Indigo

By Martin Heller

April 12, 2004

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Indigo, to make it as simple as possible, or possibly simpler, is essentially web services on steroids. While Indigo is part of Longhorn, the future Microsoft operating system, Indigo will also be available as a download for Windows XP and Windows Server 2003.

For the rest of this column, we'll be exploring exactly how "Indigo is web services on steroids" is and isn't true. We'll also, of necessity, wallow in acronyms and code words, but I hope that we'll avoid descending into incomprehensibility (or madness).

Indigo is, according to Don Box, "a unified programming model and communications infrastructure for developing connected systems." By "unified" Don means that Indigo combines what used to be different mechanisms—web services, remoting, transactions, message queues, and so on—into one whole. It's unified at the programming model level because it all lives in one .NET Framework namespace tree, System.MessageBus. (See the Longhorn SDK to explore that namespace.) Indigo has "a service-oriented programming model in which autonomous programs are composed using asynchronous message passing."

An Indigo service creates messages, represented by the Message class, which is found in the System.MessageBus namespace—the highest level namespace for Indigo services. Services send messages over connectors, which are SOAP-based transports consisting of ports, represented by the Port class, and channels, represented by the ChannelCollection, DialogChannel, DialogChannels, ReceiveChannel, SendChannel, and SendRequestChannel classes. Box explains: "A port represents a location in network space and provides a base Uniform Resource Identifier (URI) for all services that it hosts. A channel is an I/O device that services use to send and receive messages through their port." Channels can use transport protocols such as HTTP or TCP, or they can use higher-level messaging services such as message queues, which are represented by classes in the System.M

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