Programming Flash Communication Server

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Chapter 4: Applications, Instances, and Server-Side ActionScript

the one included in Flash MX Professional 2004 (Flash Pro). SSAS source files almost

always have the extension .asc (although .js is another option); SSAS is in fact

JavaScript 1.5.

When double-byte characters are required, such as for Kanji, you must

use a text editor capable of handling UTF-8 encoding. In addition,

method names containing higher-order double-byte characters must

use the array operator instead of the dot operator. For example:

obj = {};

obj.myMethodName = function ( ) {

trace("myMethodName");

};

obj["myMethodName"]( ); // Correct

obj.myMethodName( ); // Correct

obj[" "] = function ( ) {

trace(" ");

};

obj[" "]( ); // Correct

obj. ( ); // Syntax Error !!

Instances and Resources

FlashCom can run multiple instances of an application at the same time. Each

instance has its own memory, disk, stream, and shared object environment and runs

its own single-threaded copy of the main.asc script. Instances are an important way

to group users and partition the server’s resources among them. Perhaps the sim-

plest example of this is a chat application. Many instances of a chat application can

be run simultaneously—each with different users connected to it—effectively creat-

ing a set of chat rooms. Users who connect to rtmp:/courseChat/room1 can commu-

nicate with each other using the streams and shared objects associated with the

room1

instance. Other users who connect to rtmp:/courseChat/room2 can communicate

among themselves using the

room2 instance but will normally be unaware of the con-

versation occurring in the

room1 instance. If instances must share streams or shared

objects, one instance can make a network connection to another to gain access to its

resources.

Although you can build an application in which a single instance pro-

vides a lobby and multiple chat rooms simultaneously, it’s not recom-

mended. If a large number of users simultaneously use a single

instance, performance suffers. Because an instance’s ActionScript runs

in a single thread, every remote method call on the server, including

the delivery of chat messages, will execute sequentially.

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Scripting Application Instances

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It is difficult to give precise limits on how many clients are too many for one instance

because each application makes different demands on the server. An instance that

makes extensive use of Server-Side ActionScript may support fewer than 20 clients

with adequate performance, while other applications may support hundreds or even

a few thousand. Chapter 16 covers architectures that scale more effectively.

The streams and shared objects available to each instance are located by FlashCom

using a relative URI. Most often this is just the name of the stream or shared object.

For example, if a Flash movie plays a stream named

intro, then “intro” is a relative

URI to the instance to which the movie is connected. If the instance URI is:

rtmp://my.university.edu/courseLectures/algebra101

then the full URI to the

intro video would be:

rtmp://my.university.edu/courseLectures/algebra101/intro

In practice, a full URI like that is never used, but it is useful to think of stream and

shared object resources as existing in a space defined by full URIs. We’ll make use of

full URIs later in this chapter to show how streams are organized within an applica-

tion. A Flash movie requests resources associated with an application instance using

a relative URI after a connection has been attempted. For example, a movie must

first request a connection to an instance before trying to publish or play a stream or

get a remote shared object:

nc.connect("rtmp://my.university.edu/courseLectures/algebra101");

Once the connection has been attempted, the Flash movie can request that a

recorded or live stream named

intro be played:

ns = new NetStream(nc); // Create a NetStream within a NetConnection.

videoArea.attachVideo(ns); // Attach the stream to a video object on the Stage.

ns.play("intro"); // Play a stream named intro.

Similarly, a shared object named streamList, which contains a list of stream names,

may be available for the

algebra101 instance. After a connection is requested to the

algebra101 instance, access to the streamList shared object could be requested as fol-

lows:

so = SharedObject.getRemote("streamList", nc.uri, true);

so.onSync = function (list) {

trace("onSync> list.length: " + list.length);

};

so.connect(nc);

The NetStream and SharedObject classes are described in detail in Chapters 5 and 8.

In both previous examples, the name of the stream and the shared object are simple

relative URIs. The relative path to a stream or shared object within an instance can

include directory-like names that help to separate resources into groups. For exam-

ple, lectures can be grouped by subject so that a relative URI to a video stream might