One of the problems we use to see frequently on Green Felt happened when we’d update a Javascript API: We’d add some parameters to some library function and then update some other files so that they called the function with the new parameters. But when we’d push the changes to the site we’d end up with few users that somehow had the old version of one of the files stuck in their cache. Now their browser is causing old code to call new code or new code to call old code and the site doesn’t work for them. We’d then have to explain how to reset their cache (and of course every browser has different instructions) and hope that if they didn’t write back that everything went OK.

This fragility annoyed us and so we came up with a solution:

• We replaced all of our <script> tags with calls to a custom “script” function in our HTML template system (we use Template::Toolkit; [% script("sht.js") %] is what the new calls look like).
• The “script” function is a native perl function that does a number of things:
  1. Reads the Javascript file into memory. While reading it understands C style “#include” directives so we can structure the code nicely (though we actually don’t take advantage of that yet)
  2. Uses JavaScript::Minifier::XS to minify the resulting code.
  3. Calculates the SHA hash of the minified code.
  4. Saves the minified code to a cache directory where it is named based on its hash value which makes the name globally unique (it also keeps it’s original name as a prefix so debugging is sane).
  5. Keeps track of the original script name, the minified script’s globally unique name, and the dependencies used to build the image. This is stored in a hash table and also saved to the disk for future runs.
  6. It returns a script tag that refers to the globally unique Javascript file back to the template which ends up going out in the html file. For example, <script src="js/sht-bfe39ec2e457bd091cb6b680873c4a90.js" type="text/javascript"></script>
• There’s actually a step 0 in there too. If the original Javascript file name is found in the hash table then it quickly stats its saved dependencies to see if they are newer than the saved minified file. If the minified file is up to date then steps 1 through 5 are skipped.

The advantages of this approach

It solves the original problem.

When the user refreshes the page they will either get the page from their browser cache or they will get it from our site. No matter where it came from the Javascript files it references are now uniquely named so that it is impossible for the files to be out of date from each other.

That is, if you get the old html file you will reference all the old named Javascript files and everything will be mutually consistent (even though it is out of date). If you get the new html file it guarantees you will have to fetch the latest Javascript files because the new html only references the new hashed names that aren’t going to be in your browser cache.

It’s fast.

Everything is cached so it only does the minification and hash calculations once per file. We’re obviously running FastCGI so the in memory cache goes across http requests. More importantly the js/ dir is statically served by the web server so it’s exactly as fast as it was before we did this (since we served the .js files without any preprocessing). All this technique adds is a couple filesystem stats per page load, which isn’t much.

It’s automatic.

There’s no script to remember to run when we update the site. We just push our changes up to the site using our version control and the script lazily takes care of rebuilding any files that may have gone out of date.

So you might be thinking, isn’t all that dependency stuff hard and error prone? Well, it’s really only one line of perl code:

sub max_timestamp(@) { max map { (stat $_)[9] || 0 } @_ } # Obviously 9 is mtime

It’s stateless.

It doesn’t rely on incrementing numbers (“js/v10/script.js” or even “js/script-v10.js”). We considered this approach but decided it was actually harder to implement and had no advantages over the way we chose to do it. This may have been colored by our chosen version control system (darcs) where monotonically increasing version numbers have no meaning.

It allows aggressive caching.

Since the files are named by their contents’ hash, you can set the cache time up to be practically infinite.

It’s very simple to understand.

It took less than a page of perl code to implement the whole thing and it worked the first time with no bugs. I believe it’s taken me longer to write this blog post than it took to write the code (granted I’d been thinking about it for a long time before I started coding).

No files are deleted.

The old js files are not automatically deleted (why bother, they are tiny) so people with extremely old html files will not have inconsistent pages when they reload. However:

The js/ dir is volatile.

It’s written so we can rm js/* at any point and it will just recreate what it needs to on the next request. This means there’s nothing to do when you unpack the source into a new directory while developing.

You get a bit of history.

Do a quick ls -lrt of the directory and you can see which scripts have been updated recently and in what order they got built.

What it doesn’t solve

While it does solve the problem of Javascript to Javascript API interaction, it does not help with Javascript to server API interaction–it doesn’t even attempt to solve that issue. The only way I know to solve that is to carefully craft the new APIs in parallel with the old ones so that there is a period of time where both old and new can work while the browser caches slowly catch up with your new world.

And… It seems to work

I’ve seen similar schemes discussed but I’ve not seen exactly what we ended up with. It’s been working well for us–I don’t think I’ve seen a single bug from a user in a couple months that is caused by inconsistent caching of Javascript files by the browser.

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We’ve added a new high score option today. It’s the checkbox at the bottom of the high score table that reads “Show a player’s first score, not their best.” The high score table shows only one score for other players (though it always shows all of your own scores) and this option allows you to set which score of theirs you want to see—the best or the first.

Before we added this option, the high score tables showed the first game for other players. Along with adding the option, we’ve changed it so that is no longer the default.

Why did we do this?

Because of the way the high score tables used to work, certain people were “gaming” the system—they’d play a game over and over anonymously (or with different alternate user accounts) and then log into their main user account and play it one last time to get a ridiculously fast run time. Jim and I talked about different ways to combat this but in the end we decided to cater to that style of play instead of trying to ban it outright. With the new default high score settings, there’s no need to log out, play, log in, play again. Just play a game repeatedly while logged in and your score will move up the ranks.

We’ve left the option to show the first score there for people who just want to see how well they can do against a level playing field.

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We’re upgrading our database (PostgreSQL 9.2 to 9.3, if you’re interested). Scores will be kept but the high score tables will not be able to be viewed and no one will be able to log in until it is finished.

I’m trying to stay up (by watching a movie) until it is finished upgrading so I can start everything back up as quickly as possible, but I’m fading fast and may fall asleep before it’s done. If that happens it’ll be down until I wake up and finish the (manual) process.

Update: Jim and I finished the upgrade this morning at about 10. Everything should be working at this point. The games that happened during the DB upgrade are still being loaded into the new DB, so things might be a little slow for a couple hours.

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You’ve probably noticed over the past two or three months that certain parts of the site aren’t working well. Specifically, the high scores and leader board pages are often slow and even completely unresponsive. It’s happening for two reasons. The first reason is that we don’t really know squat about database design. Actually I should say “I know nothing about database design” instead of “we” and leave Jim out of this. There are a number of, uh, questionable design decisions that were made when I first set up the database and we think they might be causing the problems.

Or maybe not. See, we run our site at a hosting company (originally it ran off my cable modem at home but it was too unreliable when we started to get lots of people playing). The problem is that our hosting company runs the database on a server we don’t have access to. So when things are locking up there’s no way for us to go in and figure out what exactly is going wrong. We’ve emailed them and tried to solicit help but they aren’t very responsive. They most they’ll do is just reset the server–They don’t try to help us figure out why the problems are happening in the first place.

So what is the solution? Well, we’re attacking both problems. First we are going to move to a new database that will run on a server that we have access to. Second, we are going to start making some changes to our database that will make it smaller and more streamlined. This should also help make things faster.

So hang in there! Bear with us. We’re working on it and we’ll hopefully have something soonish. Hopefully you won’t notice the transition except that the non-responsiveness will go away (forever, ideally). We’ll probably mention something here–I’m even considering a beta test, but no guarantees.

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The following post was originally written in August of 20066. For some reason I never posted it to the blog, though reading it now it seems interesting. It’s fairly technical, unlike some of the other posts. Hope you enjoy it:

I wasn’t sure how to best link in our new blog. After thinking about it a while I decided that a little box in the corner of the site could list the last 2 or 3 blog posts. I then figured I should be able to list the last forum posts too. Once I had the idea, I had to figure out how I was going to get the info from two diverse pieces of code into our green felt page template.

Then I noticed that the blog we set up (wordpress) has an RSS feed. This is a little XML data file that always has the latest posts in it. Nice. The forum didn’t have any RSS support though. Luckily, it’s one of the most popular forums out there so there were at least 3 add-ons that would add RSS functionality. It took me 2 tries to get a good one, but in the end I liked the way it worked. I was able to customize it and add in a couple XML fields that they didn’t set by default.

After seeing how slow the RSS feeds were to respond I decided that it would be best for the client to go get the data and transform it onto the screen. Nobody wants to wait 2 extra seconds for their page to load just for some integrated RSS.

I then had to coax the XML from the RSS into a form that could be displayed on the page. I started with the standard javascript DOM interface, but it was a big pain and the code was growing way too big way too fast. I then realized that it would be best to parse it on the server side and send JSON over to the javascript client. So it was time for a quick CGI script.

Let’s hear it for perl! I whipped up something that parsed the XML adequately in about 5 minutes. The key to the whole script is this line here:

print objToJson(XMLin($response->content, ForceArray=>["item", "channel"]));

That uses perl’s XML::Simple to parse the XML into a perl structure. The “ForceArray” part makes sure that all the “item” and “channel” tags in the XML get interpreted as array items. It then outputs that structure as JSON, ready to be interpreted by the javascript side.

The javascript side is easy since it has everything in a native structure. Want the title of the first item in the first channel? rss.channel[0].item[0].title. Once I had the right data structures, the rest of the code basically wrote itself.

In the end I’m happy with the way the code turned out, but I’m not sure whether an extra little box in the corner of the page use useful enough to balance out the amount of clutter it adds.

-David

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