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copied!<p>Web servers can't push unsolicited data to the client; they obey the request-response cycle. The alternative is to use Message Queuing at a significant increase in complexity.</p> <p>Polling from the client is not so bad; web servers are adept at handling many short requests, and a polling interval of 2 or 3 seconds should be fast enough.</p> <p>Here is a polling method I like to use. It asynchronously waits for the response to come back before polling again (requires jQuery):</p> <pre><code>function poll(url, task, progressBar, resultsCallback, timeoutMillis, pollIntervalMillis) { $.ajax({ url: url, type: 'GET', dataType: 'json', timeout: timeoutMillis, data: 'action=poll&task='+task, success: (function(response, status, xhr) { if ('progress' in response) { // update the UI with the progress progressBar.setValue(response.progress); } if ('status' in response) { if (response.status == 'pending') { // task is not finished, continue polling setTimeout((function() { poll(url, task, progressBar, resultsCallback, timeoutMillis, pollIntervalMillis); }), pollIntervalMillis); } else { // task completed if (response.status == 'cancelled') { progressBar.setColor('red'); progressBar.setText("Task '"+task+"' was cancelled"); } else { progressBar.setColor('green'); progressBar.setText("Task '"+task+"' complete"); } // GET the results $.ajax({ url: url, type: 'GET', timeout: timeoutMillis, data: 'action=results&task='+task, success: (function(response, status, xhr) { resultsCallback(response, status, xhr); }), error: error }); } } }), error: error }); function error(xhr, status, err) { alert('Failure to communicate with server: ' + status + ', ' + err); } } </code></pre> <p>And your server-side code should be responding to polls with something like this:</p> <pre><code>{"progress" : 42, "status" : "pending"} </code></pre>

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