Getting a python-like shell for Perl

When learning a new language, I find it really helpful if I can test out new syntax and language constructs without going through the trouble of creating a file and all the boilerplate along with it. Perl doesn’t have this capability built-in, as far as I know, but there’s this great CPAN module that claims to do the same thing. It’s a short little module (just about 100 SLOC), and it gives you a familiar prompt:

Perl 5.14.2 (Tue Feb  4 23:09:53 UTC 2014) [linux panlong 2.6.42-37-generic #58-ubuntu smp thu jan 24 15:28:10 utc 2013 x86_64 x86_64 x86_64 gnulinux ]
Type "help;", "copyright;", or "license;" for more information.
>>> 

It’s not very obvious how to set up the thing though. I installed the module and its dependencies via cpanm, and then created this little snippet in a file called perlthon.pl:

#!/usr/bin/perl
use 5.010;
use strict;
use warnings;

use Perl::Shell;
Perl::Shell::shell();

Then, the shell started right up.

Exporting comments from Facebook Comments to Disqus

About 14 months ago, The Daily Californian switched from Disqus to Facebook Comments in order to clean up the comments section of the website. But recently, the decision was reversed, and it was decided that Disqus would make a come back. I worked with one of my Online Developers to facilitate the process.

One of the stipulations of the Facebook to Disqus transition was that the Online Team would transfer a year’s worth of Facebook comments over to Disqus so that they would still remain on the website. Facebook doesn’t natively support any kind of data export feature for their commenting platform at the time of writing. I don’t expect that they will in the future. However, they provide a reliable API to their comments database. With a bit of ingenuity and persistence, we were able to successfully import over 99% of existing Facebook comments into Disqus.

Overview of the process

Disqus supports imports in the form of custom WXR files. These are WordPress-compatible XML files that contain things about posts (titles, dates, preview, id, etc.) and comments (name, date, IP, content, etc.).

The Daily Cal uses WordPress and the official Disqus WordPress plugin. The plugin identifies threads with a combination of WordPress’s internal post ID and a short permalink. Thread identifiers look like this:

var disqus_identifier = '528 http://code.rogerhub.com/?p=528';

This one is taken right from the source code of this post (you can see for yourself).

Facebook, on the other hand, identifies threads by the content page’s URL. After all, their system was created for arbitrary content, not just blogs. The Facebook Open Graph API provides a good amount of information about comments. There’s enough information to identify multiple comments posted by a single user. There’s accurate timestamp and reply relationships. There isn’t any personal information like IP addresses, but names are provided.

The overall process looked like this:

  1. On dailycal.org, we needed an API endpoint to grab page URLs along with other information about threads on the site.
  2. For each of these URLs, we check Facebook’s Open Graph API for comments that were posted on that URL. If there are any, then we put them into our local database.
  3. After we are done processing comments for all of the articles ever published on dailycal.org, we can export them to Disqus-compatible WXR and upload them to Disqus.

This seems like a pretty straight-forward data hacking project, but there were a couple of issues that we ran into.

Nitty-gritty details

The primary API endpoint for Facebook’s Comment API is graph.facebook.com/comments/. This takes a couple of GET parameters:

  • limit — A maximum number of comments to return
  • ids — A comma-delimited list of article URLs
  • fields — For getting comment replies

The API supports pagination with cursors (next/prev URLs), but to make things more simple, we just hardcoded a limit parameter of 9999. By default, the endpoint will return only top-level comments. To get comment replies, you need to add a fields=comments parameter to the request.

You can make a few sample requests to get a feel for the structure of the JSON data returned.

Disqus supports a well-documented XML-based import format. In our case, we decided that names were sufficient identification, although Disqus will support some social login tagging in the import format as well. The format specifies a place for the article content, which is an odd request, since article content is usually quite massive. We decided to supply just an excerpt of the article content rather than the entire page.

There were a few more precautions we took before we started development. In order to lower suspicion around our scraping activity on Facebook as well as our own web application firewall (WAF), we grabbed the user agent of a typical Google Chrome client running on Windows 7 x86-64, and used that for all of our requests. We also created a couple of test import “sites” on Disqus, since the final destination of our generated WXR was the Disqus site that we used a year ago before the switch to Facebook. There isn’t any way to copy comments or clone a Disqus site, so we didn’t want to make any mistakes.

Unicode support and escape sequences

The first version of our program had terrible support for any kind of non-ASCII character. It’s not that our commenters were all typing in Arabic or something (actually, we had a couple of comments that really were in Arabic). Smart quotes are used in plain English, and they ruin the process as well.

Facebook’s API spits out JSON data, and uses JSON’s encoding. For example, the double left quotation mark, otherwise known as lrquo in HTML/XML, is encoded as \u201c using JSON’s unicode standard. However, the JSON data also contains HTML-encoded entities like &. (Update: It appears that Facebook has corrected this issue.)

Python’s JSON library will take care of JSON escape sequences as it decodes the string into a native dictionary. However, the script applies HTML entity decoding on that result, in case there are any straggling escape sequences left. Since Disqus’s WXR format suggests that you throw the comment content into a CDATA block, all you need to escape is the CDATA ending sequence, ]]>. You can do this by splitting it up into 2 CDATA sections (e.g. ]]]><![CDATA[]>)

HTTP exceptions

Our API endpoint would timeout or throw an error every once in a while. To make our scraper more robust, we set up handlers for HTTP error responses. The scraper would retry an API request at least 5 times before giving up. If none of the attempts are successful, the URL is logged to the database for further debugging.

Commenter email addresses

Every commenter listed in the WXR needs an email address. Comments with the same email address will be tied together, and if somebody ever registers a Disqus account with that email address, they can claim the comments as their own (and edit/delete them). Facebook provides a unique ID that will associate multiple comments by the same person. But since Facebook comments also allows AOL and Yahoo! logins, not every comment has such an ID. Our script used the Facebook-provided ID when it was present, and generated a random one outside of Facebook’s typical range when it wasn’t. All of the emails ended with @dailycal.org, which meant that we would retain control over the registration verification, in case we needed it.

Edge cases

Disqus requires that comments be at least 2 characters long. There were a couple of Facebook comments that consisted of just one word: “B” or “G” or the like. These had to be filtered out before the XML export process.

We also ran into a case where a visitor commented “B<newline>” on the Facebook comments. For Disqus, this still counts as one character, since the CDATA body is stripped of leading and trailing whitespace before processing. The first version of our script didn’t strip whitespace before checking the length, so it failed to filter out this erroneous comment.

Timezone issues

After a couple of successful trials, we created a test site in Disqus and imported a sample of the generated WXR. Everything looked good until we went and cross-referenced Disqus’s data with the Facebook comments that were displayed on the site. The comment times appeared to be around 5 hours off!

Here in California, we’re GMT -0800, so there wasn’t a clear explanation why the comment times were delayed. WXR specified GMT times, which we verified were correct. The bug seemed to be on Disqus’s end. We contacted Disqus support and posted on their developer’s mailing list, but after around a week, we decided that it would be easiest to just counteract the delay with an offset in the opposite direction.

import datetime
time_offset = datetime.timedelta(0, -5*60*60)
comment_date = (datetime.datetime.strptime(
    comment['created_time'], "%Y-%m-%dT%H:%M:%S+0000"
    ) + time_offset).strftime("%Y-%m-%d %H:%M:%S")

Conclusion

After a dozen successful test runs, we pulled the trigger and unloaded the WXR onto the live Disqus site. The import process finished within 5 minutes, and everything worked without a hitch.

How to write fast-rendering HTML and CSS

RogerHub’s most popular page contains over 50,000 HTML nodes. It’s no easy task for a browser to chew through all of that HTML and render it quickly. However, most modern browsers are now able to render that particular page without any performance issues, including mobile and tablet web browsers. When it comes to website performance, most people are concerned with app server response time or JavaScript performance. There really aren’t a lot of things that you can do to make gigantic pages load more quickly, but here are some tricks I learned along the way.

Cut down on the CSS3

Drop shadows and gradients look great when you use them correctly, but when your document is 600,000px tall, they create serious browser lag no matter how modern your hardware may be. You get the fastest renderer performance with simple solid colors. Aesthetics are a small sacrifice when you’re trying to squeeze more speed out of your markup.

Hide unnecessary things

I keep all of my calculator’s comments on a single page because 1) that’s the way it has always been, and 2) SEO. However, many users never even look at those comments. It improves browser performance if you simply instruct the browser not to display most of the markup until they are requested, which brings me to the next point..

Limit the number of tree children

Rather than applying a universal .comment-hidden class to all older hidden comments, put hidden elements under a single unifying parent and apply styles to the parent instead. It’s much faster to style one parent element than a thousand children.

Fake it

The comments on RogerHub no longer support Gravatar-based photos, since literally hundreds of unique avatars were being requested and rendered on each page load. Since I didn’t want to take out the images entirely, everybody now gets a generic anonymous avatar. Much of the original comment markup has also been taken out, leaving only the bare necessities required to properly apply styles.

Use simple, non-nested classes

I don’t have any hard data behind this practice, but intuitively I feel that simple unnested classes are the fastest way to go about styling an arbitrary number of elements. Avoid tag selectors, deeply nested selectors, wildcard selectors, attribute/value selectors, etc.

Archiving and compressing a dynamic web application

From 1999 to mid-2011, the Daily Cal used an in-house CMS to run the website. It contains around 65,000 individual articles and thousands of images. But ever since we moved to WordPress, the old system has been collecting dust in the corner. It was about time that all of the old CMS content was archived as static HTML so that it could be served indefinitely in the future as server software evolves. To accomplish this, I set up a linux virtual machine with my trusty vagrant up utility on my spare home server.

Retrieving the application data

The CMS’s production server did not actually have enough free disk space to create an tarball copy of the application. But since there were on the order of 10,000 files involved, a recursive copy via scp would be too slow. In order to speed up the transfer process, I used a little gnu/linux philosophy and piped a few things together:

$ screen
$ ssh root@domain "tar -c /srv/archive" | tar xvf -

I decided that compression was not going to be very helpful because most of the data was jpeg and png images, which are not very compressible. Enabling compression would just slow things down, since the bottleneck would become the CPU rather than the network.

Preparing the application

The CMS had very few dependencies, which is not surprising given the state of PHP 10 years ago. I set up a simple nginx+php-fpm+mysql configuration with a single PHP worker thread. The crawl operation would be executed in serial anyway, so multiple workers would not be useful.

I also added an entry to the VM’s hosts file for the hostname of the production server. The server’s hostname was hardcoded in a few places, and I didn’t want the crawl operation sending requests out to Internet. Additionally, I set up a secondary web server configuration that served generated HTML from the output directory and static assets from the application data, so that I could preview the results as they were being generated.

Crawling the application

Generating the static pages was the hardest part of the archival process, and it took me around 5 retries to get it right. The primary purpose of the whole archival operation was to remove the dynamic portions of the web application. This meant that static versions of every single conceivable page request had to be run against the application and saved. I picked wget as my archival tool.

Articles in the CMS were stored in one of two places. However, the format of the URL was luckily the same for both. I dumped article IDs from the MySQL database and created a seed file of article URLs:

$ echo "SELECT article_id FROM dailycal.article;" | mysql -u root > article_ids
$ echo "SELECT id FROM dailycal.h_articles;" | mysql -u root  >> article_ids
$ sed -e 's/^/http:\/\/archive.dailycal.org\/article.php?id=/' -i article_ids

I didn’t see much point in setting a root password for the local MySQL installation, since this was a single-use VM anyway. Sed ate through the 65,326 article IDs in seconds. I then created a second seed file containing just the URL of the application root, from where (nearly) all other pages would be crawlable.

On the crawler’s final run, I set the following command line switches:

  • --trust-server-names - Sets the output file name according to the last request in a redirection chain. By default, wget uses the first request.
  • --append-output=generator.log - Outputs progress information to a file, so that I can run the main process in screen and monitor it with tail in follow mode.
  • --input-file=source.txt - Specifies the seed file of URLs.
  • --timestamping - Sets the file modification time according to HTTP headers, if available.
  • --no-host-directories - Disables the creation of per-host directories.
  • --default-page=index.php - Defines the name for request paths that end in a slash.
  • --html-extension - Appends the html file extension to all generated pages, even if another extension already exists.
  • --domains archive.dailycal.org - Restricts the crawl to only the application domain.

Additionally, I set the following switches to crawl through the links on the application’s root page.

  • -r - Enables recursive downloading.
  • --mirror - Sets infinite recursion depth and other useful options.

In total, 543MB of non-article HTML and 2.1GB of article HTML were generated. These are reasonable sizes, given how many URLs were crawled in total, but they were still a bit unwieldy to store. I looked for a solution.

Serving from a compressed archive

I knew a couple of data facts about the generated HTML:

  1. There was tons of redundancy. The articles shared much of their header and footer markup.
  2. Virtually all of the data consisted of printable characters and whitespace, which means considerably less unique information than 8 bits per byte.

Both of these factors made the HTML a good candidate for archive compression. My first thought was tar+gzip, but tar+gzip compression works on blocks, not files. To extract a single file, you’d need to parse all the data up till that file. A request for the last file in the archive could take 15 to 20 seconds!

Luckily, the zip file format maintains an index of individual files and compresses them individually, which means that single file extraction is instantaneous no matter where in the archive it is located. I opted to use fuse-zip, an extension for FUSE that lets you mount zip files onto the file system. Fully compressed, the 543MB of pages became a 92MB zip archive (83% deflation), and the 2.1GB of articles became a 407MB zip archive (81% deflation).

After everything was finished, I uploaded the newly created HTML archives to a new production server and shut down the old CMS. From there, a decade’s worth of archived articles can be maintained indefinitely for the future.

Self-contained build environments with Vagrant

Vagrant is a nifty piece of Ruby software that lets you set up Virtual Machines with an unparalleled amount of automation. It interfaces with a VM provider like VirtualBox, and helps you set up and tear down VM’s as you need them. I like it better than Juju because there isn’t as much hand-holding involved, and I like it better than vanilla Puppet because I don’t regularly deploy a thousand VM’s at a time. At the Daily Cal, I’ve used Vagrant to help developers set up their own build environments for our software where they can write code and test features in isolation. I also use it as a general-purpose VM manager on my home file server, so I can build and test server software in a sandbox.

You can run Vagrant on your laptop, but I think that it’s the wrong piece of hardware for the job. Long-running VM batch jobs and build environment VM’s should be run by headless servers, where you don’t have to worry about excessive heat, power consumption, huge amounts of I/O, and keeping your laptop on so it doesn’t suspend. My server at home is set up with:

  • 1000GB of disk space backed by RAID1
  • Loud 3000RPM fans I bought off a Chinese guy four years ago
  • Repurposed consumer-grade CPU and memory (4GB) from an old desktop PC

You don’t need great hardware to run a couple of Linux VM’s. Since my server is basically hidden in the corner, the noisy fans are not a problem and actually do a great job of keeping everything cool under load. RAID mirroring (I’m hoping) will provide high availability, and since the server’s data is easily replaceable, I don’t need to worry about backups. Setting up your own server is usually cheaper than persistent storage on public clouds like AWS, but your mileage may vary.

Vagrant configuration is a single Ruby file named Vagrantfile in the working directory of your vagrant process. My basic Vagrantfile just sets up a virtual machine with Vagrant’s preconfigured Ubuntu 12.04LTS image. They offer other preconfigured images, but this is what I’m most familiar with.

# Vagrantfile

Vagrant.configure("2") do |config|
  # Every Vagrant virtual environment requires a box to build off of.
  config.vm.box = "precise32"

  # The url from where the 'config.vm.box' box will be fetched if it
  # doesn't already exist on the user's system.
  config.vm.box_url = "http://files.vagrantup.com/precise32.box"

  config.vm.network :forwarded_port, guest: 8080, host: 8080

  # Enable public network access from the VM. This is required so that
  # the machine can access the Internet and download required packages.
  config.vm.network :public_network

end

For long-running batch jobs, I like keeping a CPU Execution Cap on my VM’s so that they don’t overwork the system. The cap keeps the temperature down and prevents the VM from interfering with other server processes. You can add an execution cap (for VirtualBox only) by appending the following before the end of your primary configuration block:

# Adding a CPU Execution Cap

Vagrant.configure("2") do |config|
  ...

  config.vm.provider "virtualbox" do |v|
    v.customize ["modifyvm", :id, "--cpuexecutioncap", "40"]
  end

end

After setting up Vagrant’s configuration, create a new directory containing only the Vagrantfile and run vagrant up to set up the VM. Other useful commands include:

  • vagrant ssh — Opens a shell session to the VM
  • vagrant halt — Halts the VM gracefully (Vagrant will connect via SSH)
  • vagrant status — Checks the current status of the VM
  • vagrant destroy — Destroys the VM

Finally, to set up the build environment automatically every time you create a new Vagrant VM, you can write provisioners. Vagrant supports complex provisioning frameworks like Puppet and Chef, but you can also write a provisioner that’s just a shell script. To do so, add the following inside your Vagrantfile:

Vagrant.configure("2") do |config|
  ...

  config.vm.provision :shell, :path => "bootstrap.sh"
end

Then just stick your provisioner next to your Vagrantfile, and it will execute every time you start your VM. You can write commands to fetch package lists and upgrade system software, or to install build dependencies and check out source code. By default, Vagrant’s current working directory is also mounted on the VM guest as a folder named vagrant in the file system root. You can refer to other provisioner dependencies this way.

Vagrant uses a single public/private keypair for all of its default images. The private key can usually be found in your home directory as ~/.vagrant.d/insecure_private_key. You can add it to your ssh-agent and open your own SSH connections to your VM without Vagrant’s help.

Even if you accidentally mess up your Vagrant configuration, you can use VirtualBox’s built-in command-line tools to fix boot configuration issues or ssh daemon issues.

$ VBoxManage list vms
...
$ VBoxManage controlvm <name|uuid> pause|reset|poweroff|etc
...
$ VBoxHeadless -startvm <name|uuid> --vnc
...
(connect via VNC)

The great thing about Vagrant’s VM-provider abstraction layer is that you can grab the VM images from VirtualBox and boot them on another server with VirtualBox installed, without Vagrant completely. Vagrant is a excellent support tool for programmers (and combined with SSH tunneling, it is great for web developers as well). If you don’t already have support from some sort of VM infrastructure, you should into possibilities with Vagrant.