Saturday, July 19, 2014

Tailing a file - Spring Websocket sample

This is a sample that I have wanted to try for sometime - A Websocket application to tail the contents of a file.

The following is the final view of the web-application:

There are a few parts to this application:

Generating a File to tail:

I chose to use a set of 100 random quotes as a source of the file content, every few seconds the application generates a quote and writes this quote to the temporary file. Spring Integration is used for wiring this flow for writing the contents to the file:

<int:channel id="toFileChannel"/>

<int:inbound-channel-adapter ref="randomQuoteGenerator" method="generateQuote" channel="toFileChannel">
	<int:poller fixed-delay="2000"/>

<int:chain input-channel="toFileChannel">
		<int:header name="file_name" value="quotes.txt"/>
	<int-file:outbound-channel-adapter directory="#{systemProperties['']}" mode="APPEND" />

Just a quick note, Spring Integration flows can now also be written using a Java Based DSL, and this flow using Java is available here

Tailing the file and sending the content to a broker

The actual tailing of the file itself can be accomplished by OS specific tail command or by using a library like Apache Commons IO. Again in my case I decided to use Spring Integration which provides Inbound channel adapters to tail a file purely using configuration, this flow looks like this:
<int:channel id="toTopicChannel"/>

<int-file:tail-inbound-channel-adapter id="fileInboundChannelAdapter"

<int:outbound-channel-adapter ref="fileContentRecordingService" method="sendLinesToTopic" channel="toTopicChannel"/>
and its working Java equivalent

There is a reference to a "fileContentRecordingService" above, this is the component which will direct the lines of the file to a place where the Websocket client will subscribe to.

Websocket server configuration

Spring Websocket support makes it super simple to write a Websocket based application, in this instance the entire working configuration is the following:
public class WebSocketDefaultConfig extends AbstractWebSocketMessageBrokerConfigurer {

	public void configureMessageBroker(MessageBrokerRegistry config) {
		//config.enableStompBrokerRelay("/topic/", "/queue/");
		config.enableSimpleBroker("/topic/", "/queue/");

	public void registerStompEndpoints(StompEndpointRegistry registry) {

This may seem a little over the top, but what these few lines of configuration does is very powerful and the configuration can be better understood by going through the reference here. In brief, it sets up a websocket endpoint at '/tailfileep' uri, this endpoint is enhanced with SockJS support, Stomp is used as a sub-protocol, endpoints `/topic` and `/queue` is configured to a real broker like RabbitMQ or ActiveMQ but in this specific to an in-memory one.

Going back to the "fileContentRecordingService" once more, this component essentially takes the line of the file and sends it this in-memory broker, SimpMessagingTemplate facilitates this wiring:

public class FileContentRecordingService {
	private SimpMessagingTemplate simpMessagingTemplate;

	public void sendLinesToTopic(String line) {
		this.simpMessagingTemplate.convertAndSend("/topic/tailfiles", line);

Websocket UI configuration

The UI is angularjs based, the client controller is set up this way and internally uses the javascript libraries for sockjs and stomp support:

var tailFilesApp = angular.module("tailFilesApp",[]);

tailFilesApp.controller("TailFilesCtrl", function ($scope) {
    function init() {
        $scope.buffer = new CircularBuffer(20);

    $scope.initSockets = function() {
        $scope.socket.client = new SockJS("/tailfilesep);
        $scope.socket.stomp = Stomp.over($scope.socket.client);
        $scope.socket.stomp.connect({}, function() {
            $scope.socket.stomp.subscribe("/topic/tailfiles", $scope.notify);
        $scope.socket.client.onclose = $scope.reconnect;

    $scope.notify = function(message) {
        $scope.$apply(function() {

    $scope.reconnect = function() {
        setTimeout($scope.initSockets, 10000);


The meat of this code is the "notify" function which the callback acting on the messages from the server, in this instance the new lines coming into the file and showing it in a textarea.

This wraps up the entire application to tail a file. A complete working sample without any external dependencies is available at this github location, instructions to start it up is also available at that location.


Spring Websockets provides a concise way to create Websocket based applications, this sample provides a good demonstration of this support. I had presented on this topic recently at my local JUG (IndyJUG) and a deck with the presentation is available here

Friday, July 4, 2014

Scala Tail Recursion confusion

I was looking at a video of Martin Odersky's keynote during Scala Days 2014 and there was a sample tail recursion code that confused me:

private def sameLength[T, U](xs: List[T], ys: List[U]): Boolean = {
  if (xs.isEmpty) ys.isEmpty
  else ys.nonEmpty && sameLength(xs.tail, ys.tail)

On a quick glance, this did not appear to be tail recursive to me, as there is the && operation that needs to be called after the recursive call.

However, thinking a little more about it, && is a short-circuit operator and the recursive operation would get called only if the ys.nonEmpty statement evaluates to true, thus maintaining the definition of a tail recursion.

The decompiled class clarifies this a little more, surprisingly the && operator does not appear anywhere in the decompiled code!:

public <T, U> boolean org$bk$sample$SameLengthTest$$sameLength(List<T> xs, List<U> ys)
    for (; ys.nonEmpty(); xs = (List)xs.tail()) ys = (List)ys.tail();
      xs.isEmpty() ? ys.isEmpty() : 

If the operator were changed to something that does not have short-circuit behavior, the method of course will not be a tail-recursion at that point, say a hypothetical method with the XOR operator:

private def notWorking[T, U](xs: List[T], ys: List[U]): Boolean = {
  if (xs.isEmpty) ys.isEmpty
  else ys.nonEmpty ^ notWorking(xs.tail, ys.tail)

Something fairly basic that tripped me up today!