[Haskell-cafe] STM and FFI

[Sterling Clover]

I've been playing with this, and on top of STM as it exists, managed  
to neatly interleave it with sqite3 and postgres. To do so with  
postgres, however, required setting the locking mode to be a bit more  
restrictive than it is out-of-the-box. Clever use of encapsulation  
and monad transformers gets you 90% of the way there quite easily.  
Note, however, that unsafeIOToSTM is *much* more unsafe at the moment  
than you would expect -- in fact there is no "safe" way to use it at  
all, due to the interaction of exceptions and rollbacks at the  
moment. The thread about this on glasgow-haskell-users[1], along with  
my initial note, has a very useful reply by Simon Marlow where he  
both explains some things about the STM implementation and logic  
behind it that I didn't understand, and also describes how the GHC  
team intends to fix this at some point in the future.

Regards,
Sterl.

[1] http://www.nabble.com/Where-STM-is-unstable-at-the-moment%2C-and- 
how-we-can-fix-it-tc19236082.html#a19236082


On Sep 9, 2008, at 6:08 AM, Arnar Birgisson wrote:

> On Tue, Sep 9, 2008 at 11:58, Jules Bean <jules at jellybean.co.uk>  
> wrote:
>>> Maybe this is an idea for an extension to the STM system, adding
>>> something like unsafeIOToSTM, except that in addition to the main IO
>>> action, it also takes two more IO actions that are invoked on  
>>> rollback
>>> and commit, respectively.
>>>
>>> This might allow for integration with transactional systems (e.g. a
>>> remote transaction on an rdbms), although to support two-phased  
>>> commit
>>> we'd need a third action for the "prepare" step.
>>
>> That would be an absolutely killer feature.
>>
>> A common problem in large systems is that the underlying RDBMS  
>> supports
>> transactionality, but then the software layer has to handle its own
>> rollbacks. I've seen some nasty bugs when the DB rolled back and the
>> software didn't.
>>
>> If we could have a transactional RDBMS linked into STM with matching
>> semantics, that would be a very nice thing.
>
> I think this is entirely doable. For comparison we already have done
> this with another STM framework, the DSTM2 library for Java. I.e. we
> hooked into prepare, commit and rollback and integrated with both
> MySQL transactions and a transactional file system library from Apache
> Commons.
>
> I'm not yet involved enough with the GHC library code, but I guess
> this would require the addition of a "prepare" phase to the STM code.
>
> There's also the question of what to do when the remote TX system
> indicates failure, should the transaction be retried or aborted? In
> the DSTM2 case we make it abort and throws an exception encapsulating
> the remote error to the code that initiated the TX (in Haskell's case,
> the caller of atomically).
>
> On a related note, we do have a paper on utilizing the STM system for
> authorization and policy enforcement in general. The paper is to be
> presented at CCS'08, and has an implementation on top of DSTM2, but we
> have a technical report in the works that implements this on top of
> the Haskell STM and gives operational semantics for the whole thing.
>
> You can find the conference paper on my website:
> http://www.hvergi.net/arnar/publications
>
> cheers,
> Arnar
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