Distributed Events and the Browser-Server Dialogue - Decisions about Distributed Events
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Will you publish a history or just the current state?
Given the nature of HTTP, server data must be retrieved periodically--the server can't directly call the browser when something happens. If the server exposes only the current state, there are two problems:
Change detection
The browser can only infer a change has occurred
by comparing the previous value, or perhaps by
using a version ID or timestamp.
Intermediate states
The browser will miss values that occurred between updates.
Neither of these are showstoppers, so exposing only the current state may well be feasible. The alternative is to publish a history of states along with timestamps. That's more work to output and parse, and it also requires some form of storage on the server. If you are following that option, it's worth formatting the changes using a standard feed-based approach such as RSS or Atom. You'll benefit from the abundance of libraries for both browser and server. As a bonus, the service will be generic enough to be used by external clients, if that's a requirement.
For observer-style events, will you propagate the details of whats changed, or just point to the object thats changed?
Often, events concern an object that's changed state. Sometimes, you only need to propagate a pointer to the object; the recipient will then interrogate the object as required. Other times, you need to send the change itself. The latter approach is useful for functionality that requires not just the object's current state, but the nature of the change. For example, imagine you have an auditing function that logs whenever a budget balance has been increased. It will be much easier if the change event indicates that an increase has occurred. Otherwise, it will have to manually compare the budget against its previous state. For server-to-browser events, indicating the change may be better for performance since it may alleviate a follow-up call to the server.
What information will accompany the event notification?
When an event occurs, there's usually several pieces of information to pass to listeners--for example, a source object (an object that's just changed), the nature of the change or action that's occurred, and meta-information such as event time and unique ID. You can pass this information in different ways:
String message
Pass the information as a single string.
Single event object
Pass in a single event object containing attributes
for each piece of information.
Parameter list
Pass the information as separate parameters.
Each style has its strengths. A string message is the most flexible approach and has the benefit of being a portable format that will work on the server as well. Unfortunately, a string message must often be parsed and formatted to convert to and from useful JavaScript values. A single event object is easier to manipulate and, like a string message, can usually be extended without breaking existing code--the callback function still takes a single value. You can create a factory function to create the event and expose properties, so its interface can be made explicit if you so desire. Finally, a parameter list makes for a cleaner callback function implementation, since you don't have to extract variables from a wrapper object. However, a long list of parameters is cumbersome and difficult to maintain.
Will events be processed synchronously or asynchronously?
The simplest way to handle events is synchronously. As soon as something happens, the event manager immediately notifies all interested parties. All this happens in the same thread of execution, so each event handler becomes a bottleneck--the main program flow that triggered the event won't be able to proceed until each event handler has executed.
If some event handlers are slow, you can get more stable performance by handling events asynchronously. Here, the manager maintains a collection of pending events. Each time a new event arises, it simply adds it to the collection and returns--a very quick operation. Using a repeating timer (see Scheduling [Chapter 7]), the manager periodically pulls off pending events and notifies listeners.
There are various decisions involved in asynchronous event handling. First, what sort of collection do you use? A queue is most common, to ensure that events are handled in the order they arise. But sometimes a stack is more appropriate, so that if the manager falls behind, at least the most recent events will have been handled. Another decision is scheduling of the event handler, the object that picks events off the collection. The simplest style is a pure repeating timer, but if the handling takes too long (longer than the timer interval), you'll end up with multiple processes picking off events. One way to prevent this situation is to have the event handler monitor its own progress and cease activity after a certain time has elapsed.
Next: Real-World Examples of Distributed Events >>
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 This article is excerpted from chapter 10 of the book Ajax Design Patterns, written by Michael Mahemoff (O'Reilly, 2006; ISBN: 0596101805). Check it out today at your favorite bookstore. Buy this book now.
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