Self Modifying.

The MagLev alpha was released recently.  Before I get too far in to this post I need to make it clear that I’m not affiliated with the MagLev development team.  I’m not really even much of a Ruby interpreter hacker.  I’m a curious ruby developer that has heard some interesting things about the project and wanted to get it up and running now that it’s available.  I decided to make this post because the install and setup procedure is anything but standard.  It’s not complicated, just not what you normally would expect.

First lets get the code:

$ git clone git://github.com/MagLev/maglev.git

Initialized empty Git repository in /Users/rgarver/Sources/maglev/.git/
remote: Counting objects: 28955, done.
remote: Compressing objects: 100% (12671/12671), done.
remote: Total 28955 (delta 15669), reused 28427 (delta 15200)
Receiving objects: 100% (28955/28955), 14.97 MiB | 539 KiB/s, done.
Resolving deltas: 100% (15669/15669), done.
Checking out files: 100% (2180/2180), done.
Initialized empty Git repository in /Users/rgarver/Sources/maglev/.git/remote: Counting objects: 28955, done.remote: Compressing objects: 100% (12671/12671), done.remote: Total 28955 (delta 15669), reused 28427 (delta 15200)Receiving objects: 100% (28955/28955), 14.97 MiB | 539 KiB/s, done.Resolving deltas: 100% (15669/15669), done.Checking out files: 100% (2180/2180), done.

$ cd maglev

Great, we have the code.  Next step is to do a base install.  This installs the base libraries and GemStone which is the fabled persistence layer that MagLev has integrated.  GemStone is a object persistence layer originally built for Smalltalk.  If you haven’t ever played with Smalltalk or some of the variants (eg: Squeak) I recommend it.  It will turn your head upside down.

$ ./install.sh
[Info] Starting installation of MagLev-22578.MacOSX on sirius.local
Password:
Sat Nov 21 09:22:44 PST 2009
[Info] Setting up shared memory
  Total memory available is 4096 MB
  Max shared memory segment size is 4 MB
  Max shared memory allowed is 4 MB
[Info] Increasing max shared memory segment size to 2048 MB
kern.sysv.shmmax: 4194304 -> 2147483648
[Info] Increasing max shared memory allowed to 2048 MB
kern.sysv.shmall: 1024 -> 524288
[Info] Adding the following section to /etc/sysctl.conf
# kern.sysv.shm* settings added by MagLev installation
kern.sysv.shmmax=2147483648
kern.sysv.shmall=524288
kern.sysv.shmmin=1
kern.sysv.shmmni=32
kern.sysv.shmseg=8
[Info] Setting up GemStone netldi service port
[Info] Adding "gs64ldi  51456/tcp" to /etc/services
[Info] Downloading GemStone archive using /opt/local/bin/wget
--2009-11-21 09:22:44--  http://glass-downloads.gemstone.com/maglev/GemStone-22578.MacOSX.zip
Resolving glass-downloads.gemstone.com... 207.171.185.197
Connecting to glass-downloads.gemstone.com|207.171.185.197|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 74858717 (71M) [application/zip]
Saving to: `GemStone-22578.MacOSX.zip'

100%[==========================================================================>] 74,858,717   847K/s   in 1m 45s

2009-11-21 09:24:32 (694 KB/s) - `GemStone-22578.MacOSX.zip' saved [74858717/74858717]

[Info] Uncompressing GemStone archive into /Users/rgarver/Sources
[Info] Linking gemstone to /Users/rgarver/Sources/GemStone-22578.MacOSX
[Info] updating MSpec, RubySpec, and RBS submodules
Submodule 'benchmark' (git://github.com/acangiano/ruby-benchmark-suite.git) registered for path 'benchmark'
Submodule 'spec/mspec' (git://github.com/rubyspec/mspec.git) registered for path 'spec/mspec'
Submodule 'spec/rubyspec' (git://github.com/rubyspec/rubyspec.git) registered for path 'spec/rubyspec'
Initialized empty Git repository in /Users/rgarver/Sources/maglev/benchmark/.git/
remote: Counting objects: 7332, done.
remote: Compressing objects: 100% (5521/5521), done.
remote: Total 7332 (delta 1595), reused 6917 (delta 1274)
Receiving objects: 100% (7332/7332), 9.90 MiB | 578 KiB/s, done.
Resolving deltas: 100% (1595/1595), done.
Submodule path 'benchmark': checked out 'd807eea7f7b2f38240bc177a0c22e599081882ea'
Initialized empty Git repository in /Users/rgarver/Sources/maglev/spec/mspec/.git/
remote: Counting objects: 2745, done.
remote: Compressing objects: 100% (1080/1080), done.
remote: Total 2745 (delta 1848), reused 2484 (delta 1644)
Receiving objects: 100% (2745/2745), 378.57 KiB | 383 KiB/s, done.
Resolving deltas: 100% (1848/1848), done.
Submodule path 'spec/mspec': checked out 'bcec47c70e0678a29fd0c1345358c4daf7b971a3'
Initialized empty Git repository in /Users/rgarver/Sources/maglev/spec/rubyspec/.git/
remote: Counting objects: 26787, done.
remote: Compressing objects: 100% (8705/8705), done.
remote: Total 26787 (delta 18332), reused 25672 (delta 17482)
Receiving objects: 100% (26787/26787), 3.71 MiB | 520 KiB/s, done.
Resolving deltas: 100% (18332/18332), done.
Submodule path 'spec/rubyspec': checked out 'b0a18cf80dc706d39ee550831b8b941224b60fb6'
[Info] Creating new default 'maglev' repository
[Info] Generating the MagLev HTML documentation
[Info] Finished upgrade to MagLev-22578.MacOSX on sirius.local

[Info] MagLev version information:
maglev 0.6 (ruby 1.8.6) (2009-11-20 rev 22578-1067) [x86_64-linux]
GEMSTONE: 3.0.0 Build: 64bit-22578
MONTICELLO: MagLev-ao.1067.mcz
MAGLEV: commit e2a4fe2e0f7ca85cdcb141e6b56913eba802eefd
        Author: Allen Otis <otisa@abaco.gemstone.com>
        Date:   Thu Nov 19 19:57:09 2009 -0800
[Info] GemStone version information:
GemStone/S 64 Bit
3.0.0 Build: 64bit-22578
Fri Nov 20  8:22:00 2009

[Info] Adding these to your .bashrc will make it easier to run MagLev
export MAGLEV_HOME=/Users/rgarver/Sources/maglev
export PATH=$MAGLEV_HOME/bin:$PATH

[Info] After you complete this upgrade and verify MagLev is working, run
  rake stwrappers
to generate the .rb files for the GemStone/Smalltalk FFI
in MAGLEV_HOME/lib/ruby/site_ruby/1.8/smalltalk/

As you can see on OS X it will build everything for 64bit which is pretty cool.  It also downloaded a bunch of support libraries and updated all of the submodules.  If you ever update the code locally you are supposed to run ‘$ ./update.sh’ to rebuild everything and get it all up and running.

Once you have it installed you should add the following lines to your .profile or .bashrc

export MAGLEV_HOME=/Users/rgarver/Sources/maglev
export PATH=$MAGLEV_HOME/bin:$PATH

You’ll need to make sure you run those lines on the command line also.  Once the environment is setup you can run ‘$ rake maglev:start’.  This command apparently boots up the core MagLev engine.

$ rake maglev:start
(in /Users/rgarver/Sources/maglev)
startstone[Info]: Starting Stone repository monitor "maglev".
startstone[Info]: GemStone server 'maglev' has been started.

Once that is started you are good to go:

$ maglev-irb
/Users/rgarver/.irbrc
error , no such file to load -- readline,
          during /Users/rgarver/Sources/maglev/lib/ruby/1.8/irb/completion.rb
error , no such file to load -- readline,
          during /Users/rgarver/.irbrc
irb(main):001:0> puts 'hi'
hi
=> nil
irb(main):002:0>

I’ve been playing around more with Red.  I alternate between really liking it and totally pulling my hair out.  On a order form app that I’m building I made some very quick work of a dynamically updating billing summary div.  It came together very quickly until I added the AJAX piece.  AJAX support in Red is actually very good, but JSON support is non-existent.  This made Ryan a sad boy.  I dug around to see what other people have done and found that they haven’t done much.  There is some stub code for a Request::JSON that my be what I’m looking for at some point, but I need it now.  So here is what I did:

class Module
  def define_method(sym, &block)
    `this.prototype['m$'+sym.__value__]=block.__block__.__unbound__`
    `Red.updateChildren(this)`
    `Red.updateIncluders(this)`
    return `block`
  end
end

class Object
  def self.from_json(text)
    ret = Object.new
    meths = []
    `var v = eval("("+#{text}.__value__+")")`
    `for(var member in v){#{meths}.push(new Array($q(member), $q(v[member])))}`
    meths.each do |meth|
      ret.class.send(:define_method, meth[0]) do
        meth[1]
      end
    end
    return ret
  end
end

Now doing a simple AJAX call that returns JSON becomes really easy:

@req = Request.new(:url => '/orders')
@req.upon(:response) do |response|
  new_data = Object.from_json(response.text)
  @summary = new_data.summary
  @subtotal = new_data.subtotal
  @total = new_data.total
end
@req.execute(:data => {'plan_id' => @plan, 'promo' => @promo})

A few things to note here.  The section that defines `define_method` on `Module` is actually back ported from the current master branch of Red.  This works pretty well, though I haven’t tried it with a JSON string that describes a living object.  But I don’t think it’s too far from being able to handle that.

No.

If you are in a position to ask this question I humbly recommend you default to: “No, I do not need this as an instance variable right now.” There is a tendency to plan for every potentiality when doing software development and this is a prime example of such a situation. Resist the temptation.

Instance variables represent state associated with an object in memory. When you declare an instance variable you are saying to all future developers: “This piece of state needs to last for the life time of this object because it will be used elsewhere.” Think very hard about that. We are saying loudly here that this variable is coupled to other fragments of code. Which code? Dunno, do a global search .

This is called tight coupling. I will go so far to say that coupling is the source of all evil and complexity that we deal with as programmers. With every step that we, as programmers, take in building a system should be with an eye to reduce or at least not increase the coupling in the system that we are working on. The fewer instance variables that we have in a Rails controller the smaller the potential coupling between the view and other method calls that have access to those instance variables. Each method, each action in a controller, should be as self contained as possible. The less leakage of state out of a function the better. Generally your view doesn’t need to see that temp variable of the calculations that you made to set a model attribute keep it local, drop the “@”. Worst case scenario you go back and refactor your code to include that “@”.

If you had asked me about doing this 6 months ago I would have said that it would be worth it.  It would be too hard and be a ton of work to get it functional and even then it wouldn’t be good enough to actually use.  After seeing this video I’m glad I am wrong.  This is totally sweet.