This article shows you how to use a jQuery UI component and Web Sockets to implement a progress bar for a deterministic asynchronous server-side job in a Play framework web application. This is an example of a user-interface component that is not part of the web’s stateless request-response paradigm, but is now possible with new HTML5 features.
This follows on from earlier jQuery UI component articles: jQuery UI Ajax autocomplete with Play, Lazy loading page content with jQuery UI, Ajax and Play and Selector dialogue with jQuery UI, Ajax and Play. The examples and source code for all of these articles are available as part of the Play framework jQuery UI module.
For this example, we are going to start a long-running job in our web application on the server, and display progress in real-time:
Note that this time we are not using a time zones example, although the recent time zone database debacle seems to have been largely resolved.
Architecture
The Architecture of the World Wide Web is about resources and their identifiers and representations, and while we are used to the original web of HTML documents and URLs, resources and representations can be richer. The key resource in this progress bar example is a server-side process.
The Play framework includes support for starting asynchronous server-side jobs - processes that continue running outside the scope of the HTTP request-response cycle. This is useful for important tasks that must be completed independently of the user’s navigation behaviour, such as sending hundreds of party invitation e-mails. The challenge with these long-running jobs is to use URIs to identify them as ‘resources’ in the web architecture sense, and to provide useful representations.
For this example, we will use a web socket URI to address an asynchronous job, and on the server-side generate a representation consisting of only a single number - the job’s completion percentage. Play’s web sockets support is ideal for updating the progress bar, and Play’s asynchronous jobs are suitable for long-running jobs.
Note that web sockets support on both server-side and client-side is still evolving. This example works with Play 1.2.3 and Safari 5.1.1; Play 1.2.4 adds Chrome support.
Progress bar
The jQuery UI progress bar widget shows progress for a server-side process for which the completion percentage can be calculated. Progress bars are useful for processes that take longer than a few seconds to complete, provided that they are deterministic, i.e. processes whose progress can be measured or predicted.
Client
The widget is based on an empty HTML div element.
<div id="progressbar" data-url="@@{jqueryui.ProgressSocket.progress(processId)}"></div>
The data-url attribute value specifies the URL for a web socket connection. Note that the template uses the @@ syntax to generate an absolute URL that includes the web socket protocol - ws://localhost:9000/progressbar/progress.
The progressbar.js JavaScript applies the jQuery UI dialog plug-in to the div element and opens a web socket connection. Finally, the JavaScript uses web socket messages to update the progress bar value.
$(function() {
// Initialise the progress bar.
var $progressbar = $('#progressbar').progressbar();
// Open the web socket connection.
var serverUrl = $progressbar.data('url');
var socket = new WebSocket(serverUrl);
// Use web socket messages to update the progress bar.
socket.onmessage = function(event) {
$progressbar.progressbar('value', parseInt(event.data));
};
});
Server
The server-side has three parts. First, we define an implementation of a long-running job, which has a generated UUID identifier and an event stream that the job will publish progress to. The implementation in this example simply waits for short intervals, publishing progress to the event stream after each one.
package models.jqueryui;
import play.jobs.Job;
import play.libs.Codec;
import play.libs.F;
import java.io.Serializable;
import java.util.HashMap;
import java.util.Map;
/**
* Example job that publishes progress as an event stream.
*/
public class Process extends Job implements Serializable {
public static Map<String, Process> registry = new HashMap<String, Process>();
public String id = Codec.UUID();
/**
* Event stream for events that report job completion percentage.
*/
public F.EventStream<Integer> percentComplete = new F.EventStream<Integer>();
public void doJob() {
// Report completion percentage every 100 ms.
for (int i = 1; i <= 50; i++) {
try {
Thread.sleep(100l);
final int percent = i * 2;
this.percentComplete.publish(percent);
} catch (InterruptedException e) {
// ignore
}
}
}
}
The second server-side class is a conventional action that starts an asynchronous long-running job, and stores a reference to the job in a ‘registry’ singleton. This action then renders the same page with a processId argument that indicates that a job has been started.
package controllers.jqueryui;
import models.jqueryui.Process;
/**
* Progressbar example.
*/
public class Progressbar extends JQueryUI {
/**
* Start a job, cache it and re-render the page
*/
public static void startJob() {
final Process process = new Process();
process.now();
final String processId = process.id;
Process.registry.put(processId, process);
renderTemplate("jqueryui/Progressbar/index.html", processId);
}
}
The last part is a web socket controller that retrieves the running job from the registry, using its ID, and waits for events on the job’s event stream. When the job publishes a completion percentage to its event stream, the controller sends the data in a web socket message to the browser client.
package controllers.jqueryui;
import models.jqueryui.Process;
import play.libs.F;
import play.mvc.Http;
import play.mvc.WebSocketController;
import static play.libs.F.Matcher.ClassOf;
/**
* Web socket controller for use by the progress bar.
*/
public class ProgressSocket extends WebSocketController {
public static void progress(final String processId) {
final Process process = Process.registry.get(processId);
final F.EventStream<Integer> progress = process.percentComplete;
// Loop while the socket is open
while (inbound.isOpen()) {
// Wait for either an inbound socket event or a process progress event.
F.Either<Http.WebSocketEvent, Integer> e = await(F.Promise.waitEither(
inbound.nextEvent(),
progress.nextEvent()
));
// Case: The socket has been closed
for (Http.WebSocketClose closed : Http.WebSocketEvent.SocketClosed.match(e._1)) {
disconnect();
}
// Case: percentComplete published - send the value to the client.
for (Integer percentComplete : ClassOf(Integer.class).match(e._2)) {
outbound.send(percentComplete.toString());
if (percentComplete >= 100) {
disconnect();
}
}
}
}
}
Note that this design means that the job is started by a conventional HTTP POST request to the Progressbar.startJob() action, which means that the job is executed even if the client does not have web socket support and is therefore unable to update the progress bar.
One possible enhancement would be to add JavaScript to detect the case where web sockets are not supported, and send a single Ajax request to get the current progress value to update the progress bar.
Conclusion
Web sockets are an HTML5 technology that you can now use in your web applications, thanks to browser support for the JavaScript API and Play’s server-side support. This is good news for developers who can see opportunities to improve user-experience in their applications with more sophisticated applications.
Peter Hilton
is a senior software developer at Lunatech Research and committer on the Play open-source project.
