Persistent Code

MagLev persists Ruby code as well as Ruby objects. You can install code once, and share with all VMs. MagLev lets you have either persistent or non-persistent code. By default (i.e., if you do nothing), when MagLev reads a ruby file, the code is loaded transiently. This is just like regular ruby: each VM loads code, and when the VM exits, the code disappears.

Maglev.persistent do ... end

To follow along with this example, go to the hat trick directory:

$ cd $MAGLEV_HOME/examples/persistence/hat_trick

rabbit.rb defines a simple class with one method:

# This class defines the Rabbit, which just prints an ASCII art
# representation of a rabbit.
class Rabbit
  def inspect
    "\n () ()\n( '.' )\n(\")_(\")\n"
  end
end

If you run this file, not much happens. A VM starts up, reads in the code, defines a transient class named Rabbit, then exits, destroying all of that work. You cannot access Rabbit from a new VM:

$ maglev-ruby rabbit.rb 
$ maglev-ruby -e 'puts Rabbit.new.inspect'
ERROR 2730 , uninitialized constant Rabbit (NameError)

maglev-ruby provides a -Mcommit option that will run within the context of a Maglev.persistent block, and do a commit on exit. So we can do this:

$ maglev-ruby -Mcommit rabbit.rb 
$ maglev-ruby -e 'puts Rabbit.new.inspect'

 () ()
( '.' )
(")_(")

The maglev-ruby -Mcommit rabbit.rb line is equivalent to:

Maglev.persistent do
  load 'rabbit.rb'
end
Maglev.commit_transaction

We can also persistently monkey patch a class. Create a new file, breed.rb:

Maglev.persistent do
  # Monkey patch the Rabbit class
  class Rabbit
    def self.breed
      5.times.map { Rabbit.new }
    end
  end
end
Maglev.commit_transaction

Since this version has the persistent block baked in, we just run the file:

$ maglev-ruby breed.rb

And now we start a fresh VM, and we will immediately have access to the new method:

$ maglev-irb
irb(main):001:0> Rabbit.breed
=> [
 () ()
( '.' )
(")_(")
,
 () ()
( '.' )
(")_(")
,
 () ()
( '.' )
(")_(")
,
 () ()
( '.' )
(")_(")
,
 () ()
( '.' )
(")_(")
,
]
irb(main):002:0>

Details

For details, read the Persistence Documentation.