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Sync Versus Async. Asynchronous interaction is a key component of loosely coupled systems. Here's an example I've found helpful to explain its implications:
Synchronous Interaction
To illustrate the differences between the synchronous and asynchronous styles of interaction, let's use the analogy of holiday gift shopping. Here's the situation: You have a list of three people for whom you want to purchase gifts, and you've got two choices. You can head for the shopping mall, or you can shop online. For the sake of the analogy, think of yourself (the buyer) and the clerk or online store (the merchant) as a pair of distributed systems. The figure below illustrates the interaction of these systems when you shop at the mall.
The shopping-mall model is synchronous because it's composed of a series of predictable steps that cannot be re-ordered. Specifically, you and the sales clerks alternate roles: You select a gift, a clerk sells it to you, and you repeat the process for the remaining gifts. Your actions are synchronized with those of the clerks, and you won't move on to the next clerk before completing the exchange with the current one.
Asynchronous Interaction
Next, let's examine the interaction between the two systems when you shop online, as illustrated below.
This is an example of the asynchronous interaction model, which differs from the synchronous model in two ways. First, the three ordering actions are predictable, but the three shipping actions are not. The shipments are initiated at seemingly random times and out of sequence, take different lengths of time, and arrive in yet another unpredictable sequence. Second, in the asynchronous model you can place all three orders, one immediately after another. The merchants send email acknowledgements in response to your orders, but you don't need to wait for one order to be acknowledged before placing the next. In other words, the buyer and merchant are no longer synchronized.
[Excerpted from Loosely Coupled--The Missing Pieces of Web Services.]
Posted Sunday, March 09, 2003 12:42:58 AM 
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