scala-meteor-chat

This project is now an example of Atmosphere working with Scala in a relatively simple way. This project is based on the meteor-chat example provided with Atmosphere.

Unlike the project this was based on, this project uses jQuery rather than Prototype to match the use of the atmosphere jQuery plugin.

Along the way I had quite a few simple but frustrating problems. These are documented a little below to help prevent anyone else having them!

 Running the example

• Install sbt 0.11.

Built and run with the commands:

sbt
> jetty-run

Problems along the way

Here are some problems I ran across and found fixes to.

404 error in the browser trying to access /atmosphere-meteor-chat/Meteor?0

I was getting browser errors trying to access /atmosphere-meteor-chat/Meteor?0 and realised that the path in the web.xml file did not match the path in the application.js file. So I changed line 3 of application.js from:

url: '/atmosphere-meteor-chat/Meteor',

to

url: '/Meteor',

This was just because I was serving it as a stand-alone site which is different from the original.

NoSuchMethodError for HttpServletRequest.isAsyncStarted using Jetty 7.3.1.v20110307

To start off using Jetty 7.3.1.v20110307 I was getting a no such method error for javax.servlet.http.HttpServletRequest.isAsyncStarted. This error went away when I upgraded to Jetty 8.0.0.M3.

Here was the error stacktrace:

[warn] Error for /Meteor
java.lang.NoSuchMethodError: javax.servlet.http.HttpServletRequest.isAsyncStarted()Z
    at org.atmosphere.container.Servlet30Support.suspend(Servlet30Support.java:136)
    at org.atmosphere.container.Servlet30Support.service(Servlet30Support.java:91)
    at org.atmosphere.cpr.AtmosphereServlet.doCometSupport(AtmosphereServlet.java:1182)
    at org.atmosphere.cpr.AtmosphereServlet.doPost(AtmosphereServlet.java:1164)
    at org.atmosphere.cpr.AtmosphereServlet.doGet(AtmosphereServlet.java:1150)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:707)
    at org.eclipse.jetty.websocket.WebSocketServlet.service(WebSocketServlet.java:86)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
    at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:534)

However, having read more about the version of Jetty here: http://java.dzone.com/articles/roadmap-jetty-6-jetty-7-and I would prefer to run under Jetty 7 which seems more aligned to likely deployment at the present time.

I tried what appeared to be the latest official release: 7.5.0.v20110901 but this failed with a different error.

I tried the latest official release from one of the maven indexes: 7.4.5.v20110725 but this failed with the same isAsyncStarted error which I now understand to be due to the lack of Servlet 3.0 support. So I tried this save version with Atmosphere 0.7.2. This still generated the isAsyncStarted error. However, given that this was trying to use the Servlet 3.0 API I noticed that I was including:

"org.apache.geronimo.specs" % "geronimo-servlet_3.0_spec" % "1.0",

and changed this back to:

"org.apache.geronimo.specs" % "geronimo-servlet_2.4_spec" % "1.0",

and it worked. Then I noticed that the scalatra codebase chose to include:

"javax.servlet" % "javax.servlet-api" % "3.0.1" % "provided"

instead, so I tried that and, although it is the Servlet 3.x API it also works!

I also tried changing the version of Atmosphere back to 0.7.2 and found that this worked fine again.

Eventually I discovered that if I set the jetty libraries as "provided,jetty" then they were included during the compilation and supplied for actual web serving and this seemed to solve everything removing any need for either javax.servlet-api or geronimo-servlet_X.X_spec.

Delayed output on Webkit browsers

I encountered delayed output on Webkit based browsers which seems to have been resolved by explicitly setting the content type of the response to:

Content-Type: text/plain;charset=UTF-8

or

Content-Type: text/html;charset=UTF-8

Missing the content type appears to result in a delay in display on the client if the client is running Webkit. However, in another example this wasn't enough and further investigation found this issue raised in detail here: http://code.google.com/p/chromium/issues/detail?id=2016.

The first workaround is to pad the output with 256 characters of output before sending anything that matters. This apparently works, but doesn't seem very elegant so say the least! Atmosphere does this automatically in some situations.

The next workaround (which I haven't tried, but is listed in the explanation above) is to set:

Content-Type: application/octet-stream
Transfer-Encoding: chunked

However, setting the transfer encoding to 'chunked' doesn't appear to be so easy with the Servlet API. Apparently if the output buffering is switched off and the size is not sent then it should default to chunked but I haven't been able to find a way of making that work yet!

The calls to Atmosphere now seem to successfully including this padding of characters so I'm not entirely sure where I went wrong before.

Websocket support not working in Firefox 8 and Opera 11

This stackoverflow question answers the question of which browsers support WebSockets: http://stackoverflow.com/questions/1253683/websocket-for-html5.

One of the links: http://techdows.com/2010/12/turn-on-websockets-in-firefox-4.html explains how to enable WebSocket support in Firefox releases that have it implemented but disabled.

However, to add to the confusion the websocket object has been renamed in the later versions of Firefox so you use MozWebSocket rather than WebSocket

I therefore modified the atmosphere.jquery.js library to check for either WebSocket or MozWebSocket before falling back to Comet, and to use MozWebSocket rather than WebSocket to create the socket if the latter was not available.

This has now been incorporated into the latest release of the jquery.atmosphere.js plugin.

Websockets don't establish or establish and immediately fail on Safari and Chrome

This problem held me back for ages and was finally resolved by the simple addition of these two parameters to the web.xml configuration:

<init-param>
    <param-name>org.atmosphere.useWebSocket</param-name>
    <param-value>true</param-value>
</init-param>
<init-param>
    <param-name>org.atmosphere.cpr.WebSocketProcessor</param-name>
    <param-value>org.atmosphere.cpr.HttpServletRequestWebSocketProcessor</param-value>
</init-param>

ngrep debugging didn't seem to work

Latterly I tried to debug what was going on using ngrep which I hadn't used before, but couldn't get it to see the communications. It turns out that it made a big difference to connect to my local server on address http://127.0.0.1:8080 rather than http://localhost:8080. Having found that out, the following command line was very helpful for tracing what went on:

    sudo ngrep -d lo0 -q -W byline port 8080

Atmosphere's websocket Meteor implementation doesn't handle www form encoding of parameters

I found that when you sent parameters www form encoded they were received and interpreted as request parameters by the web server except when the transport was websocket. To work around this I JSON encoded all the data for all transmissions.