as WS-Security and WS-Atomic Transactions. Your service may need to interoperate with legacy clients over raw MSMQ messages, or you may want to restrict your service to interoperate only with another WCF service or client.
If you start counting all the possible communication and interaction options, the number of permutations is probably in the tens of thousands. Some of those choices may be mutually exclusive, and some may mandate other choices. Clearly, both the client and the service must be aligned on all these options in order to communicate properly. Managing this level of complexity adds no business value to most applications, and yet the productivity and quality implications of making the wrong decisions are severe.
To simplify these choices and make them more manageable, WCF groups together a set of such communication aspects in bindings. A binding is merely a consistent, canned set of choices regarding the transport protocol, message encoding, communication pattern, reliability, security, transaction propagation, and interoperability. Ideally, you would extract all these "plumbing" aspects out of your service code and allow the service to focus solely on the implementation of the business logic. Binding enables you to use the same service logic over drastically different plumbing.
You can use the WCF-provided bindings as is, you can tweak their properties, or you can write your own custom bindings from scratch. The service publishes its choice of binding in its metadata, enabling clients to query for the type and specific properties of the binding because the client must use the exact same binding values as the service. A single service can support multiple bindings on separate addresses.
The Standard Bindings
WCF defines nine standard bindings:
Offered by the BasicHttpBinding class, this is designed to expose a WCF service as a legacy ASMX web service, so that old clients can work with new services. When used by the client, this binding enables new WCF clients to work with old ASMX services.
Offered by the NetTcpBinding class, this uses TCP for cross-machine communication on the intranet. It supports a variety of features, including reliability, transactions, and security, and is optimized for WCF-to-WCF communication. As a result, it requires both the client and the service to use WCF.
Peer network binding
Offered by the NetPeerTcpBinding class, this uses peer networking as a transport. The peer network-enabled client and services all subscribe to the same grid and broadcast messages to it. Peer networking is beyond the scope of this book since it requires an understanding of grid topology and mesh computing strategies.
Offered by the NetNamedPipeBinding class, this uses named pipes as a transport for same-machine communication. It is the most secure binding since it cannot accept calls from outside the machine and it supports a variety of features similar to the TCP binding.
Web Service (WS) binding
Offered by the WSHttpBinding class, this uses HTTP or HTTPS for transport, and is designed to offer a variety of features such as reliability, transactions, and security over the Internet.
Federated WS binding
Offered by the WSFederationHttpBinding class, this is a specialization of the WS binding, offering support for federated security. Federated security is beyond the scope of this book.
Duplex WS binding
Offered by the WSDualHttpBinding class, this is similar to the WS binding except it also supports bidirectional communication from the service to the client as discussed in Chapter 5.
Offered by the NetMsmqBinding class, this uses MSMQ for transport and is designed to offer support for disconnected queued calls. Using this binding is the subject of Chapter 9.
MSMQ integration binding
Offered by the MsmqIntegrationBinding class, this converts WCF messages to and from MSMQ messages, and is designed to interoperate with legacy MSMQ clients. Using this binding is beyond the scope of this book.
Format and Encoding
Each of the standard bindings uses different transport and encoding, as listed in Figure.
Having a text-based encoding enables a WCF service (or client) to communicate over HTTP with any other service (or client) regardless of its technology. Binary encoding over TCP or IPC yields the best performance but at the expense of interoperability, by mandating WCF-to-WCF communication.
Choosing a Binding
Choosing a binding for your service should follow the decision-activity diagram shown in Figure.
Choosing a binding
The first question you should ask yourself is whether your service needs to interact with non-WCF clients. If the answer is yes, and if the client is a legacy MSMQ client, choose the MsmqIntegrationBinding that enables your service to interoperate over MSMQ with such a client. If you need to interoperate with a non-WCF client and that client expects basic web service protocol (ASMX web services), choose the BasicHttpBinding, which exposes your WCF service to the outside world as if it were an ASMX web service (that is, a WSI-basic profile). The downside is that you cannot take advantage of most of the modern WS-* protocols. However, if the non-WCF client can understand these standards, choose one of the WS bindings, such as WSHttpBinding, WSFederationBinding, or WSDualHttpBinding. If you can assume that the client is a WCF client, yet it requires offline or disconnected interaction, choose the NetMsmqBinding that uses MSMQ for transporting the messages. If the client requires connected communication, but could be calling across machine boundaries, choose the NetTcpBinding that communicates over TCP. If the client is on the same machine as the service, choose the NetNamedPipeBinding that uses named pipes to maximize performance. You may fine-tune binding selections based on additional criteria such as the need for callbacks (WSDualHttpBinding) or federated security (WSFederationBinding).
Using a Binding
Each binding offers literally dozens of configurable properties. There are three modes of working with bindings. You can use the built-in bindings as is if they fit your requirements. You can tweak and configure some of their properties such as transaction propagation, reliability, and security. You can also write your own custom bindings. The most common scenario is using an existing binding almost as is, and merely configuring two or three of its aspects. Application developers will hardly ever need to write a custom binding, but framework developers may need to.