Monday, August 1, 2011

Super Sized JavaScript

The most technically challenging improvements in Robot Framework 2.6 are the new logs. They are about 1/10 of the size of the old logs and they are completely generated from very large JSON objects. One of the challenges in the new format is that the log is a single file that includes all the generated JavaScript, HTML and CSS code.

The largest new style log-files so far have been about 100 MB (that is a lot of JavaScript) and I believe that because we are working with such a big JavaScript objects we've encountered many difficulties that others have not yet have to deal with.

Although these files take time to download when loaded from servers, once loaded they work very well. Actually after we had first figured out how to make these almost 100 MB log-files work the reality came in and we had to figure out a way to split those large files to reasonable sized pieces (the method that we used is also explained in this post)..

Here's a list of tricks that we have learned during the process of super sized JavaScript development.

Doing large computations in JavaScript and how to prevent browser from freezing.

One of the first problems we had was that the extra large html/JavaScript files would freeze almost all browsers while expanding all the log elements in logs tree view. The final solution to this problem required some thinking and experimentation as the second point (not putting everything to the event queue) wasn't obvious.

JavaScript engines are one threaded and event based. This means that a big task will freeze everything. You can split your big task with setTimeout function to smaller parts. But if you split your task in too many parts and queue them all to the task queue the browser will again freeze as there is no space for users tasks.

The solution is to have a separate queue for the tasks that are generated during execution and to have only one task in the real event queue in any given time.

function timerTasker() {
var currentTask = tasksQueue.nextTask();;
tasksQueue.appendAll(currentTask.tasks()); //add tasks generated during current task execution
if (!tasksQueue.isEmpty()) {
setTimeout(timerTasker, 0);

Lazy domain objects.

Everything should be as lazy as possible when dealing with a huge serialized data. It would be very sad to run out of memory or CPU when generating thousands of useless objects that no one will ever use.

IE9 JavaScript parsing out of memory.

Everything seemed to work ok even with very large files (Over 20 MB) but then we tried them in the IE9 and some of the logs just didn't work. After hard debugging we found that IE9 had a very odd problem that none of the other browsers had.

For some reason IE9 JavaScript parsing with reasonable small sized lists containing nested lists and integers and strings (the total size in our case was "only" 1.5 MB) will run in to out of memory error. This can be prevented by not mixing integers and strings.. In my opinion this could be a bug in IE9 as IE8 doesn't have these problems.

Too Large JavaScript.

After IE9 problem was fixed everything was again OK. Until we finally tried to generate extremely big JavaScript log files. This time the problems were with Firefox. Luckily these were simple problems with easy fixes -- just had to invent a clever way to split our data.

Firefox 4 (and 5) will at some point between 40MB - 80MB start to say that your JavaScript block is too large.. To prevent this use multiple JavaScript blocks instead of just one. The memory errors seem to occur during parsing, thus you can handle extremely large objects but you just have to keep the parser happy.

<script type="text/javascript"> = [.. [big data subelement] ..];

-- transform this to: --

<script type="text/javascript">
window.dataSubElement = [big data subelement];
<script type="text/javascript"> = [.. window.dataSubElement ..];

Loading more JavaScript on the fly + Chrome safety.

100 MB log files were not something that all of our users wanted to work with. So we had to find a way to split the logs. The method had to work also locally. After brainstorming with ideas and spiking with techniqus we finally found a solution that works.

There are many ways to load more JavaScript while a page is open. I think that AJAX request to a server is the most common way but in our case there is no server.

There is a convenient function in jQuery for downloading new scripts ($.getScript) but it doesn't work for local files when using Chrome (@see Chrome issue 40787).

Our solution was to insert new script blocks to dom-tree on the fly (How to Dynamically Insert JavaScript). This works at least in our case.

Robot Framework 2.6 with new logs and reports has been finally released and everything seems to run smoothly (knock on wood). It was very interesting experience (with a lot of unexpected problems) to develop them. Respect to all my teammates!

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