Michael Bayne
e0aea87f62
pages which give us something useful to link to when we want to point to all of the javadocs for a particular subtree of the Narya packages. git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@5411 542714f4-19e9-0310-aa3c-eee0fc999fb1
607 lines
25 KiB
HTML
607 lines
25 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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<html>
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<head>
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<!-- $Id: package.html 617 2001-11-13 00:12:20Z mdb $ -->
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<link rel="stylesheet" href="../stylesheet.css" type="text/css"/>
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</head>
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<body bgcolor="white">
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<h2>Presents Distributed Object System</h2>
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The Presents Distributed Object System is a framework for distributing
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information between multiple separate applications (over a network) and for
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coordinating control flow between those applications in the form of remote
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procedure call services. The normal configuration of the Presents system is
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client/server; generally with many clients connecting to a single
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server. All information transfer takes place through the server using the
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distributed object system documented below.
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<ul>
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<li><a href="#distributed_objects">Distributed Objects</a>
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<li><a href="#event_listeners">Event Listeners</a>
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<li><a href="#distributed_collections">Distributed Collections</a>
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<li><a href="#invocation_services">Invocation Services</a>
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<li><a href="#ant_tasks">Ant Tasks</a>
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</ul>
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<p> <em>A note to the reader:</em> the Presents system is a complex
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one and though a great deal of code is provided in explaining the
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services it provides, it is not the intent that one should start from
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only these examples and build a working system. A better approach is
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to read through this documentation to come to an understanding of the
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concepts and mechanisms that define the system and then take a look at
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some working sample code which is provided in the <code>tests</code>
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directory of this distribution.
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<h3><a name="distributed_objects">Distributed Objects</a></h3>
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The Presents services allow applications to access and update shared
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information through a mechanism known as distributed objects.
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Distributed objects are maintainedon the server and clients
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"subscribe" to the objects and are provided with proxy copies which
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are updated by a stream of events sent by the server when any state
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changes in the objects.
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<p> Clients cannot modify their proxy distributed objects directly,
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instead they make use of setter methods which package up the requested
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change into an event and send that event to the server for processing.
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After performing access control checks, the server will apply the
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event to the primary distributed object instance and then dispatch
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that event to all subscribed clients. Those clients (including the
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original change requesting client) then apply the event to their proxy
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copy of the object and in this way all clients maintain an up to date
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copy of the object's data.
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<p align="center"> <img src="images/dobject.png">
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<h4>Defining an object</h4>
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A distributed object is defined just like a regular Java object and is
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then run through a post-processor which inserts methods and constants
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into the object definition which are needed by the distributed object
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system. Here is a distributed object as originally defined:
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<pre class="example">
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public class CageObject extends DObject
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{
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/** The number of monkeys in the cage. */
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public int monkeys;
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/** The name of the owner of this cage. */
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public String owner;
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}</pre>
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Note that all distributed fields, or attributes (fields in a
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distributed object are frequently referred to as <i>attributes</i> in
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this documentation and elsewhere in the system), are public fields in
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our distributed object. Non-public fields will be ignored by the
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system and not transmitted when a proxy object is delivered over the
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network to a subscriber. Further, fields marked <code>transient</code>
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will also be ignored by the system.
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<p> We then run our class definition through a post-processor which
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turns it into the following:
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<pre class="example">
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public class CageObject extends DObject
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{
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<b>// AUTO-GENERATED: FIELDS START
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/** The field name of the <code>monkeys</code> field. */
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public static final String MONKEYS = "monkeys";
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/** The field name of the <code>owner</code> field. */
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public static final String OWNER = "owner";
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// AUTO-GENERATED: FIELDS END</b>
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/** The number of monkeys in the cage. */
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public int monkeys;
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/** The name of the owner of this cage. */
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public String owner;
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<b>// AUTO-GENERATED: METHODS START
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/**
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* Requests that the <code>monkeys</code> field be set to the
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* specified value. The local value will be updated immediately and an
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* event will be propagated through the system to notify all listeners
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* that the attribute did change. Proxied copies of this object (on
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* clients) will apply the value change when they received the
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* attribute changed notification.
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*/
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public void setMonkeys (int value)
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{
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int ovalue = this.monkeys;
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requestAttributeChange(
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EVEN_BASE, new Integer(value), new Integer(ovalue));
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this.monkeys = value;
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}
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/**
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* Requests that the <code>owner</code> field be set to the
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* specified value. The local value will be updated immediately and an
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* event will be propagated through the system to notify all listeners
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* that the attribute did change. Proxied copies of this object (on
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* clients) will apply the value change when they received the
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* attribute changed notification.
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*/
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public void setOwner (String value)
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{
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String ovalue = this.owner;
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requestAttributeChange(
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ODD_BASE, value, ovalue);
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this.owner = value;
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}
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// AUTO-GENERATED: METHODS END</b>
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}</pre>
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The contents of the methods are not too important, the main things to
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note are that setter methods for the two attributes were generated and
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constants were defined that will be used to identify which attribute
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changed if we choose to inspect an event notifying us of such a
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change. Note also that additional methods may be added to a
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distributed object class as long as nothing is modified in the
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<code>AUTO-GENERATED</code> section. As new fields are added and the
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post-processing tool re-run, everything outside the auto-generated
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section will be preserved.
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<p> One may also notice that attribute change requests result in the
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new value of the attribute being immediately written to the local copy
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of the object. This is a convention that was decided upon after
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repeatedly running into trouble when users of the system would set a
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value in an object and immediately assume it held the new value rather
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than realizing that an event would have to propagate back from the
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server before the value was in fact updated. By setting the value
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immediately, these problems are avoided and the opposite assumption is
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almost never made. This is further justified by the fact that, in
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general, attribute changes never originate on a client but instead
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originate on the server after processing a request from the client
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(via the below documented <a href="#invocation_services">invocation
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services</a>) to do something application-specific that results in one
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or more attribute changes taking place.
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<p> See the section on <a href="#ant_tasks">Ant Tasks</a> for
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information on how to configure and run this post-processor.
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<h4>Creating an object</h4>
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Generally, some entity on the server will choose to create a new
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instance of a distributed object. Rather than simply instantiate the
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object directly, one must create the object through the {@link
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com.threerings.presents.dobj.DObjectManager}:
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<pre class="example">
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public class ServerEntity implements Subscriber {
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public void init (DObjectManager omgr) {
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omgr.createObject(CageObject.class, this);
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}
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// inherited from interface Subscriber
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public void objectAvailable (DObject object) {
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// yay! we created our object
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_object = (CageObject)object;
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}
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// inherited from interface Subscriber
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public void requestFailed (int oid, ObjectAccessException cause) {
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// oh the humanity, we failed to create our object; in
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// general this would only happen if we did something silly like
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// passed in a DObject class that didn't extend DObject
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}
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protected CageObject _object;
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}</pre>
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You'll notice that we provide an instance of a <code>Subscriber</code>
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when creating our object. This subscriber instance is in fact
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subscribed to the newly created object in the same manner as is
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described below for all additional subscribers to the object. It is
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possible to instruct an object to automatically destroy itself when
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all subscribers have unsubscribed. (See the not very terse {@link
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com.threerings.presents.dobj.DObject}.setDestroyOnLastSubscriberRemoved()).
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<h4>Subscribing to an object</h4>
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<p> The client obtains a proxy of the object by a process called
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subscription, which is accomplished via {@link
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com.threerings.presents.dobj.DObjectManager}.subscribeToObject():
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<pre class="example">
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public class ObjectUser implements Subscriber {
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public void init (Client client, int objectId) {
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client.getDObjectManager().subscribeToObject(objectId, this);
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}
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// inherited from interface Subscriber
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public void objectAvailable (DObject object) {
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// yay! we got our object
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_object = (CageObject)object;
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}
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// inherited from interface Subscriber
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public void requestFailed (int oid, ObjectAccessException cause) {
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// oh the humanity, we failed to subscribe
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}
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protected CageObject _object;
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}</pre>
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<p> Later a client would relinquish its subscription to the object
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using a similar mechanism:
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<pre class="example">
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public class ObjectUser implements Subscriber {
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// ...
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public void shutdown (Client client) {
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client.getDObjectManager().unsubscribeFromObject(
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_object.getOid(), this);
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_object = null;
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}
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// ...
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}</pre>
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However, this is a fine time to point out the dangers of working in an
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asynchronous distributed environment. There is no guarantee that your
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object subscription request will be completed before the client
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decides to call shutdown() on its <code>ObjectUser</code>. Thus, in
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the previous code, we could get a null pointer exception, and even
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worse, we could remain subscribed to the object even though we didn't
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want to be. To avoid these sorts of problems, the {@link
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com.threerings.presents.util.SafeSubscriber} class is provided:
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<pre class="example">
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public class ObjectUser implements Subscriber {
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public void init (Client client, int objectId) {
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<b>_safesub = new SafeSubscriber(objectId, this);
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_safesub.subcribe(client.getDObjectManager());</b>
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}
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// inherited from interface Subscriber
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public void objectAvailable (DObject object) {
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// yay! we got our object
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_object = (CageObject)object;
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}
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// inherited from interface Subscriber
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public void requestFailed (int oid, ObjectAccessException cause) {
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// oh the humanity, we failed to subscribe
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}
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public void shutdown (Client client) {
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<b>_safesub.unsubscribe(client.getDObjectManager());</b>
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_object = null;
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}
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<b>protected SafeSubscriber _safesub;</b>
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protected CageObject _object;
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}</pre>
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The safe subscriber will pass the object availability on to your
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subscriber and when the time comes to unsubscribe, it will cope with
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the case where the original subscription was not fully processed and
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stick around long enough to ensure that once it is, the request to
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unsubscribe is also dispatched. It will also cope with a request to
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<code>unsubscribe()</code> even if the original subscription request
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failed.
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<h3><a name="event_listeners">Event Listeners</a></h3>
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Once a client has subscribed to a distributed object, all events
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pertaining to that object will be delivered to the client. Frequently,
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it is useful to respond dynamically to changes in distributed object
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values and this is accomplished using listeners. A client can register
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any number of listeners on an object and when the object is finally
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unsubscribed from and garbage collected, the listener registrations
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all go away as well.
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<p> The basic listener is the {@link
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com.threerings.presents.dobj.AttributeChangeListener} which is
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informed of all simple attribute changes (setting a primitive field to
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a new value is called an attribute change). We return to our trusty
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example:
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<pre class="example">
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public class ObjectUser
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implements Subscriber, <b>AttributeChangeListener</b> {
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// ...
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public void init (Client client, int objectId) {
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_safesub = new SafeSubscriber(_subscriber, objectId);
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_safesub.subcribe(client.getDObjectManager());
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}
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// inherited from interface Subscriber
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public void objectAvailable (DObject object) {
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// yay! we got our object
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_object = (CageObject)object;
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<b>_object.addListener(this);</b>
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}
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// inherited from interface Subscriber
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public void requestFailed (int oid, ObjectAccessException cause) {
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// oh the humanity, we failed to subscribe
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}
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<b>// inherited from interface AttributeChangeListener
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public void attributeChanged (AttributeChangedEvent event)
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{
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System.out.println("Wow! The " + event.getName() +
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" field changed to " + event.getValue() + ".");
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}</b>
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public void shutdown (Client client) {
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_safesub.unsubscribe(client.getDObjectManager());
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<b>if (_object != null) {
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// removing our listener not necessary as we are
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// unsubscribing, but it's a good habit to develop as
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// frequently listeners will come and go during the
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// lifetime of an object subscription
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_object.removeListener(this);
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_object = null;
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}</b>
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}
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protected SafeSubscriber _safesub;
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protected CageObject _object;
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}</pre>
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The <code>attributeChanged()</code> method of our registered listener
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will be called whenever an event is received as a result of one of the
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setter methods being called on the <code>CageObject</code> by
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<em>any</em> participant in the distributed system. The setter creates
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an event which is sent to the server, the server dispatches the event
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to all subscribers of the object and the Presents system dispatches
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the event notification to all registered listeners when the event is
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received on the client. Note that listeners are also used on the
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server as entities on the server also frequently need to respond to
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attribute changes. They are notified immediately after the server has
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dispatched the event (over the network) to all subscribed clients.
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<p> It is useful to note that listeners are notified of a changed
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attribute <b>after</b> the change has been applied to the object. The
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previous value of the attribute is available through the {@link
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com.threerings.presents.dobj.AttributeChangedEvent#getOldValue}
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method, though in spite of many years of experience using this system
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in a variety of circumstances, we have rarely found that we cared to
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know the previous value.
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<h3><a name="distributed_collections">Distributed collections</a></h3>
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One soon discovers that primitive object fields do not make for a very
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useful information distribution mechanism and that more complex data
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structures are necessary. Two collection types, sets and arrays, are
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supported, and a mechanism is provided for allowing whole objects to
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be passed around in toto as if they were a primitive field.
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<p><b>Distributed Arrays</b><br>
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Arrays of primitive types can be used in a distributed object and the
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system will detect their use and provide a mechanism for updating the
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entire array and an additional mechanism for updating a single element
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at a time:
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<pre class="example">
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public class ChessObject extends DObject
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{
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// AUTO-GENERATED: FIELDS START
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/** The field name of the <code>state</code> field. */
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public static final String STATE = "state";
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// AUTO-GENERATED: FIELDS END
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/** Used to track our board state. */
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public int[] state;
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// AUTO-GENERATED: METHODS START
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/**
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* Requests that the <code>state</code> field be set to the
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* specified value. The local value will be updated immediately and an
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* event will be propagated through the system to notify all listeners
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* that the attribute did change. Proxied copies of this object (on
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* clients) will apply the value change when they received the
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* attribute changed notification.
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*/
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public void setState (int[] value)
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{
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int[] ovalue = this.state;
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requestAttributeChange(
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STATE, value, ovalue);
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this.state = (value == null) ? null : (int[])value.clone();
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}
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/**
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* Requests that the <code>index</code>th element of
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* <code>state</code> field be set to the specified value.
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* The local value will be updated immediately and an event will be
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* propagated through the system to notify all listeners that the
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* attribute did change. Proxied copies of this object (on clients)
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* will apply the value change when they received the attribute
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* changed notification.
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*/
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public void setStateAt (int value, int index)
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{
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int ovalue = this.state[index];
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requestElementUpdate(
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STATE, index, new Integer(value), new Integer(ovalue));
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this.state[index] = value;
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}
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// AUTO-GENERATED: METHODS END
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}</pre>
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To correspond with what is called an "element update" (the
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modification of a single element in an array), there is the {@link
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com.threerings.presents.dobj.ElementUpdateListener}. When an element
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is updated, listeners implementing that interface will be notified.
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Remember that if the whole array is changed using
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<code>setState()</code>, the normal {@link
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com.threerings.presents.dobj.AttributeChangeListener} is the interface
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one uses to hear about it.
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<p> <em>Note</em> that distributed arrays are <em>not</em>
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automatically resized. If a request is made to update the element at
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index 9 of an array, the array must be of at least size 10 or an array
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index out of bounds exception will be thrown (as should be evident
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from inspecting the code above). For more dynamic collections of
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objects, see the documentation below about distributed sets.
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<p> This mechanism is not actually limited to arrays of primitive
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types. It also works for arrays of objects that implement the {@link
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com.threerings.io.Streamable} interface which is documented next.
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<p><b>Streamable and its good friend SimpleStreamableObject</b><br>
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The {@link com.threerings.io.Streamable} interface is used to mark
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objects that can be sent over the network by using them in distributed
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object fields by using arrays of such objects as a field. This
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interface functions in much the same way that {@link
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java.io.Serializable} does in that it simply marks the class and an
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underlying mechanism uses reflection to actually marshall and
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unmarshall the object on the network. In fact, all
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non-<code>transient</code> fields of a streamable object are included
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during the marhsalling process. Here's an example:
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<pre class="example">
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public class Player implements Streamable
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{
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/** This player's name. */
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public String name;
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/** This player's rating. */
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public int rating;
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}
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public class ChessObject extends DObject
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{
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/** A record for each player in the game. */
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public Player[] players;
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}</pre>
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The generated methods are ommitted for the sake of brevity, but as you
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would expect, both a <code>setPlayers(Player[] value)</code> and a
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<code>setPlayersAt(Player value, int index)</code> method will be
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generated and do just what you expect.
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<p> It should be pointed out that streamable objects sent over the
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network are sent in their entirety. No mechanism is provided for
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updating just a single field in a streamable instance both because
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that would increase the complexity of the system tremendously and
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because it is generally not very useful. If conservation of bandwidth
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is of extreme importance, special {@link
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com.threerings.presents.dobj.DEvent} derived classes can be created to
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transmit precisely what is desired and nothing more. Doing so is
|
|
beyond the scope of this introduction, but will hopefully be covered
|
|
in an additional tutorial.
|
|
|
|
<p> The {@link com.threerings.io.SimpleStreamableObject} class is a
|
|
convenient way to create a simple record like the <code>Player</code>
|
|
record above that implements <code>Streamable</code> and provides a
|
|
default implementation of <code>toString()</code> that uses reflection
|
|
to print out the actual values of the fields in the object (a boon
|
|
when logging and debugging).
|
|
|
|
<p><b>Distributed Sets</b><br>
|
|
|
|
In developing a distributed system, one frequently encounters
|
|
situations where one wants distributed collection of objects where
|
|
order is generally not important but the ability to fluidly add and
|
|
remove elements is. For such occasions we provide the distributed set
|
|
or {@link com.threerings.presents.dobj.DSet}.
|
|
|
|
<p> A <code>DSet</code> contains entries (called entries rather than
|
|
elements to avoid confusion with array "element updating") which must
|
|
implement the {@link com.threerings.presents.dobj.DSet.Entry}
|
|
interface. This automatically makes them {@link
|
|
com.threerings.io.Streamable} and requires that they provide a {@link
|
|
java.lang.Comparable} key which is used to distinguish them from other
|
|
entries in the set (and look them up via an efficient binary search).
|
|
|
|
<p> When using a <code>DSet</code> one is provided with three new
|
|
operations: <code>addToFoo()</code>, <code>updateFoo()</code> and
|
|
<code>removeFromFoo()</code>. Once again an example is in order:
|
|
|
|
<pre class="example">
|
|
public class Monkey implements DSet.Entry
|
|
{
|
|
/** The monkey's name. */
|
|
public String name;
|
|
|
|
/** The monkey's age. */
|
|
public int age;
|
|
|
|
// documentation inherited from interface DSet.Entry
|
|
public Comparable getKey ()
|
|
{
|
|
return name;
|
|
}
|
|
}
|
|
|
|
public class CageObject extends DObject
|
|
{
|
|
// AUTO-GENERATED: FIELDS START
|
|
/** The field name of the <code>monkeys</code> field. */
|
|
public static final String MONKEYS = "monkeys";
|
|
// AUTO-GENERATED: FIELDS END
|
|
|
|
/** A collection of monkeys. */
|
|
public DSet monkeys;
|
|
|
|
// AUTO-GENERATED: METHODS START
|
|
/**
|
|
* Requests that the specified entry be added to the
|
|
* <code>monkeys</code> set.
|
|
*/
|
|
public void addToMonkeys (DSet.Entry elem)
|
|
{
|
|
requestEntryAdd(MONKEYS, monkeys, elem);
|
|
}
|
|
|
|
/**
|
|
* Requests that the entry matching the supplied key be removed from
|
|
* the <code>monkeys</code> set.
|
|
*/
|
|
public void removeFromMonkeys (Comparable key)
|
|
{
|
|
requestEntryRemove(MONKEYS, monkeys, key);
|
|
}
|
|
|
|
/**
|
|
* Requests that the specified entry be updated in the
|
|
* <code>monkeys</code> set.
|
|
*/
|
|
public void updateMonkeys (DSet.Entry elem)
|
|
{
|
|
requestEntryUpdate(MONKEYS, monkeys, elem);
|
|
}
|
|
|
|
/**
|
|
* Requests that the <code>monkeys</code> field be set to the
|
|
* specified value.
|
|
*/
|
|
public void setMonkeys (DSet value)
|
|
{
|
|
requestAttributeChange(MONKEYS, value, this.monkeys);
|
|
this.monkeys = (value == null) ? null : (DSet)value.clone();
|
|
}
|
|
// AUTO-GENERATED: METHODS END
|
|
}</pre>
|
|
|
|
It is possible to set the entire set (which is necessary to establish
|
|
its original value even if one decides to set it to the empty set),
|
|
but more commonly one will simply add entries to the set, update those
|
|
entries and remove them using the provided methods.
|
|
|
|
<p> In conjunction with the <code>DSet</code> there exists the {@link
|
|
com.threerings.presents.dobj.SetListener} which is notified when
|
|
changes are made to a distributed set. This functions in the same was
|
|
as the previously documented listeners, so I will refrain from boring
|
|
you with yet more sample code.
|
|
|
|
<h3><a name="invocation_services">Invocation Services</a></h3>
|
|
|
|
TBD
|
|
|
|
<h3><a name="ant_tasks">Ant Tasks</a></h3>
|
|
|
|
TBD
|
|
|
|
</body>
|
|
</html>
|