On 11 January 2011 07:18, Paolo Giarrusso <p.giarru...@gmail.com> wrote:
> Hi all,
> On Mon, Jan 10, 2011 at 09:22, William ML Leslie
> <william.leslie....@gmail.com> wrote:
>> On 10 January 2011 15:24, Nathanael D. Jones <nathanael.jo...@gmail.com> 
>> wrote:
>>> 4) Dynamic code loading. Users will be able to 'branch' their own version of
>>> the world and share it with others. There may be thousands of versions of a
>>> class, and they need to be able to execute in separate sandboxes at the same
>>> time. Source code will be pulled from a Git repository or some kind of
>>> versioning database.
>> Quite like this idea.
>> You do have to deal with a bunch of (fairly well known) problems,
>> which any specific implementation of dynamic code loading is going to
>> need to solve (or not).  Pypy doesn't currently implement any
>> hot-schema-change magic, and reloading has always been error prone in
>> the presence of state.  First-class mutable types make it particularly
>> difficult (there is no sensible answer to what it means to reload a
>> python class).
> You might want to reuse the solutions to those issues used in the Java
> (and maybe .NET) world. Java allows reloading a class in a different
> classloader, and that has been used inside OSGi (see
> http://www.osgi.org/About/Technology).
> Not sure about the solution in OSGi, but Java serialization allows to
> serialize an instance of version 1 of a class, and to de-serialize it
> with version 2 of that class, if v2 takes extra care for that; one
> could use this to convert existing instances.

Sure, and there is also Dynamic Class Evolution for Hotspot, and many
other VMs support some form of reloading and redefinition (goops is an
exceptional and accessible example, see
for a post with lots of diagrams).

What I am trying to say is that the *ability* to do reloading does not
mean your code will work in the presence of interface changes.  You
have to decide whether updating a closure in a way that captures more
variables or changes the signature updates existing instances of that
closure: if it does, there will be entries missing in the closure
slot, if it doesn't, older closures won't be callable in the same way
as the new closures.  No matter what, you here need to write code to
be explicitly reloadable, or provide some way to instruct the runtime
what to do for each schema change.

>> The one issue that interests me is where you implement the persistence
>> boundary - do you go with orthogonal persistence and act as if
>> everything is preserved, or assume all user code is run within some
>> sort of (fast and loose) transaction that can be re-entered at will,
>> providing an API for persistent data access?  The second case makes
>> the reloading question a bit more reasonable, because you can always
>> throw away the current delta and replay the external effects, assuming
>> the interface for the external events hasn't changed significantly.
> The key question is: when would you start and commit such transactions?
> Apart from that, your idea looks very similar to Software
> Transactional Memory (STM). STM restarts explicitly-marked
> transactions in a thread when some other thread modifies the affected
> data (which would be a data race) and commits its transaction. In your
> case, a transaction is restarted when some other thread modifies the
> involved code.

There is a bit of ambiguity in the "some other thread modifies" here.
I don't know what synchronisation and communication is going on in
your game, but I suspect that it only rarely interacts with reloading
code in an interesting way.  I'll reply to this properly in another
email, I'd better get back to work :)

William Leslie

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